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SUMMARY TECHNICAL REPORT 
OF THE 

NATIONAL DEFENSE RESEARCH COMMITTEE 


This document contains information affecting the national defense of 
the United States within the meaning of the Espionage Act, 50 U. S. C., 
31 and 32, as amended. Its transmission or the revelation of its con¬ 
tents in any manner to an unauthorized person is prohibited by law. 

This volume is classified RESTRICTED in accordance with security 
regulations of the War and Navy Departments because certain chap¬ 
ters contain material which was RESTRICTED at the date of 
printing. Other chapters may have had a lower classification or none. 
The reader is advised to consult the War and Navy agencies listed on 
the reverse of this page for the current classification of any material. 





. 12 0 43 


1 


LA 


ilMY 


H7 











Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the 
Columbia University Division of War Research under con¬ 
tract OEMsr-1131 with the Office of Scientific Research and 
Development. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC 
has been made by the War and Navy Departments. Inquiries 
concerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer¬ 
ence material should be addressed to the War Department 
Library, Room 1A-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten¬ 
tion : Reports and Documents Section, Washington 25, D. C. 


Copy No. 


5 


This volume, like the seventy others of the Summary Tech¬ 
nical Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 

Please report errors to: 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make 
this book more useful to other readers and will be of great 
value in preparing any revisions. 





8 " v 3 ; 1 1 . V i few \ I] 





SUMMARY TECHNICAL REPORT OF DIVISION 16, NDRC 


VOLUME 1 


OPTICAL INSTRUMENTS 


OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 16 

GEORGE R. HARRISON, CHIEF 

/ 


WASHINGTON, D. C., 1946 






















NATIONAL DEFENSE RESEARCH COMMITTEE 


x -JAN12 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative 1 

Frank B. Jewett Navy Representative 2 

Karl T. Compton Commissioner of Patents 3 

Irvin Stewart, Executive Secretary 


1 Army representatives in order of service: 


Maj. Gen. G. V. Strong 
Maj. Gen. R. C. Moore 
Maj. Gen. C. C. Williams 
Brig. Gen. W. A. Wood, Jr. 

Col. E. A. 


Col. L. A. Denson 
Col. P. R. Faymonville 
Brig. Gen. E. A. Regnier 
Col. M. M. Irvine 
Routheau 


2 Navy representatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Furer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 

3 Commissioners of Patents in order of service: 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suit¬ 
able projects and research programs on the instrumen¬ 
talities of warfare, together with contract facilities for 
carrying out these projects and programs, and (2) to 
administer the technical and scientific work of the con¬ 
tracts. More specifically, NDRC functioned by initiating 
research projects on requests from the Army or the 
Navy, or on requests from an allied government trans¬ 
mitted through the Liaison Office of OSRD, or on its 
own considered initiative as a result of the experience 
of its members. Proposals prepared by the Division, 
Panel, or Committee for research contracts for perform¬ 
ance of the work involved in such projects were first re¬ 
viewed by NDRC, and if approved, recommended to the 
Director of OSRD. Upon approval of a proposal by the 
Director, a contract permitting maximum flexibility of 
scientific effort was arranged. The business aspects of 
the contract, including such matters as materials, clear¬ 
ances, vouchers, patents, priorities, legal matters, and 
administration of patent matters were handled by the 
Executive Secretary of OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 

These were: 

Division A—Armor and Ordnance 
Division B—Bombs, Fuels, Gases, & Chemical Problems 
Division C—Communication and Transportation 
Division D—Detection, Controls, and Instruments 
Division E—Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization was 
as follows: 

Division 1—Ballistic Research 

Division 2—Effects of Impact and Explosion 

Division 3—Rocket Ordnance 

Division 4—Ordnance Accessories 

Division 5—New Missiles 

Division 6—Sub-Surface Warfare 

Division 7—Fire Control 

Division 8—Explosives 

Division 9—Chemistry 

Division 10—Absorbents and Aerosols 

Division 11—Chemical Engineering 

Division 12—Transportation 

Division 13—Electrical Communication 

Division 14—Radar 

Division 15—Radio Coordination 

Division 16—Optics and Camouflage 

Division 17—Physics 

Division 18—War Metallurgy 

Division 19—Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 












NDRC FOREWORD 


As events of the years preceding 1940 re- 
/\vealed more and more clearly the serious¬ 
ness of the world situation, many scientists in 
this country came to realize the need of organ¬ 
izing scientific research for service in a national 
emergency. Recommendations which they made 
to the White House were given careful and 
sympathetic attention, and as a result the Na¬ 
tional Defense Research Committee [NDRC] 
was formed by Executive Order of the Presi¬ 
dent in the summer of 1940. The members of 
NDRC, appointed by the President, were in¬ 
structed to supplement the work of the Army 
and the Navy in the development of the instru¬ 
mentalities of war. A year later, upon the 
establishment of the Office of Scientific Re¬ 
search and Development [OSRD], NDRC be¬ 
came one of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to pre¬ 
sent it in a useful and permanent form. It 
comprises some seventy volumes broken into 
groups corresponding to the NDRC Divisions, 
Panels, and Committees. 

The Summary Technical Report of each Di¬ 
vision, Panel, or Committee is an integral sur¬ 
vey of the work of that group. The first volume 
of each group’s report contains a summary of 
the report, stating the problems presented and 
the philosophy of attacking them, and sum¬ 
marizing the results of the research, develop¬ 
ment, and training activities undertaken. Some 
volumes may be “state of the art” treatises 
covering subjects to which various research 
groups have contributed information. Others 
may contain descriptions of devices developed 
in the laboratories. A master index of all these 
divisional, panel, and committee reports which 
together constitute the Summary Technical 
report of NDRC is contained in a separate 
volume, which also includes the index of a 
microfilm record of pertinent technical labora¬ 
tory reports and reference material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by Di¬ 


vision 14 and the monograph on sampling in¬ 
spection by the Applied Mathematics Panel. 
Since the material treated in them is not dupli¬ 
cated in the Summary Technical Report of 
NDRC, the monographs are an important part 
of the story of these aspects of NDRC research. 

In contrast to the information on radar, 
which is of widespread interest and much of 
which is released to the public, the research on 
subsurface warfare is largely classified and is 
of general interest to a more restricted group. 
As a consequence, the report of Division 6 is 
found almost entirely in its Summary Technical 
Report, which runs to over twenty volumes. The 
extent of the work of a division cannot there¬ 
fore be judged solely by the number of volumes 
devoted to it in the Summary Technical Report 
of NDRC: account must be taken of the mono¬ 
graphs and available reports published else¬ 
where. 

Division 16 carried out a broad program in 
the fields of light and optics. Among the studies 
undertaken were a number involving the prin¬ 
ciples and techniques of camouflage, and per¬ 
haps the outstanding success achieved in this 
field was the development of the “black widow” 
finish for night-flying aircraft. Significant im¬ 
provements were made in aerial mapping and 
photography. Devices depending on the use of 
infrared light were developed for the detection 
of enemy craft, the recognition of friendly ones 
and for intercommunication by voice and code. 
The sniperscope, using image-forming infrared 
rays, was a spectacular weapon which enabled 
our troops to fire accurately on an enemy 100 
yards away in utter darkness. 

The Division 16 Summary Technical Report, 
prepared under the direction of the Division 
Chief, George R. Harrison, describes the tech¬ 
nical achievements of the Division personnel 
and its contractors, and is a record of their 
slpll, integrity, and loyal cooperation. To all of 
them, we extend our grateful praise. 

Vannevar Bush, Director 
Office of Scientific Research and Development 

J. B. Conant, Chairman 
National Defense Research Committee 



























FOREWORD 


At the time of its formation late in 1942, Di- 
vision 16, the Optics Division of the NDRC 
was assigned both the general task of stimulat¬ 
ing and supervising OSRD research in optics 
and the immediate problem of overseeing a 
large number of contracts which had previously 
been initiated by the Instruments Section. Inas¬ 
much as the new Division consisted to a large 
extent of personnel associated with the Instru¬ 
ments Section during 1940 and 1941, the re¬ 
organization which took place in the fall of the 
latter year involved few important changes. 

The present Summary Technical Report de¬ 
scribes the accomplishments of both Division 16 
and Section D-3, and covers the principal devel¬ 
opments in optics made in America during 
World War II. These reports should be consid¬ 
ered as intermediate in character between the 
detailed contractors’ reports of Division 16, to 
which reference is frequently made herein, 
which are complete scientific reports of the in¬ 
vestigations carried on, and the historical vol¬ 
ume entitled Optics and Applied Physics in 
World War II, which presents in less technical 
form the accomplishments of the Division and 
its contractors, and assigns credit to those who 
took part. 

The contents of the present volume demon¬ 
strate impressively the great contribution made 
by the optical industry of America and the uni¬ 
versity optical laboratories to the war effort. 
Although less glamorous than some of the 
newer fields brought into existence during the 
war, optics nevertheless made significant con¬ 
tributions which were by no means confined to 
mere extension or application of optical meth¬ 


ods or apparatus previously in use. The stress 
of the emergency produced many new optical 
developments, and the genesis of a large pro¬ 
portion of these will be found recorded in the 
following pages. 

The science of optics and the optical industry 
have both benefited greatly by the intensive re¬ 
search which took place during the war. Many 
of the new devices developed under emergency 
conditions have contributed and will contribute 
more to our fundamental understanding of op¬ 
tics, and many of them will have peacetime 
applications. New lines along which optical re¬ 
search should be directed have been made ap¬ 
parent. In particular, the infrared field has 
benefited greatly, and the art of infrared phos¬ 
phor development and utilization has been ele¬ 
vated to an entirely new level. 

Consideration of the developments in optics, 
as in other fields, emphasizes that once ade¬ 
quate immediate defense has been insured, 
more important than having weapons for a pos¬ 
sible future war is having available a large 
body of trained personnel who can step into any 
breach that occurs and be available to produce 
the new devices that may be needed. 

The Optics Division of NDRC is especially 
indebted to the chiefs and members of its Sec¬ 
tions whose names are listed elsewhere in this 
volume. They have provided the essential lead¬ 
ership, combined with scientific knowledge 
without which the work of the Division could 
not have been planned or completed. 

George R. Harrison 
Chief, Division 16 



vii 
































































































































































































. 





































PREFACE 


S ECTION 16.1, Optical Instruments, of NDRC 
was organized in December 1942 to continue 
the work on optical instruments for which 
Section D-3, Instruments, had been responsible, 
along with many other types of physical equip¬ 
ment, from July 1940 to December 1942. The 
work of Section 16.1 continued until June 28, 
1946. 

The membership of Section 16.1 was drawn 
from universities and research institutions. 
Ira S. Bowen, Professor of Physics at the Cali¬ 
fornia Institute of Technology, and after Janu¬ 
ary 1, 1946, Director of the Mount Wilson 
Observatory, contributed his wide knowledge of 
physical optics and instrument design, particu¬ 
larly in the field of aerial photography. William 
V. Houston, Professor of Physics at the Cali¬ 
fornia Institute of Technology, who was re¬ 
cently appointed President of the Rice Insti¬ 
tute, brought his extensive experience in phys¬ 
ics to bear on many problems, particularly 
those relating to aerial photography and binoc¬ 
ular performance. Robert R. McMath, Director 
of the McMath-Hulbert Observatory and Pro¬ 
fessor of Astronomy at the University of Michi¬ 
gan applied his unusual experience in optical 
engineering to many of the projects, particu¬ 
larly aerial photography, stabilized mounts and 
phototheodolites. Frederick E. Wright, of the 
Geophysical Laboratory of the Carnegie Insti¬ 
tution of Washington, who was Technical Ad¬ 
viser to the Joint Optics Committee of the 
Army-Navy Munitions Board during World 
War II and who was responsible for matters 
relating to the production of optical instru¬ 
ments during the first world war contributed 
many of the ideas which guided decisions re¬ 
garding ordnance projects, particularly those 
relating to optical inspection and tropical de¬ 
terioration. George W. Morey, who is also a 
member of the Geophysical Laboratory, applied 
his knowledge of methods of growing and test¬ 
ing crystals to the project aimed at developing 
artificial fluorite. All of these men took an ac¬ 
tive part in planning and supervising individual 
projects. The aim was to obtain basic data by 
quick intensive studies whenever necessary, 
while pushing ahead developments for Service 
applications as rapidly as possible. Theodore 
Dunham, Jr., of the Mount Wilson Observatory 
of the Carnegie Institution of Washington, 
served as chief of the Optical Instruments Sec¬ 


tion and devoted full time to studying Army 
and Navy requirements, implementing deci¬ 
sions reached at meetings of the Section, visit¬ 
ing contractors’ laboratories, operating the 
Section Office and, after the end of the war, 
editing this volume. 

Harold F. Weaver of the Yerkes Observa¬ 
tory, Sidney W. McCuskey of the Case School 
of Applied Science, and Lillian R. Elveback of 
the University of Minnesota, served as Techni¬ 
cal Aides to Section 16.1 for various periods 
of time. Dr. Weaver and Miss Elveback were 
largely responsible for supervising the binocular 
programs, whereas Dr. McCuskey spent much 
time on the optical plastics program. The suc¬ 
cess of the Section was largely due to the 
imagination and effort of the Technical Aides. 

Herman F. Mark, who is well known for his 
work on high polymers, served as consultant 
on the project for developing optical plastics. 
He made many stimulating suggestions regard¬ 
ing the program and regarding the report on 
this subject. Dr. Morey served as consultant on 
artificial fluorite and on optical plastics before 
his appointment as a member of the Section. 
Harold F. Weaver served as consultant on the 
project relating to binocular performance. 

No account of the activities of the Section 
would be complete without referring to the 
strikingly efficient work and splendid spirit of 
the secretarial staff, headed by Margaret M. 
Connolly. Without this unusual group of young 
women, who throughout the war did far more 
than was expected of them, the results would 
have been much less effective. 

A large part of the program has been carried 
out in close contact with the Services. The work 
on aerial photography required close coopera¬ 
tion with the Photographic Laboratory at 
Wright Field and the use of Army aircraft for 
the testing program at Bedford. The work on 
optical inspection required even closer coopera¬ 
tion with the Frankford Arsenal, where many 
officers supplied basic ideas. The Office of the 
Coordinator of Research and Development 
(later known as the Office of Research and In¬ 
ventions), the Hydrographic Office and the 
Bureau of Aeronautics in the Navy provided 
valuable informational mapping requirements 
and methods. The Section is pjplfcularly in¬ 
debted to H. Noble of Wb^LO ajy^To H. G. Dyke 
of the C n n r rl i n ntrrr*m Uffiriii f (It the imagination 



REGRADED UNCLASSIFIED 
ORDER SEC ARMY BY TAG ?£R .fo 0 4 3 


3 






X 


PREFACE 


and assistance which they gave in the solution 
of a vast number of problems involving liaison 
which arose in connection with the projects. 
Differences of opinion with the Services fre¬ 
quently developed, both regarding the planning 
of projects and regarding the testing of pro¬ 
cedures and equipment, but in almost every case 
these were resolved without difficulty. 

The contractors and individual workers in 
the various laboratories deserve the greatest 
credit for the success of the projects on which 
they worked. Without exception they dropped 
other scientific and commercial undertakings 
not related to the war effort, and concentrated 
all of their resources on achieving solutions to 
NDRC problems. The experience of all divisions 
of NDRC shows that successful results can be 
achieved quickly by balanced teams of scien¬ 
tists and engineers working in accordance with 
a carefully formulated plan, toward a common 
objective. The extent to which similar joint 
efforts are indicated, and can be carried out in 
peacetime without stifling individual initiative 
may be usefully considered on the basis of the 
detailed history of NDRC projects. 

The present volume represents an attempt to 
give in condensed form a technical report on 
the activities of Section 16.1. Almost every 
project, even those of apparent minor impor¬ 
tance has been at least briefly described. Ex¬ 
perience has shown that projects which seemed 
relatively unimportant while work was in prog¬ 
ress often assumed unexpected importance 
later in planning other projects. Full references 
are given to OSRD reports in which the original 
work is described. Microfilms of all of these 
reports are available. It is unfortunate that 
time has not been adequate for discussing fully 
many of the projects described in this report. 
The aim has been to present at least representa¬ 
tive samples of all results, so that the reader 
may draw his own conclusions regarding the 
significance of the work. 


The contractors’ reports have served as the 
basis for the text of this volume. In general 
they have been summarized in approximately 
30 per cent of the original number of words. 
Frequently parts of the original text have been 
used unchanged. In such cases, quotation marks 
ordinarily have not been used in order to sim¬ 
plify the final text as much as possible. In all 
cases, however, references are given to the 
original sources. 

Recommendations for further work on those 
projects where more remains to be done have 
been given careful consideration by the Sec¬ 
tion. They are grouped together at the ends of 
the chapters to which they apply under the 
heading “Recommendations by NDRC.” The 
Section, not individual writers, is responsible 
for these recommendations, although they have 
been discussed with the writers, who have 
themselves suggested many of the recommenda¬ 
tions. 

The following individuals have compiled 
chapters or parts of chapters in this volume: 
James G. Baker, Harvard College Observatory; 
J. S. Chandler, Eastman Kodak Co.; Theodore 
Dunham, Jr., Chief, Section 16.1 (also editor), 
NDRC; John W. Evans, University of Roches¬ 
ter; Howard S. Coleman, Pennsylvania State 
College; Hobert W. French, Argus, Inc.; Leo 
Goldberg, McMath-Hulbert Observatory; H. K. 
Hartline, Johnson Foundation, University of 
Pennsylvania; Duncan MacDonald, Physics De¬ 
partment, Boston University; Sidney W. Mc- 
Cuskey, Case School of Applied Science, 
Warner and Swasey Observatory; Robert R. 
Singleton, Merrill Flood and Associates. 

The splendid cooperation of those who have 
prepared material for this volume in the lim¬ 
ited time available is very much appreciated. 


Theodore Dunham, Jr. 
Chief, Section 16.1 







CONTENTS 


PAGE 


CHAPTER 

Summary by Theodore Dunham, Jr . 1 

1 Equipment for Aerial Photography by James G. Baker and 

/. S. Chandler .23 

2 Resolution in Aerial Photography by Duncan Macdonald, 

Theodore Dunham, Jr., and James G. Baker .147 

3 Mapping Methods Employing High Oblique Photographs by 

Robert Singleton .175 

4 Optical Testing Methods by Roderic M. Scott .204 

5 Binoculars as Aids to Vision by H. K. Hartline .... 263 

6 Harmonization of B-29 Guns and Sights by Theodore Dun¬ 
ham, Jr .289 

7 Optical Fluorite by Sidney W. McCuskey and James G. Baker 312 

8 Optical Plastics by Sidney W. McCuskey .342 

9 Optical Techniques by James G. Baker and H. F. Weaver . 389 

10 Optical Systems for Telescopes and Binoculars by James G. 

Baker .435 

11 Projecting Systems and Other Special Optical Developments 

by Theodore Dunham, Jr., and James G. Baker .... 472 

12 Reflex Sights by John W. Evans .477 

13 Stadiameters by Theodore Dunham, Jr .505 

14 Antioscillation Mounts for Optical Instruments by Hobart W. 

French, Jr .510 

15 Phototheodolites by Leo Goldberg .528 

16 Optical Scanning by Theodore Dunham, Jr .551 

17 Antiglare Shutter for Night Binoculars by Sidney W. 

McCuskey .565 

18 Rapid Processing Equipment for Periscope Photography by 

James G. Baker .572 

19 Two-Star Navigating Device by Theodore Dunham, Jr. . . 577 

Appendix.582 

Glossary.591 

Bibliography.597 

OSRD Appointees.605 

Contract Numbers.607 

Service Project Numbers.608 

Index.611 








XI 













































































































































































































































































* 






\ ' 





< 


- 




















SUMMARY 


By Theodore Dunham, Jr. a 


T he work of Section 16.1 was extremely 
varied. It included (1) basic studies, such 
as binocular performance tests at low levels of 
illumination, the investigation of factors limit¬ 
ing resolution in aerial photography, and the 
development of methods for preventing tropical 
deterioration in optical instruments, (2) 
studies of existing methods, followed by the 
development of improved methods, such as the 
work on mapping and optical inspection, (3) 
the development of new laboratory techniques, 
such as those for growing large artificial fluo¬ 
rite crystals, for high efficiency coating, milling 
and figuring of roof prisms, glass molding, and 
casting of optical plastic elements, (4) the de¬ 
velopment of special procedures, such as those 
for harmonizing B-29 guns and sights in the 
field, and (5) the development of a wide va¬ 
riety of optical and mechanical devices to meet 
specific military needs, such as precision aerial 
camera lenses and mounts, special telescopes, 
wide-field binoculars, improved periscopes and 
projecting systems, reflex sights, stadiameters, 
antioscillation mounts for optical instruments, 
phototheodolites, optical scanning devices, an 
antiglare shutter for night binoculars, rapid 
processing equipment for periscope photog¬ 
raphy, and a two-star navigating device for 
use in aircraft. 

The work was done under eighteen contracts 
with research institutions and under twelve 
contracts with industrial laboratories. The de¬ 
velopment of methods for preventing tropical 
deterioration in optical instruments was trans¬ 
ferred to the Tropical Deterioration Adminis¬ 
trative Committee in 1945. b 

Particular attention was devoted to aerial 
photography which provides in wartime a large 
part of all available information about enemy 
territory, the disposition of men, equipment 
and shipping, and the damage inflicted as the 
result of raids, all of which serves to guide fu- 

a Chief, Section 16.1, NDRC (Mount Wilson Observa¬ 
tory) . 

b These methods are described in Summary Technical 
Report of the Tropical Deterioration Administrative 
Committee. 


ture military operations. The usefulness of an 
aerial photograph depends largely on the detail 
which it records, and ordinarily the ability of 
an interpreter to distinguish and identify ob¬ 
jects on the ground increases very rapidly with 
improving quality of the photograph. 

The work on aerial photography was carried 
out under Project AC-29 which covered the de¬ 
velopment of a number of special cameras, 
lenses, and shutters, and under Project AC-88 
which covered studies of the factors which 
limit resolution. The aim was to increase the 
linear scale of the photographs by using lenses 
of longer focal length than had previously been 
employed, and at the same time to improve the 
resolution as much as possible. Photographs of 
high quality can be made only with a lens of 
high intrinsic resolution, accurate location of 
the film in the focal plane, freedom from angu¬ 
lar vibration and from differential linear vibra¬ 
tion of the camera and lens elements, and suffi¬ 
cient aperture to permit optimum exposure 
with an emulsion having good resolution. 

Several lenses of unusually high perform¬ 
ance were designed and constructed at Har¬ 
vard University. The most significant of these 
was, undoubtedly, the 40-in. //5 lens with auto¬ 
matic control of focus and temperature. This 
lens was procured by the Army Air Forces 
[AAF] and some units were used overseas. The 
performance was markedly superior to that of 
standard lenses previously used and demon¬ 
strated convincingly the striking improvement 
that can be achieved when every effort is made 
not only to design the lens to concentrate all the 
rays of light from a distant point in the small¬ 
est possible region at the focus, but also to de¬ 
sign a mount which will hold the lens elements 
accurately in position without strain. Several 
other high-performance lenses were developed, 
including a color-free glass-fluorite 36-in. f/S 
lens, a 36-in. f/S 9x18 lens, and a 100-in. //10 
lens. At the end of World War II, a 60-in. //5 
9x18 lens, which incorporated all of the new 
experience with op tical and mechanical designs, 
was under This lens \ya&*4;o be 


REGRADED unclassified. i 

sec army by tag 0 4 3 13 








2 


SUMMARY 


sealed at the front and back with plates of 
optical glass, and the system was to be evacu¬ 
ated to eliminate changes of focus due to vari¬ 
ation in barometric pressure and to prevent 
condensation of moisture and tropical deterio¬ 
ration. It would be very desirable to finish this 
lens and to test it thoroughly. 

Extensive programs covering tests of the 
laboratory performance of lenses have been 
carried out at the Eastman Kodak Company 
and the Mount Wilson Observatory. The re¬ 
sults are important, not only for guidance in 
the selection of lenses for special application by 
the Services, but for providing the lens designer 
with information regarding the overall lens- 
film performance that is given by various de¬ 
signs. 

Considerable attention has been given to the 
development of Schmidt cameras (Mount Wil¬ 
son, the Massachusetts Institute of Technology 
[MIT] and Harvard) primarily to provide fast 
systems for night flash photography. A particu¬ 
larly promising Schmidt camera developed at 
Harvard operates at //1.3 and uses continu¬ 
ously running motion-picture film with inter¬ 
mittent Edgerton flashes to produce a series of 
exposures which can later be fitted together to 
make a long strip photograph. An //1.0 lens 
was developed by the University of Rochester 
which covers 40 degrees with good definition. 
The field is strongly curved, but air pressure is 
used to make the film conform to the required 
surface. 

Improved camera mounts have been devel¬ 
oped (Eastman and Mount Wilson) to reduce 
as far as possible the transmission of vibration 
from the fuselage to the camera and to compen¬ 
sate image motion due to forward motion of the 
aircraft. Experiments have also been carried 
out with gyrostabilized mounts to eliminate the 
effects of long-period roll and pitch. 

Several developments on camera shutters 
were carried out by Mount Wilson, Eastman, 
and Technicolor Motion Picture Corporation. 
The speed of the between-the-lens shutter of 
the 6-in. Metrogon camera was considerably in¬ 
creased by reducing the inertia of the driving 
cam, and by mounting its shaft on ball bear¬ 
ings. Some improvement in the speed of the 24- 
in. between-the-lens shutter was also achieved, 


but not enough to warrant changes in produc¬ 
tion. A two-blind multiple-slit Langer shutter 
was developed at Mount Wilson and at Techni¬ 
color in an attempt to increase speed, make all 
parts of a photograph represent more nearly 
simultaneous exposures, and increase life of the 
ordinary focal plane shutter. These efforts were 
only partially successful. The shutter problem 
is a difficult one, but it deserves a concentrated 
attack. A more efficient and a faster shutter is 
one of the most immediate means for achieving 
better resolution on aerial photographs, with¬ 
out waiting to improve mounts. 

Unfortunately the resolution of high-quality 
lenses, when used under standard flight condi¬ 
tions, appears to be only slightly more than half 
as much as the resolution which the same lenses 
are capable of giving under laboratory condi¬ 
tions. This means that much information of 
great potential value is being missed on photo¬ 
graphs, and can only be regained at present by 
nearly doubling the focal length of the camera. 
An extensive investigation of this situation was 
carried out at Harvard, Eastman, and Mount 
Wilson. Laboratory studies were supplemented 
by flight tests at Bedford, Mass., including the 
recording of camera vibrations under operating 
conditions. The results show clearly that for¬ 
ward motion of the aircraft is the most serious 
factor that limits resolution, but that roll and 
pitch of the fuselage, vibration transmitted to 
the cameras through their mounts, and vibra¬ 
tion caused by shutter recoil and low optical 
quality of standard photographic windows com¬ 
bine to reduce to a marked extent the detail 
which can be detected on photographs. Each of 
these factors can unquestionably be reduced to 
an unimportant level by suitable engineering of 
the accessory equipment. The resolving power 
(altitude of aircraft divided by spacing of high- 
contrast black and white lines resolved on the 
ground) of the standard 24-in. camera under 
average operating conditions probably rarely 
exceeds 8,000. A suitable mount and better win¬ 
dows would probably increase this value at least 
to 12,000. The 40-in. Harvard-NDRC lens in an 
improved mount has given an average resolv¬ 
ing power of 30,000, and this can undoubtedly 
be increased still further. The 60-in. and 100-in. 
lenses, properly mounted, should have resolv- 



SUMMARY 


3 


ing powers of at least 45,000 and 70,000, respec¬ 
tively. 

It must be emphasized that, while it is easy 
to produce photographic equipment with a re¬ 
solving power of 10,000, this is about the limit 
that can be attained without special effort. 
Higher resolving powers require larger and 
much heavier cameras with special mounts and 
temperature control. The production and oper¬ 
ation of such equipment in aircraft appears, 
however, to be entirely practical, and the tre¬ 
mendous importance of the information which 
would be gained by using such equipment for 
reconnaissance fully justifies the effort involved 
in the undertaking. 

It cannot be expected that personnel with 
average training can operate special equipment 
to full advantage. A group should therefore be 
trained in the operating and servicing of preci¬ 
sion photographic equipment. Special squad¬ 
rons of aircraft with precision equipment of the 
latest type should be reserved for exclusive use 
by this group over special targets. This group 
should report directly to a high level in the 
AAF, and should be in direct touch with all 
laboratories where development work is in 
progress. In response to special requests, such 
a group could unquestionably provide military 
information of great importance. 

Improved mounts for gun cameras were de¬ 
veloped by the Eastman Kodak Company under 
Project AC-99. Tests with high-speed cameras 
and mirrors attached to various parts of the 
Martin upper turret indicated that angular vi¬ 
bration is extreme, even in the gun receiver, 
and that improvements in the linkage to the 
sight would do little to improve the present low 
quality of photographs. A gimbal mount was 
developed and tested by the Navy at Patuxent 
with encouraging results. An antioscillation 
mirror mount for the Gun Sight Aiming Point 
camera [GSAP] on the K-15 sight was devel¬ 
oped. The logical solution would be to incorpo¬ 
rate the camera in the sight. Much could be 
done to produce sharper individual photographs 
by merely shortening the exposure time. Under 
most operating conditions the //3.5 lens would 
permit reducing the exposure by a factor of 
several times, which could be accomplished by 
reducing the sector opening of the shutter. 


A method for mapping from high oblique 
photographs was developed by Merrill Flood 
and Associates under Project NA-124. A survey 
of the requirements of the Navy and of avail¬ 
able methods in Washington and at Pearl Har¬ 
bor showed that mapping from vertical photo¬ 
graphs could be carried out effectively with 
multiplex equipment, but that in many cases it 
would be advantageous to be able to use oblique 
photographs, either because of difficulties en¬ 
countered in making verticals due to clouds or 
enemy interference, because of the reduction in 
the required amount of flying when obliques are 
used, or because of the advantages which 
obliques offer for certain purposes, such as re¬ 
lating islands which are widely separated by 
water and covering wide areas of land in single 
photographs. 

A pinhole photographic rectifier was devel¬ 
oped by the Aero Service Corporation, and was 
taken to Pearl Harbor for testing. A simple lens 
designed at Harvard greatly improved the reso¬ 
lution. This was followed by a Hypergon lens 
which gave still better results. 

The method of mapping developed by Merrill 
Flood and Associates is based on the use of a 
fixed-angle rectifying camera with a Hypergon 
lens, designed for photographs of 60 degrees 
tilt. Variations in this tilt are compensated by 
a preliminary reduction of the scale of the 
original negative by the appropriate ratio to 
adapt the perspective index to the rectifying 
camera. This procedure permits great simplifi¬ 
cation of equipment. Following rectification, 
the photograph is enlarged or reduced to the 
desired scale. A precision variable-ratio printer 
was developed by the Aero Service Corporation 
and a fixed-angle rectifying camera was de¬ 
signed and built by Merrill Flood and Associ¬ 
ates. An orthographic plotter was also devel¬ 
oped for drawing contours from the photo¬ 
graphs. Tests of the method show that it is en¬ 
tirely practical and that it gives the desired 
accuracy. Production tests will be required to 
evaluate the effectiveness and cost of the 
method in comparison with other methods. 

The determination of depth of shallow water 
is a matter of great importance in the planning 
of amphibious assault operations. Many meth¬ 
ods have been proposed. One of the most prom- 




4 


SUMMARY 


ising of these is based on the use of the Sonne 
stereostrip camera at altitudes of less than 500 
ft, and was developed by the Photographic In¬ 
terpretation Center. An alternative method was 
proposed by Merrill Flood and Associates, 
based on the use of two standard strip cameras 
mounted in the wings of an airplane, so that 
the base line and relative orientation are 
known. This method seems promising, provided 
that the flexure of the inner parts of the wings 
where the cameras would be mounted, is not 
too great. A test installation was under way at 
the Naval Aircraft Factory at the end of World 
War II. 

It is desirable that the Army and Navy main¬ 
tain mapping organizations with well-trained 
personnel at all times, and that other Govern¬ 
ment and civilian agencies be asked to cooper¬ 
ate in a program aimed at mapping wide areas 
of territory and developing improved methods 
and equipment. The Navy should maintain mo¬ 
bile fully equipped units for mapping special 
distant regions. Pilots and other personnel 
should be trained especially for mapping, as 
distinguished from reconnaissance. A well- 
planned program for further development of 
mapping methods should be undertaken with 
Government funds, so that this country will be 
in a strong position regarding the application 
of photogrammetric techniques if another 
emergency should arise. 

A study of optical inspection methods used 
to control production was made by the Pennsyl¬ 
vania State College under Project OD-138. A 
report was made on the methods which were in 
use in 1945 at various Government and com¬ 
mercial production plants. A program was also 
undertaken aimed at developing methods of in¬ 
spection which would eliminate, as far as pos¬ 
sible, the variable factor of human judgment. 

The staff of the Frankford Arsenal gave 
much valuable advice and assistance through¬ 
out the programs. An impersonal method for 
measuring the overall resolution of a telescope 
system was developed. The equipment, known 
as the Kinetic Definition Chart [KDC] appa¬ 
ratus provides images of a parallel-line test 
object in which the apparent angular spacing 
can be varied continuously by changing the dis¬ 
tance between the object itself and a microscope 


objective. The observer compares the minimum 
resolvable angular spacing for the telescope 
under test with that for a telescope of nearly 
perfect optical quality, thus establishing the 
resolution of the instrument on a percentage 
basis. The KDC apparatus was adopted by the 
Ordnance Department and was used to control 
the production of a large number of tank tele¬ 
scopes and of some other optical instruments. 
The accuracy and consistency of measures 
made with the equipment were studied exten¬ 
sively at Penn State. Further investigations are 
needed to determine the most effective value for 
the auxiliary magnification which is ordinarily 
employed with the KDC apparatus. 

Several additional laboratory studies and de¬ 
velopments were made. The usefulness of the 
interferometer for inspecting individual optical 
elements and subassemblies was established 
and an improved design for an inspection inter¬ 
ferometer was developed. An improved diop¬ 
tometer was also developed and applied to a 
number of inspection problems. 

The resolving power of the eye with Foucault 
test patterns was measured, using stops of vari¬ 
ous diameters. The observed values exceeded 
those indicated by the Rayleigh criterion for 
optical instruments when the diameter of the 
pupil was less than 1 mm. This result is, how¬ 
ever, explained to a large extent by the fact 
that a Foucault pattern with ten parallel black 
lines was used, whereas the Rayleigh criterion 
is based on the use of only two lines. Experi¬ 
ments showed that when two lines are used the 
resolving power of the eye is reduced by about 
30 per cent, which makes it slightly less than 
that indicated by the Rayleigh criterion. 

Equipment was devised for measuring the 
amount of light scattered into the image and 
the resulting reduction of contrast in telescope 
systems. The performance of a telescope in this 
respect may be as important as its resolving 
power at low levels of illumination. It is desir¬ 
able that a comparison be made of scattered 
light in coated and uncoated telescope systems, 
since there are indications that in some cases 
coating increases the amount of light scattered. 

In order to establish the physical perform¬ 
ance of telescope systems on a strictly imper¬ 
sonal basis, measures have been made of the in- 



SUMMARY 


5 


tensity distribution of light in the image of a 
distant line source, using a modification of the 
photographic wedge method developed by Jones 
and Wolfe, and also using a traveling slit and 
photoelectric recorder. Both methods give data 
which are of fundamental importance, not only 
for evaluating various methods of optical in¬ 
spection, but also for establishing the perform¬ 
ance of new optical designs. It would be desir¬ 
able to have measures of light distribution 
made in images formed by a number of repre¬ 
sentative fire-control instruments, on and off 
axis, and at various focal settings, to provide 
a basis for comparing different methods of in¬ 
spection. 

Studies of binocular performance were car¬ 
ried out, under Project NO-210, at the Dart¬ 
mouth Eye Institute, at Brown University, and 
at the University of Pennsylvania, to determine 
the optimum design characteristics of hand¬ 
held binoculars intended for use in detecting 
targets by night lookouts on ships. The effects 
of varying magnification and exit pupil diame¬ 
ter separately and simultaneously were investi¬ 
gated by observing the ranges at which small 
black targets could be detected at low levels of 
illumination under laboratory conditions. Time 
did not permit a study of the effects of these 
factors on target recognition or a study of the 
effect of the diameter of the field of view on de¬ 
tection. 

It is well known that an increase in the mag¬ 
nification of a binocular leads to an increase in 
the range at which objects can be detected 
at night, and that this is usually the most im¬ 
portant single factor in the design of a binocu¬ 
lar which controls the range of detection. There 
is, however, a limit to the magnification that 
can be used in practice, since if the diameter of 
the exit pupil is held constant, the size, weight, 
and cost of the instrument increase rapidly 
because of the need for increasing the diameter 
of the objectives in the same proportion as the 
magnification. The results at Dartmouth and 
at Brown show that an increase in the diameter 
of the exit pupil leads to a rapid increase in the 
range of detection up to 6 mm, with a much 
slower increase up to about 8 mm, after which 
there is no further gain. Maximum range of de¬ 
tection with 50-mm objectives is attained at 


10X magnification in the laboratory, but the 
gain as compared with the standard 7x50 bin¬ 
ocular amounts to only about 10 per cent. The 
failure of higher magnifications to give in¬ 
creased ranges is only partly due to the reduced 
exit pupil. The inevitable reduction in eye relief 
and the more restricted field of view are also 
important factors. The results of the binocular 
testing program show that it is not possible to 
achieve any striking gain in range by changing 
the magnification or exit pupil of a hand-held 
binocular, which must be kept within reasonable 
dimensions. It is nevertheless possible that 
moderate gains may be important from a mili¬ 
tary point of view, since the purpose of a lookout 
is to detect the enemy before being himself de¬ 
tected. Under these conditions, even a small ad¬ 
vantage may be decisive. 

The gain in the range at which targets can be 
detected with binoculars at night would, theo¬ 
retically, be expected to equal numerically the 
value of the magnification, provided the exit 
pupil is larger than the pupil of the eye, and 
provided there is no loss of light in the optical 
system of the binocular. However, the results 
at Brown show that the gain in range is usually 
only a little more than half the value to be ex¬ 
pected from the magnification after light losses 
are taken into account. 

In an attempt to uncover the reason for this 
failure of the gain in range due to binoculars 
to measure up to expectations, several ex¬ 
periments were carried out at Brown. It was 
found that alidade mounting of the binoculars 
increased range about 8 per cent, as compared 
with hand-held instruments. Headrests for the 
binoculars, which might be expected to aid in 
holding the exit pupil in line with the pupil of 
the eye, made little difference under laboratory 
conditions. Rolling the observer in a Scoresby 
machine with an amplitude of 14 degrees and a 
period of 9 sec to simulate motion on shipboard 
reduced range by 15 per cent. A comparison be¬ 
tween monocular and binocular vision showed 
that binocular vision increases range by 15 per 
cent with the instrument, as compared with 
slightly more than 20 per cent when the naked 
eyes alone are used. None of these results was 
adequate to explain the low gain in range due 
to binoculars in laboratory tests. 





6 


SUMMARY 


Experiments carried out at the University 
of Pennsylvania showed that the loss in ex¬ 
pected range can be entirely explained as the 
result of “clipping” (lack of alignment of the 
exit pupil of the instrument and the pupil of 
the eye) and of angular tremor due to unsteadi¬ 
ness of holding. Binoculars fixed rigidly to the 
skull by means of a bite, in the correct position 
of alignment, gave the full gain in range that 
was to be expected on the basis of magnifica¬ 
tion. This suggests that eye guards in contact 
with the face and with the side of the head 
would be helpful on shipboard where wind and 
vibration increase the difficulty of maintaining 
proper alignment of binoculars and cause con¬ 
siderable angular motion of the image. 

Infrared photographs taken through the ob¬ 
jectives in the laboratory at the University of 
Pennsylvania showed the relation of the two 
apertures (the exit pupil of the instrument and 
the pupil of the eye) at the moments when ex¬ 
posures were made, and gave a basis for calcu¬ 
lating the average light loss due to clipping. 
Direct measurements of tremor in hand-held 
binoculars have also been made by photograph¬ 
ing a distant lamp with a light camera attached 
to the binocular. The resulting irregular path 
recorded by the image on the film showed that 
in the laboratory the tremor of large and small 
binoculars was much the same. The radius of 
the area covered by angular motion was about 
1 mil, and the period of the tremor was about 
1 sec, with higher frequencies superposed. Tests 
on shipboard showed that the amplitudes were 
about three times as large. 

The effect of tremor is to spread the image, 
as the eye sees it, over an area the radius of 
which is equal to the mean radius of tremor of 
the binocular multiplied by M — 1, where M is 
the magnification. If the tremor is fast enough, 
the result is that the signal is spread over a 
considerable area of the retina, and is less 
effective in producing a response than if it were 
concentrated in one place. Data on the visibility 
of large targets of low contrast, as compared 
with smaller targets of higher contrast but with 
the same integrated brightness difference, indi¬ 
cated that the tremor disk of hand-held binocu¬ 
lars has a radius of about 1 mil. Since misalign¬ 
ment is partly responsible for the loss in range. 


the indicated radius of the tremor disk should 
probably be reduced somewhat. Moreover, the 
retina cannot integrate perfectly an image wan¬ 
dering within a limited area when the period is 
as long as 1 sec. Accordingly, the suggested in¬ 
terpretation is subject to appreciable correction 
when knowledge of these properties of the 
retina becomes more complete, but the general 
picture of the process involved is undoubtedly 
valid. 

Eye guards, intended for attachment to 
standard binoculars, were developed at the Uni¬ 
versity of Pennsylvania. Although these eye 
guards did not increase range of detection un¬ 
der laboratory conditions, it seems almost cer¬ 
tain that they would do so if used by lookouts 
on shipboard, partly because of the increase in 
comfort and resulting general efficiency of 
the observer, but primarily because, in the pres¬ 
ence of wind and vibration, eye guards are al¬ 
most certain to improve the alignment of the 
instrument with the eye. 

Experiments made with binoculars to which 
weighted arms had been attached to increase 
their moment of inertia showed a definite re¬ 
duction in angular tremor and suggested that it 
would be worth while to test the performance 
of such binoculars on shipboard. If a spring 
were provided to relieve the observer from 
carrying the extra weight on his elbows, and 
if vibration-filtering supports (perhaps air 
cushions) were provided for his elbows, it 
seems entirely possible that the efficiency of 
lookouts would be noticeably increased. 

Binoculars have been shown, on the basis of 
the tests at Brown, to be capable under labora¬ 
tory conditions of increasing the range at which 
objects can be detected at night by a factor of 
about 4.5 when the optimum magnification of 
10 X is used. On shipboard, it may be that the 
optimum magnification will be found to be 
slightly lower, but it is nevertheless likely that 
some gain in range would result if the mag¬ 
nification of the standard 7x50 were increased 
to 8x or 9X, without changing the present 
50-mm aperture. The effect of fatigue on the 
value of the optimum magnification, under nor¬ 
mal lookout conditions, has not yet been investi¬ 
gated. When this effect is known, it may change 
the optimum magnification appreciably. No 



SUMMARY 


7 


single factor in binocular design leads to a 
striking increase in range. A very significant 
increase in the range achieved by lookouts can, 
however, be realized by giving full attention to 
all of the following factors: magnification, exit 
pupil diameter, coated optics, careful focusing 
and adjustment of inter pupillary distance, ali¬ 
dade and antioscillation mounts or vibration¬ 
absorbing elbow rests, eye guards, comfort of 
the observer, shelter from the wind, and train¬ 
ing in the most effective use of binoculars at 
night. Even if the gain to be expected by mak¬ 
ing changes in each of these factors were only 
5 to 10 per cent, the number of individual fac¬ 
tors involved suggests that the overall gain may 
be impressive. 

Several methods for harmonizing B-29 guns 
and sights in the field were developed under 
Project AC-127 to meet the need for routine 
verification of harmonization at bases in the 
Marianas between combat missions. Accuracy 
and speed were important, but it was even more 
essential that the space required for operation 
should not exceed the very limited amount 
available on the hardstands, many of which 
had sharp declivities at their edges. 

Two methods were developed which de¬ 
pended upon the use of optical targets set up 
temporarily in front of the gun and sight of 
each pair of stations requiring harmonization. 
In the prism method (University of Rochester) 
two targets are set, with the aid of two identical 
double-image prisms, on parallel lines passing 
through the gun and sight. The equipment is 
light and compact, but about 200 ft of reason¬ 
ably level ground must be available in certain 
directions from the aircraft. The wire method 
employs two telescopes (MIT) or collimators 
(Merrill Flood and Associates) which are set 
parallel to one another in front of the gun and 
sight by reference to a level bubble and to a 
wire stretched near the bomber, just above the 
ground. The wire method requires less space 
around the aircraft than the prism method, but 
the telescopes or collimators, with their ob¬ 
servers, must be located at specified positions 
up to 15 ft above the ground. A fork-lift truck 
is used as part of the MIT equipment to carry 
the observer and his instrument with a support 
that allows fine adjustment to any desired loca¬ 


tion. A special aluminum tower, mounted on 
rubber wheels and removable jacks, with pro¬ 
vision for running the instrument vertically 
and horizontally on rollers, and with a pair of 
diagonal ladders to support the observer at any 
level, was developed by Merrill Flood and Asso¬ 
ciates. Tests at Bedford, Mass, showed that 
both the prism and the wire methods were ca¬ 
pable of harmonizing B-29 aircraft in the field 
with the required accuracy and speed. Several 
sets of equipment were sent, with trained op¬ 
erators, to the Marianas in August 1945. 

The mirror boresight method (Merrill Flood 
and Associates) employs a small mirror 
mounted accurately perpendicular to a mandrel 
which is set in the muzzle of a gun. The mirror 
is offset and faces backward, so that an opera¬ 
tor at the sight which controls the gun can ob¬ 
serve the accuracy of the harmonization setting 
directly by noting how nearly the reticle dot 
falls on the reflected image of a target at the 
sight. This simple and direct method can, un¬ 
fortunately, be used only for harmonizing the 
two top turrets to the ringsight, but it is recom¬ 
mended for use on these stations whenever the 
prism or wire methods are used for the other 
stations. Several units were sent to the Mari¬ 
anas. 

A method of harmonization based on the use 
of a group of flat mirrors, mounted accurately 
parallel to one another on a steel frame carried 
on a truck, each mirror opposite one of the sta¬ 
tions to be harmonized, was tested briefly at 
Harvard. The frame required is not unreason¬ 
ably large and the mirrors appear to retain 
their adjustment adequately in spite of tem¬ 
perature variations. The method is extremely 
simple in principle and should be studied fur¬ 
ther. 

The optical quality of the Plexiglas used in 
the B-29 is far from satisfactory and should be 
improved if harmonization is to be carried out 
with an accuracy of 1 mil, and if the guns are 
to be used with this accuracy over the entire 
range which they are capable of covering. 

Several basic studies should be carried out as 
soon as possible in connection with harmoniza¬ 
tion. It is particularly important to measure, by 
using gun cameras, the difference between har¬ 
monization settings of the aircraft in flight and 





8 


SUMMARY 


on the ground, so that an appropriate correc¬ 
tion can be applied when harmonization is done 
on the ground. It is also very desirable that 
carefully planned tests be conducted to ascer¬ 
tain the contributions which unbalance of the 
servo amplifier, disturbance in the level setting 
of sights and turrets, and temperature and 
wind have on harmonization. It seems at least 
possible that, if the balance of the servo ampli¬ 
fier and the level of the stations were adjusted 
at reasonably frequent intervals, it would only 
rarely be necessary to carry out optical har¬ 
monization and adjustment of the selsyns. 

The development of some substitutes for op¬ 
tical glass was undertaken under Project 
AC-11, partly to provide new materials for the 
design of special optical systems and partly to 
relieve the demand on optical glass production 
if difficulty should have been encountered in 
this direction during World War II. As matters 
turned out, there was never any serious prob¬ 
lem relating to optical glass, but the projects 
produced extremely useful results from entirely 
different points of view. 

A process for making artificial optical fluo¬ 
rite was developed at MIT. Crystals up to 4 in. 
in diameter and 2% in. high were grown in 
an evacuated elevator furnace with a sharp 
temperature gradient, using graphite crucibles 
and graphite heaters. Crystals up to 6 in. in 
diameter and a little more than 1 in. thick were 
grown in a pot furnace in which the necessary 
temperature gradient was established and con¬ 
trolled by differential heating. Selected natural 
fluorite gave the best results, although experi¬ 
ments showed that synthetic calcium fluorite 
could probably be used if means for removing 
absorbed water were developed. Crystals of 
barium and strontium fluoride up to 1 in. in 
diameter were also made by this process. The 
index and dispersion of all three crystals were 
measured on prisms submitted to the National 
Bureau of Standards. The results are given in 
an MIT report. (See Reference 2 of Chapter 7.) 

The crystals of fluorite were not in most 
cases single crystals, but this did not appear 
to cause any noticeable mechanical weakness or 
optical defect. The 4-in. crystals were annealed 
by reducing the temperature gradually from 
about 750 C to room temperature over a period 


of about 3 days. Some strain remained, but not 
enough to disturb high performance at focal 
lengths up to 72 in. Further studies are 
needed to determine the best annealing sched¬ 
ule for these crystals. The ends of five crystals 
were ground and polished flat to permit a study 
of internal homogeneity with an interferometer 
at the National Bureau of Standards. Varia¬ 
tions in index amounted, in most cases, to less 
than one-quarter wavelength per inch of path 
over the central 3-in. area of these crystals. 

Methods for grinding and polishing fluorite 
were worked out at Harvard and at the Perkin- 
Elmer Corporation. The material is extremely 
sensitive to thermal shock and must not be 
washed with water below room temperature. If 
attention is paid to this point, and if emery is 
used for grinding in place of silicon carbide 
which tends to start cracks, little difficulty is 
encountered. Polishing is done with Linde com¬ 
pounds A and B. Sleeks are common, and are 
not easy to avoid. Because the crystals are usu¬ 
ally multiple, it is difficult to attain a uniform 
surface across an interface between individual 
crystals which appear to differ in hardness. It 
has been found advantageous to design systems 
so that fluorite elements can be cemented be¬ 
tween glass elements, using soft cement which 
never sets. This makes the effects of any resid¬ 
ual imperfections in figure and of sleeks much 
less serious, and it allows differential expansion 
to occur without introducing stresses which 
might otherwise lead to fracture. 

Studies at Harvard showed that secondary 
color could be almost entirely eliminated in 
camera lenses and in telescope systems by using- 
one or more elements of fluorite. Barium fluo¬ 
ride is even more effective in this respect, while 
strontium fluoride shows no advantages. The 
low index of all of these materials is a disad¬ 
vantage and limits the aperture ratio of most 
systems in which they are used to about f/8. A 
72-in. collimator, which incorporated a fluorite 
element and gave outstanding performance, 
was constructed. Several aerial camera lenses 
up to 36 in. in focal length have been designed 
and constructed each with at least one fluorite 
element. These have shown theoretical resolv¬ 
ing power when tested in the laboratory and in 
the air. They should be particularly successful 



SUMMARY 


9 


for color photography when long focal lengths 
are employed. No elements have broken due to 
temperature changes resulting from flights to 
30,000 ft. Because of the relatively high tem¬ 
perature coefficient of the index of refraction of 
fluorite, all lenses which include fluorite ele¬ 
ments should be thermostated to prevent 
change in focal setting. 

An extensive program to develop methods for 
fabricating optical plastic elements was under¬ 
taken by the Polaroid Corporation. It was 
hoped that lenses and prisms of quality ade¬ 
quate for fire-control instruments might be 
made, thus relieving the requirements for op¬ 
tical glass and at the same time employing 
methods of manufacture which could use labor 
not possessing the special skills required for 
work on optical glass. It was hoped also that 
materials with new and desirable optical prop¬ 
erties might become available. Finally, there 
was the possibility that aspherical elements 
could be cast economically, thus opening up 
new opportunities for lens design. 

Fallowing extensive experiments, a method 
for casting optical plastic elements, with high 
internal homogeneity and very fair surface ac¬ 
curacy, was developed. Although the surfaces of 
a lens 3 in. in diameter usually depart from 
perfect spheres by one or two fringes, and not 
infrequently by more, this is not much inferior 
to the quality of many production glass lenses, 
and is entirely adequate for use in many fire- 
control instruments. The most successful 
method of fabrication depends on partially 
polymerizing the monomer with a catalyst 
while stirring constantly in an inert atmos¬ 
phere. When the material is thick, but can still 
be poured, it is introduced into polished Pyrex 
molds and is baked at two successive tempera¬ 
tures, usually not exceeding 80 C, to complete 
the polymerization process. The plastic ele¬ 
ments are separated from the molds by passing 
them through four successive water baths, each 
cooler than the last. A high percentage of the 
lenses and prisms emerge with almost perfect 
surfaces, free from defects. Prisms are cast in 
plate-glass molds and are welded together with 
low fusing alloy in jigs. The surface accuracy 
of prisms is not as good as that of lenses, be¬ 
cause of the nonuniform depth of the material 


below different parts of the surface. 

More than one hundred new plastics were 
synthesized, in accordance with a carefully 
planned program which sought to produce ma¬ 
terials having optical properties suited to the 
needs of lens design. The two most satisfactory 
materials for general use that have been en¬ 
countered so far are cyclohexylmethacrylate 
[CHM] and styrene. These resemble, in their 
general optical characteristics, crown and flint 
glass, respectively. They are highly transpar¬ 
ent and almost free from color. In spite of con¬ 
siderable improvements, styrene still exhibits 
enough haze to prevent its use for prisms in 
binocular erecting systems, because of the loss 
of contrast that is involved. Styrene lenses and 
prisms can be cast with greater surface accu¬ 
racy than CHM elements. 

The surfaces of all the linear polymers that 
have been studied are relatively soft and must 
be protected from scratching by glass cover 
plates if they are to be used in the field. A 
process for hardening the surface by exposing 
the material to silicon tetrachloride vapor fol¬ 
lowed by hydrolysis was developed at the Cali¬ 
fornia Institute of Technology. This treatment 
increased the length of time required to pro¬ 
duce a certain level of scratching by a factor 
of more than ten. Promising experiments on 
hardening plastic surfaces by evaporating vari¬ 
ous materials in a vacuum were carried out at 
Polaroid. Considerable work was also done at 
Polaroid on cross-linked plastics which are 
much harder than CHM and styrene, but which 
present greater difficulties in the casting of 
accurate surfaces. The future trend in optical 
plastics seems to lie in this direction, but ex¬ 
tensive experimentation will almost certainly 
be required. Success of commercial efforts to 
achieve a complete solution of the problem will 
insure that lenses and prisms of good quality 
will be available soon. As soon as surface hard¬ 
ness can be combined with good surface ac¬ 
curacy, and without the sacrifice of the present 
excellent internal homogeneity, optical plastics 
will immediately find many useful applications, 
partly due to reduction in weight and partly due 
to possible reduction in cost if they are pro¬ 
duced in large numbers. 

'The strain in cast elements of CHM and 





10 


SUMMARY 


styrene usually amounts to about 100 m \i which 
is not enough to limit optical performance in 
most optical designs of short focal length. The 
temperature coefficient of the index of refrac¬ 
tion of both materials is high and this leads to 
a considerable change in focal length with tem¬ 
perature. Some systems have been “athermal- 
ized” by the addition of a glass element which 
contributes a large part of the positive power, 
while the plastics supply the corrections. The 
overall system has a reasonably constant focal 
length in spite of temperature changes. Even 
this procedure cannot prevent poor optical per¬ 
formance when there are temperature gradi¬ 
ents within the system. 

A considerable number of optical instru¬ 
ments incorporating plastics was designed for 
special military applications. One of the most 
successful of these was the T-108 antitank tele¬ 
scope (Project OD-128), with 3X magnification 
and an exit pupil approximately 1 in. in diam¬ 
eter. A plate-glass mirror erecting system, 
mounted in low-melting alloy, was used in this 
telescope. Another significant design was that 
for a 7-in. //2.8 aerial camera lens for night 
photography. This camera lens incorporated 
four plastic elements and one element of DBC-1 
glass to athermalize the system. An //0.7 
camera lens, for photographing radar screens, 
was designed with six glass elements and an 
aspherical plastic plate. Several collimators for 
reflex sights and a number of special telescope 
systems were developed and tested by the Serv¬ 
ices. At the end of the contract, many samples 
of plastic elements and of complete instruments 
incorporating plastics were transferred to the 
Naval Research Laboratory. 

A considerable variety of optical techniques 
was developed under Section 16.1 of NDRC, 
partly in the natural course of developing spe¬ 
cific equipment for the Services, and partly to 
develop new methods as an aid in the produc¬ 
tion of critical items. 

Methods for grinding and figuring large 
lenses with high precision were devised at Har¬ 
vard. Diamond milling was used extensively to 
save time in getting the glass ready for the 
fine-grinding operations with silicon carbide 
and emery. The edges of large lenses were 
ground with spherical surfaces to improve the 


fit in their cells and to facilitate assembly. 
Every effort was made to machine the cells to 
close tolerances and to mount the elements in 
such a way that they would be both accurately 
defined and free from strain. Spacer rings were 
lapped in many cases to give the best optical 
performance. The Harvard reports contain 
many descriptions of optical and mechanical 
techniques which will be useful in other optical 
shops. 

A method for making roof prisms was de¬ 
veloped at the Mount Wilson Observatory to 
help meet the heavy demands on the industry 
for prisms which must have the roof angle ac¬ 
curate to within a very few seconds of arc. A 
No. 11 Blanchard grinder with cup-shaped Nor¬ 
ton diamond wheels was used. The glass blanks 
were waxed to special jigs. These jigs were 
so designed that the angles were established by 
turning the blanks first on one side and then on 
another side while they were in contact with a 
magnetic chuck on the rotating table of the 
grinding machine. The rate of feed was con¬ 
trolled and stopped automatically at the correct 
point. In this way, the prisms were milled in a 
series of carefully planned steps, using 100- 
and 180-grit diamond wheels. The average er¬ 
rors in the 90-degree roof angle of the milled 
prisms, measured with an air-pressure gauge, 
were usually less than 10 sec of arc. The varia¬ 
tion in errors produced by individual jigs was 
less than this. It seems clear that angles could 
be milled with an accuracy of at least 5 sec of 
arc if necessary. Present methods of blocking 
glass for polishing introduce errors of about 30 
sec. The prisms were polished on one side and 
then contacted to a plane-parallel block of glass, 
two on each side. The resulting unit was 
mounted in a metal cage and polished. Errors 
in the roof angle were observed with a collima¬ 
tor and eliminated by changing the location of 
a weight on the periphery of the cage. Milling 
operations required an average of 2.9 man- 
minutes per prism. The time for fine grinding 
and polishing was somewhat less than 1 man¬ 
hour per prism. It seems clear that one shop 
could mill enough prism blanks to supply the 
entire industry of this country in wartime. 

A method for glass molding was developed at 
the Eastman Kodak Company to explore the 



SUMMARY 


11 


possibility of producing large numbers of lens 
elements with the minimum of skilled labor. A 
press with stainless steel molds, maintained at 
specified temperatures and operating in a hy¬ 
drogen atmosphere, produced surfaces which 
were entirely free from orange-peel character¬ 
istics and which had accurately reproducible 
curvatures when cane or Pyrex glass was used. 
Thickness could not usually be adequately con¬ 
trolled to meet most specifications. Accordingly 
one surface was molded and the other was fin¬ 
ished by usual methods. If optical glass is to be 
molded satisfactorily, an automatic machine 
will be required for preheating pellets of the 
correct weight, feeding them to the press at the 
correct temperature, and then conveying them 
through an annealing oven. A large number of 
aspherical lenses for the Fly’s-Eye sight were 
made by this process with complete success. 
The process is particularly well adapted to mak¬ 
ing aspherical surfaces in production. 

Photographic methods for making reticles 
were studied by the Edward Stern Company 
and the California Institute of Technology, 
under Project NO-98. These included four 
methods developed by the British Scientific In¬ 
strument Research Association, one developed 
at the Eastman Kodak Company, and one de¬ 
veloped at the California Institute of Tech¬ 
nology. It was hoped that some of these meth¬ 
ods might be used to supplant mechanical 
methods. While the photographic processes 
were found to be more rapid than the conven¬ 
tional mechanical processes which employ pan¬ 
tographs, hand retouching was almost always 
required. The reproduction of complicated pat¬ 
terns was therefore more time consuming than 
the reproduction of simple patterns. Patterns 
involving both wide and narrow lines are par¬ 
ticularly difficult to reproduce mechanically. 
Photographically this was also the case, al¬ 
though the time was less than with the me¬ 
chanical process. The photoetching process was 
used to a considerable extent commercially dur¬ 
ing the war. The lead-sulfide process was also 
used for certain purposes. It seems likely that 
if the photographic methods are developed 
somewhat further they will find much wider 
applications. 

Methods for depositing low-reflection and 


high-efficiency films on glass were studied and 
developed at the University of Rochester, at the 
California Institute of Technology, and at Vard, 
Inc., under Projects CE-9 and NO-97. Experi¬ 
ments with multiple coats were carried out to 
investigate the possibility of achieving more 
efficient low-reflection surfaces than those ordi¬ 
narily deposited. Experiments carried out at 
the California Institute of Technology showed 
that quartz can be evaporated onto silver coats 
and offers substantial protection against tar¬ 
nishing, but that when quartz is evaporated 
from tungsten coils it dissolves enough of the 
metal to lead to considerable absorption of light. 
Evaporation from iridium is indicated, but is 
not easy to carry out experimentally. A high-effi¬ 
ciency high-reflecting film can be deposited by 
fuming titanium dioxide onto glass. Such films 
can also be made very effectively by depositing 
alternating layers of titanium dioxide and cryo¬ 
lite or by depositing alternate layers of zinc 
sulfide and cryolite. These films can be designed 
to split a beam in such a way that the trans¬ 
mitted and reflected beams are almost equal in 
intensity, with little loss by absorption. 

In response to a wide variety of Service re¬ 
quests, many new designs for optical systems 
for telescopes and binoculars were developed 
by the University of Rochester, the Yerkes Ob¬ 
servatory, and the Bausch and Lomb Optical 
Company. One of these programs involved the 
design of binoculars and monoculars for use 
at night, under Projects AC-26 and CE-8. Ap¬ 
parent fields up to 70 degrees, and in one case 
up to 90 degrees, were already available in some 
commercial instruments. These designs were 
improved at the University of Rochester and 
applied to a considerable number of instru¬ 
ments for special purposes, including in par¬ 
ticular the 6x42 antioscillation mounted binocu¬ 
lar for use in the P-61 aircraft (see Chapter 
14) and a 3x21 monocular with excellent defi¬ 
nition over the whole field. A Schmidt erector 
system was developed for use with these wide- 
field telescopes. Coating of the surfaces elimi¬ 
nated the ghosts which are ordinarily trouble¬ 
some in these erecting systems. The straight- 
in-line design makes the overall unit very com¬ 
pact. These night binocular and monocular sys¬ 
tems all had 7-mm exit pupils and maximum 



12 


SUMMARY 


fields. In addition, eye relief was made as great 
as possible, so that the systems could be 
mounted on antioscillation units without the 
eyebrows of the observer touching the eye lens. 
A monocular with 85-degree apparent field 
using an aspherical plate in the eyepiece was 
developed. A commercial production model of 
a 7x50 binocular, with 10-degree field, was 
designed by Bausch and Lomb and was pro¬ 
cured in considerable number for the Bureau 
of Aeronautics. Although this instrument was 
heavier than the Zeiss Deltar binocular, in pro¬ 
portion to the dimensions of the optical ele¬ 
ments, it was better adapted to American meth¬ 
ods of manufacture. Bausch and Lomb also 
made up two special binoculars for use in the 
testing program under Project NO-210. One 
was a binocular with 10-mm exit pupil, made 
by scaling up the standard Kellner eyepiece on 
the 7x50 instrument. The binoculars became 
5x50, with the same field as the 7x50. The other 
special instruments were “dummy” binoculars, 
which consisted of the bodies of standard 7x50 
instruments with the objectives and eyepieces 
removed, and with rhomboid prisms substituted 
for the Porro prisms to prevent inversion of 
the image. Three dummies were used with a 
series of diaphragms directly in front of the eye 
to determine the effect of exit pupil on binocular 
performance at night. This avoided the many 
complications which enter when magnification 
and the other effects of the optical system are 
present. A pressure-proof binocular was devel¬ 
oped under Project NO-127 for use on the deck 
of a submarine. This binocular is linked to a 
torpedo director. It employs a heavy metal cas¬ 
ing, capable of withstanding considerable pres¬ 
sure under water, with glass plates before the 
objectives and behind the eyepieces of the stand¬ 
ard 7x50 optical system. 

Four doublet objectives for telescopes and 
collimators were designed at Harvard in the 
course of work on various projects. They em¬ 
ploy BSC-2 and DF-2 glass and form a signifi¬ 
cant series with varying degrees of correction. 
The design data, which is given in Chapter 10, 
should be of interest to experimental labora¬ 
tories. 

A revision of the design of the T-76 (3x) 
and T-44 (5X) tank telescopes was requested 


under Project OD-119. The principal aberra¬ 
tions of these systems were spherical aber¬ 
ration, color, and curvature of field. In view 
of the urgent need to get a satisfactory tele¬ 
scope into the field at the earliest possible mo¬ 
ment, the problem was approached simultane¬ 
ously by two groups, from different points of 
view. The University of Rochester designed an 
achromatic plate and new doublets for the erec¬ 
tor system, which eliminated color and most of 
the spherical aberration, but could not change 
the curvature of field. A redesign of the erector 
doublets later permitted elimination of the 
achromatic plate. This solution of the problem 
seemed very satisfactory, since no change was 
made in the triplet objective or in the Erfle 
eyepiece, and would therefore cause the mini¬ 
mum of interference with production which 
had already started. In the case of the 5x tele¬ 
scope, it was necessary to redesign both the 
erecting system and the objective to get satis¬ 
factory performance, while retaining the orig¬ 
inal Erfle eyepiece. The Yerkes Observatory 
undertook a complete redesign of the 3X sys¬ 
tem. A widely separated Cooke triplet, with the 
entrance pupil in the middle of the objective, 
was used. The focus of the objective is 0.3 in. 
in front of the collective lens, so that the angu¬ 
lar aperture of the bundle is reduced and the 
principal ray is bent and strikes almost cen¬ 
trally on the erecting system, which consists of 
two separated doublets each with a negative 
lens in front. The overall system is well cor¬ 
rected, but no attempt was made to limit flexi¬ 
bility by correcting each subassembly sepa¬ 
rately. Spherical aberration is well corrected 
at the reticle, but astigmatism is overcorrected 
there in order to compensate for negative astig¬ 
matism in the eyepiece. All aberrations in the 
T-93 telescope were much reduced in the final 
model which was submitted to the Frankford 
Arsenal. In the meantime, improvements in the 
T-76 and T-93 telescopes had also been made 
at Frankford, and these, rather than the NDRC 
designs, were finally adopted for production. 
Several commercial firms also submitted de¬ 
signs and some were adopted for limited pro¬ 
duction. A full study of the present status of 
designs for wide-field tank telescopes with 
large exit pupil is indicated. Present require- 



SUMMARY 


13 


merits demand not only a wide field and 7-mm 
exit pupil, but also that the optical system fit 
the 2-in. tube which is in current use. A careful 
study should be made to determine just how 
far the optical requirements can be met when 
this final condition is imposed. If a satisfactory 
result cannot be achieved when the tube is 
limited to 2 in. in diameter, then consideration 
should be given either to increasing the tube 
slightly or else to reducing the field or exit 
pupil somewhat, if it is deemed necessary to 
provide good definition over the entire field. 
Only tests under service conditions can answer 
this question. 

A split-field tank telescope (1.5X and 5X) 
was developed in connection with Project 
OD-180 to provide simultaneously a relatively 
wide field with low magnification for finding 
the target, and a more limited high-power field 
for aiming. The field is divided along a diam¬ 
eter. Many unusual problems in design are in¬ 
volved, which are fully described in the Yerkes 
report (see Reference 7 of Chapter 10). Two 
objectives (telephoto and inverted telephoto) 
are used with two semicircular collective lenses 
and a common erecting system. The exit pupils 
for the two systems are circular and coincide. 
This telescope was examined by various groups 
and has been delivered to the Frankford Ar¬ 
senal for further study. 

Two antitank telescopes were developed. One 
of these was the T-118 plastic antitank telescope 
(5X) which was designed and constructed by 
Polaroid under Project OD-149. The aperture 
is 4 in. in diameter, with a mirror erecting 
system. The exit pupil is 20 mm in diameter 
and the eye relief 4 in. Performance is excel¬ 
lent. All of the lenses are plastic, except for 
one protective lens of glass at the front. The 
other telescope is a split-field antitank telescope 
(1.5X and 5x), which was developed under 
Project OD-180. The Ordnance Department had 
requested that the T-118 telescope be modified, 
with as few changes as possible, to provide two 
fields with differing magnifications. A specific 
layout for the design was proposed, but studies 
indicated that it would not give satisfactory 
results. Accordingly, a somewhat different ap¬ 
proach was followed, involving the use of two 
mirror erecting systems. This design requires 


two holes in the armor in front of the objec¬ 
tives, but no greater area of exposure is re¬ 
quired than for the T-118, or than would be 
required for the same exit pupils in the design 
proposed by the Ordnance Department. No 
model of the Yerkes design has been made. 

A periscopic binocular (T-9) for use by tank 
commanders was developed by the University 
of Rochester under Project OD-129. The optical 
system was 7x50 with wide field. A common 
prism at the top of the instrument turned the 
beam downward, and could be replaced if dam¬ 
aged in its exposed position. The objectives 
were placed in the vertical beam and were 
followed by three reflections in glass. The 
Frankford Arsenal originally requested that 
provision be made for a unit-power system be¬ 
tween the two telescope systems, but later 
asked that this be eliminated. However, after 
a model had been completed at the University 
of Rochester, it was decided that the unit-power 
system should be restored. Accordingly, the 
mechanical parts were redesigned by the Arse¬ 
nal for production. The instrument gave excel¬ 
lent optical performance over the entire field. 

A precision theodolite telescope was devel¬ 
oped by the University of Rochester under 
Project CE-21. The final design employs an 
air-spaced triplet objective of 40 mm aperture 
and 122 mm focal length. The telescope is in¬ 
verting and is focused internally by moving a 
negative lens. An orthoscopic eyepiece is em¬ 
ployed. The system is anallatic to 1 per cent, 
from 3 m to infinity. A sample showed optical 
performance at least equal to that of a Zeiss 
telescope of similar type. 

An extensive study of the optical properties 
of submarine periscopes was carried out at the 
Yerkes Observatory. This work began as a nat¬ 
ural continuation of Project NS-242 which cov¬ 
ered quick developing equipment for periscope 
photography. In the course of that work, it 
had been found that the optical performance 
of the standard submarine periscopes left much 
to be desired, particularly in the case of the 
models which employ small lenses at the upper 
part of the system. By replacing one of the 
doublets in the erector system by a quintet, 
which included two elements of optical fluorite, 
the color correction of each of three periscopes 



14 


SUMMARY 


was considerably improved. Two-thirds of the 
whole color defect arises in the erector system 
of these instruments. A model of this correct¬ 
ing unit for the 1.4-in. periscope was made at 
Harvard. The monochromatic performance is 
slightly inferior to the original design, but the 
color has been markedly improved so that it 
should be possible to utilize a much wider 
region of the spectrum photographically, thus 
reducing exposure time and diminishing the 
deleterious effects of image motion. Tests of 
the corrector were carried out under a Navy 
contract after the work at Yerkes had been 
completed. The results did not come up to ex¬ 
pectations, but time did not permit an investi¬ 
gation of the reason for the discrepancy be¬ 
tween the results of ray tracing and of photo¬ 
graphic resolution tests. In the case of the 
1.4-in. periscope, better results would be 
achieved if both erectors were replaced by 
simpler fluorite units than by using the single 
complex unit that was actually designed. In the 
case of the 1.9-in. and the Type IV periscopes, 
for which the optical systems are simpler, de¬ 
signs have been worked out for air-spaced 
doublets of BSC-1 glass and fluorite which re¬ 
place both the doublets in the standard erector 
system. In these two periscopes the objective 
and eyepiece contribute little to the secondary 
color. The use of fluorite seems entirely prac¬ 
tical for these two periscopes at least, and it 
would be desirable to have models of the Yerkes 
correctors made and tested. An eyepiece with 
longer eye relief was designed for the 1.4-in. 
periscope. 

Field-flattening camera lenses were also de¬ 
signed for the 1.4-in. periscope, both in its orig¬ 
inal form and when the fluorite corrector and 
long eye relief eyepiece have been substituted. 
It is to be expected that these camera lenses 
would give much better performance off axis 
than the usual camera lens because of the strik¬ 
ing curvature of field which exists in the peri¬ 
scope. In the case of the 1.9-in. periscope, a 
field-flattening lens can be installed within the 
camera, just in front of the film. It would re¬ 
duce the present confusion disk, 3 degrees off 
axis in object space, from 0.09x0.11 mm to 
0.01x0.01 mm when a yellow filter is used. A 
similar lens can be used with the Type IV peri¬ 


scope, but no filter is required, even without 
the fluorite corrector. Serious consideration 
should be given to redesign of the entire peri¬ 
scope systems to reduce color and to flatten the 
field as far as possible within the allowed di¬ 
mensions of the tube. 

A unit-power aircraft periscope, to be used 
by an observer in TBF aircraft with linkage 
to other equipment, was designed at the Yerkes 
Observatory under Project NR-111. Four iden¬ 
tical doublets were employed to facilitate pro¬ 
duction. A model was made at Harvard and 
successfully tested at Quonset. The optical parts 
for sixteen additional models were made at 
Harvard and turned over to the Navy. Experi¬ 
ments were later tried with the optical system 
of the 3x tank telescope designed at Yerkes. 
This was mounted as a periscope, with a prism 
ahead of the eyepiece. It was decided, as a re¬ 
sult of these tests, that 2X magnification would 
be most suitable. The Bureau of Ordnance un¬ 
dertook the design of a periscope with this 
magnification. 

A periscope for the P-51 aircraft to increase 
the vision of the pilot downward was designed 
at Yerkes, following a suggestion made at 
Mount Wilson. The periscope is 110 in. long, 
with unity magnification. The lenses, which are 
about 8 in. in diameter, could be made of plas¬ 
tic. The eye lens would be incorporated in the 
windscreen in such a way as to cause minimum 
interference when the pilot turns from looking 
through the Plexiglas to looking through the 
eye lens. The pilot can ordinarily see downward 
only about 3.5 degrees. A periscope 110 in. long 
would increase the downward view to more 
than 20 degrees. Such a periscope would be 
useful, and apparently practical, if it turns out 
that the fuselage cannot be redesigned to give 
the same result. In any case this system has 
attractive possibilities for use with a reticle 
attached to a lead-computing mechanism. The 
large aperture, the use of an opaque reticle, and 
the elimination of a reflex plate are all advan¬ 
tages. 

A foxhole periscope was designed on the basis 
of an informal request from the Signal Corps. 
The purpose was to enable an operator in a 
foxhole to watch the operation of a motion 
picture camera on a support just above the 



SUMMARY 


15 


edge of the hole. The Yerkes Observatory de¬ 
signed two unit-power systems which could be 
used as viewfinders for the camera, with 18-in. 
offset, 40-degree field, and eye reliefs of 1.6 and 
2.0 in., respectively. The exit pupils were 7 and 
10 mm in diameter, respectively. Samples of 
these instruments were made up by the Signal 
Corps, and gave satisfactory performance. 

A viewfinder for the P-80 aircraft was stud¬ 
ied by the University of Rochester in connec¬ 
tion with Project AC-131. The viewfinder was 
to be used in the photographic version of this 
aircraft to permit the pilot to guide his course 
over the target and to make exposures at the 
correct time. It was requested that this view¬ 
finder be periscopic, with a 45-degree field of 
view and a 5-in. exit pupil. An unobstructed 
vertical view was desired, if possible. An opti¬ 
cal system was worked out tentatively but was 
not fully designed because the many problems 
relating to interference with mechanical parts 
could only be settled by mutual discussion at 
the factory. The University of Rochester sys¬ 
tem employed one-quarter magnification. It is 
likely that this would be a serious disadvantage 
for a pilot at high altitude, since it would make 
identification of targets difficult. A better solu¬ 
tion might be to provide a screen of consider¬ 
able size on the instrument panel, on which a 
unit-power image would be projected by a peri¬ 
scope with large lenses and a scanning prism 
at the lower end. 

A wide-field projector for a dome trainer was 
developed for the Bureau of Aeronautics under 
Project NA-200. A field of 180 degrees hori¬ 
zontally and 90 degrees vertically was desired, 
with a relative aperture of //2. The Yerkes Ob¬ 
servatory developed a system which met the 
requirements by using a 6-element Petzval-type 
lens with a field of 14 degrees in combination 
with a convex spherical mirror 12 in. in diam¬ 
eter, which increased the field by a factor of 
ten. The mirror has little effect on spherical 
aberration and coma, and no effect on chromatic 
aberration. The curvature of field of the mirror 
is opposite in sign to that of the lens, and they 
are made to cancel one another in this respect. 
By placing the observer slightly in front of the 
projector and enough to one side to miss the 
beam, the apparent field is increased to 180x90 


degrees. Definition is entirely adequate. Some 
distortion is inherent in the system. This would 
be nearly entirely removed if the same optical 
system were used as a camera lens for taking 
the training exposures. The lens was tested suc¬ 
cessfully by the Special Devices Division of the 
Bureau of Aeronautics and was transferred 
there. 

A high-resolution projection lens was de¬ 
signed at Yerkes for use at Dartmouth in the 
binocular testing program. In this case the re¬ 
quirements were very different. It was merely 
necessary to project a small target at the center 
of the field, but it was extremely important 
that resolution be almost perfect, so that the 
amount of light scattered into the image of 
even small targets would be insignificant. The 
Yerkes Observatory designed for this purpose 
an //9 lens which was a cemented doublet cor¬ 
rected for 5,461 A, bringing F and D nearly 
together. This was done to give the best per¬ 
formance with the eye, which has maximum 
absolute sensitivity at low levels near 5,060 A. 
The effective wavelength for maximum sensi¬ 
tivity shifts toward the red when tungsten 
illumination is used, and is located at about 
5,330 A. Four of these lenses were made in a 
commercial shop. Some of them were refigured 
at Harvard to give the best possible perform¬ 
ance. 

A plastic condenser lens was made at Harvard 
for photoelectric work. The diameter was 8 in. 
and the relative aperture //1.5. Spherical aber¬ 
ration was reduced to a point where, in spite 
of the fact that no color correction was under¬ 
taken, the circle of confusion was only 3 mm. 
This is entirely adequate for most condenser 
applications. If this lens were produced in large 
numbers the elements could be molded (see 
Chapter 8). 

A coronagraphic objective was designed and 
made at Harvard for use under a Navy con¬ 
tract. A cemented triplet with good color cor¬ 
rection was employed. Great care was taken to 
reduce scattered light to the absolute minimum 
by eliminating all possible scratches. The lens 
was designed as a triplet so that all outside 
surfaces, which are necessarily exposed to dust 
and cleaning, would be of crown glass. 

Reflex sights were used extensively during 



16 


SUMMARY 


World War II for many applications. They 
make it possible to project a reticle pattern at 
infinity with considerable eye freedom and with 
minimum obscuration of vision. Most of the 
applications with which NDRC was concerned 
involved sights for aircraft, where the ideal is 
a sight which does not require the pilot to locate 
his eye closely in a special position and which 
occupies the minimum of space. An almost ideal 
arrangement would be to reflect the collimated 
beam from a large aperture on the bulletproof 
glass. Steps were being taken in this direction 
at the end of World War II. 

The Yerkes Observatory made a general 
study of collimators for reflex sights, and estab¬ 
lished what could be expected from two-, three-, 
and four-lens systems. Models were made for 
testing and demonstration. These have been 
transferred to the Armament Laboratory at 
Wright Field. A sight in which the light path 
is folded twice by reflection, so that it assumes 
a “Figure-4” arrangement and is extremely 
compact, was developed at the University of 
Rochester. This design was developed for pro¬ 
duction by Bell and Howell under direct Army 
contract. The University of Rochester also de¬ 
veloped a lens-collimated reflex sight for use 
with the M-7 computer on the 40-mm Bofors 
antiaircraft gun, under Project OD-146, to re¬ 
place the small aperture unit-power telescope 
which was hard to use. Three models were 
made, each with a half-reflecting reflex plate. 
Sky illumination is employed in these sights in 
the daytime, using a region near the target. 
At night, electric illumination is available. On 
one model, a radium-excited phosphor is used 
to provide the auxiliary illumination. For use 
on the M45 multiple machine-gun mount, wide- 
field sights to cover leads up to 280 mils were 
designed at the University of Rochester and at 
the Yerkes Observatory. A similar design was 
later used for a sight on a pantograph mount 
to be attached to the machine gun on a half¬ 
track truck. Two collimator lenses for various 
applications of reflex sights were designed at 
Mount Wilson. A special sight, known as the 
flight sight , was developed by the University of 
Rochester (Project NO-103) to permit the pilot 
of an F4U-2 airplane to see reflected in the 
reflex plate and collimated at infinity not only 


the reticle, but also the radar pips, the air¬ 
speed indicator, and the gyro horizon. Tests 
indicated that this device is capable of assisting 
the pilot of a fighter plane to a very consider¬ 
able degree in making a radar approach on a 
target at night. A sight with plastic lenses, 
having 3.5 in. aperture and operating at //1.6, 
was developed by Polaroid to replace the Navy 
Mark 8 sight. The optical system was folded 
with one reflection and mounted in a mag¬ 
nesium housing. Four models were made and 
tested by the Navy and the Army but none 
were produced, because of the increased avail¬ 
ability of glass lenses. 

The Lens-Mangin sight was developed at 
Yerkes to provide a large exit pupil. This sight 
is unusually compact. A model with 4x6-in. aper¬ 
ture had dimensions only 6.2x6.5x7.5 in., ex¬ 
clusive of the reflex plate. Light from the reticle 
is reflected by a 50 per cent dividing plate to 
a Mangin mirror which returns it through the 
dividing plate to a lens and the reflex plate. 
The usual Mangin system is well corrected for 
color and spherical aberration, but not for 
coma. The addition of the weak lens controls 
coma and leaves only astigmatism as a signifi¬ 
cant residual aberration. This is not present in 
a serious amount. At best, only 25 per cent of 
the light from the reticle can reach the reflex 
plate, because of the double passage through 
the dividing plate. This is the only serious dis¬ 
advantage of this highly ingenious sight and it 
seems likely that it can be overcome by giving 
adequate attention to the illuminating system. 
The fact that a large aperture is available in 
very limited space, with the use of only two 
simple lenses, makes this design worthy of 
further study. 

The Bowen sight represents another unusual 
design, intended to provide large aperture with¬ 
out obstructing the pilot’s view. The reticle is 
collimated by a large spherical mirror. A dia¬ 
phragm on the illuminating system is placed 
at the center of curvature of the mirror, and is 
focused, after reflection at the reflex plate, to 
form an exit pupil at the pilot’s eye. Within 
the region of this pupil, the pilot sees the col¬ 
limated reticle over a field which is limited only 
by the size of the concave mirror. The magni¬ 
tude of the spherical aberration, which is the 



SUMMARY 


17 


only aberration in this system that produces 
parallax of the reticle, depends on the diameter 
of the diaphragm of the illuminating system, 
and its reflected image of equal size, the exit 
pupil. When the aperture of the illuminating 
system (and of the exit pupil) is equal to one- 
quarter the radius of curvature of the mirror, 
that is, when the aperture ratio is //2, the max¬ 
imum aberration, which occurs when the eye 
is at the edge of the exit pupil, amounts to 
about 1 mil. The size of the reticle does not 
affect the magnitude of the aberrations. This 
is a unique property of the Bowen sight. The 
best disposition of the parts of the sight raises 
problems which depend on the design of indi¬ 
vidual cockpits. Two models have been made: 
one for the AT-6 airplane, where the mirror 
was placed above the line of sight and the reti¬ 
cle and illuminator below; the other for the 
P-51 airplane, where these locations were re¬ 
versed. In each case the supports which connect 
mirror and reticle were designed to lie in front 
of divisions in the plastic of the canopy, so that 
there was little interference with normal vision. 
Pilots expressed great interest in this sight, 
particularly because of the generous eye free¬ 
dom which it allowed, and because the two eyes 
could view the reticle simultaneously. 

A third design, which provides large aper¬ 
ture within compact dimensions, is that of the 
Fly’s-Eye sight, which was developed at the 
Eastman Kodak Company, under Division 7 of 
NDRC, for use in fighter aircraft. Many small 
reticle-collimator units are grouped together, 
in a hexagonal array, accurately aligned with 
one another so that when the eye moves across 
the combined aperture the reticle appears to 
maintain a steady position at infinity. Simple 
aspherical lenses were used, molded by the 
process described in Chapter 9. The reticles, 
which were made by etching sheet metal, were 
mounted on an Invar plate and located in the 
correct positions. Two models were made with 
apertures 4in. The illuminating systems 
consisted of six concave mirrors cut to square 
dimensions, approximately 2x2 in., so that they 
would fit together at the edges. One frosted 
electric lamp was used in each mirror. Air was 
circulated by means of a blower to cool the 
unit. The collimated beam from the multiple 


lenses was reflected from the armor glass. This 
made it possible to mount the sight ahead of 
the instrument panel in a position where it did 
not interfere with other equipment. From the 
pilot’s point of view the installation was highly 
satisfactory. 

Two solid glass sights were designed at the 
Mount Wilson Observatory. One of these was 
intended for experimental use on an M-l Gar- 
and rifle to aid men on the ground in replying 
to strafing fire from aircraft. This application 
requires a sight with large rings to estimate 
the lead. A solid glass block with an air Mangin 
lens at the lower end, and with a diagonal ce¬ 
mented surface coated for half reflection, was 
used. The aperture was %x% in. The reticle 
was cemented to a small prism at the upper 
end. Sky illumination was sufficient to give a 
satisfactory display under most conditions. A 
quarter-wave plate was incorporated just in 
front of the Mangin reflector to reduce light 
loss which resulted from polarization at the 
inclined half-reflecting interface. The sight 
gave good performance, although the require¬ 
ment for using the M-l rifle against aircraft 
was never fully established. A model is being 
transferred to the Antiaircraft Service Test 
Section, Ground Forces Board No. 1 at Fort 
Bliss. The second sight was similar in construc¬ 
tion, but was designed to fit the Bureau of 
Ordnance gunsight Mark 17. Tests showed that 
it was undesirably heavy for this application, 
in spite of the fact that the aperture was only 
lxl in. The model of this sight was also trans¬ 
ferred to Fort Bliss. 

Three models of stadiameters were developed 
at the University of Rochester, primarily for 
use in aircraft. When the distance of an object 
is measured with a rangefinder, the result de¬ 
pends on measuring differences in apparent 
direction at the ends of a known base line, 
whereas when a stadiameter is used the ap¬ 
parent angular size of an object of known di¬ 
mensions is measured, from which the distance 
can be calculated. Although rangefinders have 
the advantage that they require no information 
about the size of any object, they are large, 
cumbersome, and expensive, which seriously 
restricts their usefulness. Stadiameters are 
small and inexpensive, but they do require 





18 


SUMMARY 


knowledge of the size of the target. In many 
applications where this is known, they have 
proved to be extremely convenient. Moreover, 
no accurate aiming of the instrument is neces¬ 
sary, as with coincidence rangefinders. It is 
merely necessary to have the target somewhere 
in the field. A unit-power stadiameter was de¬ 
veloped originally to assist the pilots of aircraft 
to maintain a specified separation from one 
another in a certain tactical maneuver. The 
wing span of the leading plane was of course 
known. The optical system consisted merely of 
two flat mirrors, approximately parallel to one 
another, with a half-reflecting half-transmit¬ 
ting coat on one of them, and a fully reflecting 
coat on the other. The latter was mounted on 
a pivot with a tangent screw and divided drum 
to permit making small adjustments in the 
angle between the two mirrors. By looking 
through the half-reflecting mirror, the observer 
sees a double image of the target and sets the 
angle of the other mirror until the two images 
just touch. Settings can be reproduced to at 
least 0.5 mil. The zero point is set on a distant 
object. 

A 3X stadiameter was developed at the Uni¬ 
versity of Rochester to aid F-5 (P-38) pilots 
in maintaining a separation of 6 miles while 
flying parallel mapping courses. At this range 
some magnification was essential. A 3X wide- 
field system is used, with a mirror-prism erect¬ 
ing system in which is incorporated a beam 
splitter bringing in light which has passed 
through a variable angle prism. Color is not 
serious up to about 50 mils separation. The 
field is 23 degrees, of which 18 degrees can be 
deviated and used for stadiametric setting. The 
exit pupil is 7 mm. The setting of the variable 
angle prism, which consists of a plano-convex 
and a plano-concave lens, is controlled by a 
tangent screw with a divided head reading to 
0.1 mil with a vernier. Settings can be repro¬ 
duced to 0.1 mil. 

There is need for a quick means for setting 
range into the fire-control system of the B-29 
aircraft. Under Project AC-114, a double-image 
stadiametric attachment for the pedestal sight 
was devised, based on the use of two plane 
mirrors close to one another and nearly paral¬ 
lel. The first mirror reflects approximately one- 


third of the light to give a fixed image, while it 
transmits nearly two-thirds of the light to a 
fully reflecting mirror which can be adjusted 
in angle and which reflects a second image 
through the first mirror. The two images are 
brought edge to edge by turning a knob. It 
seemed likely that this device would facilitate 
rapid ranging, since it would not be necessary 
to have the target in any particular part of the 
field. The optical parts and the layout for the 
stadiameter were turned over to the General 
Electric Company. 

In connection with the problem of night in¬ 
terception of bombers by fighter aircraft, a 
need arose for aids to night vision. Under 
Project AC-26, the University of Rochester de¬ 
veloped wide-field binoculars (see Chapter 10) 
with 7-mm exit pupils. The use of these binoc¬ 
ulars in aircraft was limited by the effects of 
angular vibration which were greatly magnified 
by the binoculars. Accordingly, studies of meth¬ 
ods to eliminate angular vibration in optical 
instruments were undertaken. Linear vibration 
is much less serious. Antioscillation mounts of 
different types were developed by the Univer¬ 
sity of Rochester, the Eastman Kodak Com¬ 
pany, and by the Technicolor Motion Picture 
Corporation. The first of these was a ball¬ 
bearing gimbal mount with the center of grav¬ 
ity of the instrument set accurately at the inter¬ 
section of the two axes of rotation to prevent 
the conversion of linear into rotational vibra¬ 
tion. Damping was provided in each coordinate 
by air dashpots. This mount was tested exten¬ 
sively in aircraft. It was found possible for a 
pilot to fly an airplane at night while overtak¬ 
ing a target plane and while looking steadily 
through the glasses. The effect of magnification 
is to make the approach seem far more gradual 
than it really is, but experience enables the 
pilot to make due allowance for this and to 
pull out in time to avoid a collision. A produc¬ 
tion model of this antioscillation mounted bin¬ 
ocular was developed by the Eastman Kodak 
Company. These instruments were standard 
equipment in P-61 night fighter aircraft. The 
binocular is mounted on a track at the left side 
of the canopy where the pilot can reach it when 
radar indicates that a target is ahead. The 
binocular is carried on a carriage with a hinge 



SUMMARY 


19 


that permits the instrument to swing out into 
place in front of the pilot, with provision for 
locking temporarily to the top of the armor 
glass frame. An illuminated reticle is incorpo¬ 
rated in one half of the binocular, which can 
be used as a gunsight if desired. The instru¬ 
ment can be removed and stowed in less than 
2 sec. Tests have shown that the 6x42 system 
increases the range of detection at night by a 
factor of about 4.5, so that an excellent transi¬ 
tion is provided between radar and naked-eye 
detection. Images are very steady, even in the 
presence of marked aircraft vibration. Under 
Project NA-140, one of these binocular units 
was linked to other equipment by Section 16.5 
of NDRC. Tests, made under Project OD-116, 
showed that an antioscillation mount is not 
needed when binoculars are installed on a sta¬ 
tionary tank and are used with the engine run¬ 
ning. 

Antioscillation mounts for standard binocu¬ 
lars, also based on the center-of-gravity princi¬ 
ple, but simpler in design, were developed by 
the Eastman Kodak Company and by the Tech¬ 
nicolor Motion Picture Corporation. In both 
of these mounts the binocular is held at its 
center of gravity. The Eastman mount carries 
the instrument on a % 6 -in. steel ball resting in 
a synthane cone, which provides freedom of 
rotation in three coordinates. A coiled spring 
surrounding this unit supplies the necessary 
restoring force, while dry friction between the 
ball and cone supplies damping which can be 
adjusted by changing the diameter of the ball. 
Stops are added to limit the motion. The center 
of gravity is maintained at the center of the 
ball, in spite of changing the interocular sep¬ 
aration of standard binoculars, by providing a 
specially shaped cam on which the two sides 
of the instrument ride. A headrest and eye 
guards are added to aid in maintaining the 
proper relation of the eyes to the instrument. 
Several units were made for testing in aircraft. 
Several were also mounted in specially designed 
alidades for use on shipboard, under Project 
NS-105. The alidades were equipped with filters 
for linear vibration to reduce the effects of 
severe vibration on bulkheads, which often is 
converted into angular vibration even with a 
good antioscillation mount if the balancing is 


not very precise. Tests of these mounts in air¬ 
craft and on ships showed that they are capable 
of giving excellent performance at least com¬ 
parable with that of the gimbal mount, although 
boresighting characteristics (at least in the 
absence of vibration) are not as good. The de¬ 
sign is much simpler and less expensive, and 
requires less critical adjustment and servicing. 
For these reasons further studies and tests 
should be made to determine the extent to 
which it may be adequate for various applica¬ 
tions. Wide-held binoculars could, of course, be 
mounted on this type of filtering unit. 

The Technicolor mount is, in general princi¬ 
ple, similar to the ball-and-cone mount described 
above. Standard binoculars are supported at 
the center of gravity, but in the Technicolor 
mount the instrument is carried on a post at 
the lower end of which is attached a ball % in. 
in diameter, surrounded by two pure gum rub¬ 
ber washers, one above and the other below 
the ball. The rubber is enclosed by an outer 
split sphere which compresses it enough to 
provide the necessary restoring force. Damp¬ 
ing is provided by spring shoes acting on the 
outside of the upper hemisphere with adjust¬ 
able tension so that the damping can be varied. 
The outer hemisphere is carried on the base 
through a shock-absorbing unit which consists 
of a horizontal metal plate compressed between 
two sheets of sponge rubber. One model was 
designed for use in PBY aircraft, in the co¬ 
pilot’s position, attached to the roof of the 
canopy either by rubber suction cups or by 
clamping to a rigid bar fastened to two ribs 
of the canopy. In either case the unit could be 
quickly installed or removed. Another model 
was designed for use on ships, with an alidade 
mount and windscreen. These models were 
tested in aircraft and on ships. In both cases 
they showed definite reduction in the vibration, 
as compared with rigidly mounted binoculars. 
Further tests will need to be conducted to com¬ 
pare adequately the performance of the Tech¬ 
nicolor and Eastman mounts. It seems possible 
that the base filtering unit in the Technicolor 
mount impairs its performance under condi¬ 
tions where linear vibration is severe, since 
such vibration is converted into rotational vi¬ 
bration by the base unit which is located below 



20 


SUMMARY 


the center of gravity, and thereby increases 
the filtering required of the central antioscilla¬ 
tion unit. Tests should be made to establish 
the effectiveness of the base filtering unit. 

A precision phototheodolite was developed by 
the Eastman Kodak Company under Project 
OD-48. Many needs for such instruments were 
apparent, even before the beginning of World 
War II. These included the testing of height- 
finders, testing the performance of the oper¬ 
ators of heightfinders and rangefinders, con¬ 
struction of range and fuse tables for use with 
antiaircraft guns, construction of bombing ta¬ 
bles, recording of aircraft motions, and finally 
testing the overall accuracy of fire-control 
equipment. Tests of the Askania instruments 
showed that they were almost, but not quite, 
capable of giving the 0.1 mil overall accuracy 
desired. Accordingly, the design of entirely new 
instruments was undertaken. Cameras with 
focal lengths of 12, 24, and 48 in. were pro¬ 
vided. The 12-in. camera was equipped with a 
conventional lens, but for the cameras of longer 
focal length two-mirror achromatic Mangin 
systems were used. The primary mirror was 
pierced by a hole to transmit the beam from 
the secondary mirror to the focus which was 
just behind the primary. Although these cam¬ 
eras involved many complications relating to 
optical figuring of the achromatic components 
and relating to mounting the elements, they 
have the advantage that the overall length is 
less than can be achieved with telephoto lenses. 
In the case of the 48-in. camera, an f/11 lens 
of telephoto type was provided as an alternative 
in the event that any difficulty be experienced 
with the large Mangin camera. Thirty-five mil¬ 
limeter film is carried in a camera mechanism 
of the single-frame type, capable of operating 
up to 10 frames per second. For faster opera¬ 
tion, the camera is run continuously at 20 
frames per second. A series of neutral and color 
filters is provided close to the focus. Two-man 
tracking is provided. Dual-lead worms are used 
to reduce backlash. Azimuth and elevation are 
recorded by photographing drums and counters 
on the worm shafts, which are illuminated by 
Edgerton flashes at the mid-point of each pho¬ 
tographic exposure. Screws on the base permit 
leveling one instrument and misleveling the 


other of a pair, to allow for the curvature of 
the earth. The instruments have been installed 
in domes, at the ends of a base line 11,000 yd 
long, at Fort Bliss, Texas by the Antiaircraft 
Service Test Section, Ground Forces Board 
No. 1. Extensive tests are being conducted to 
measure the accuracy of the instruments. These 
tests will include measures of the positions of 
fixed targets when tracking is being done at 
various rates and also measures of the positions 
of stars which provide targets accurately placed 
over a wide range in azimuth and elevation. 

Optical scanning devices were developed 
under Section 16.1 in an attempt to improve 
the effectiveness of lookouts in searching for 
both aircraft and submarines. The purpose of 
these devices is to permit the observer, while 
seated in reasonable comfort, to look into fixed 
eyepieces and to scan systematically and with 
full coverage an assigned sector of sky or hori¬ 
zon. In all of the scanning devices, the optical 
path is caused to sweep, in one or in two co¬ 
ordinates, by rotating mirrors or prisms which 
in some designs are part of the telescope erector 
system and in other designs are reflectors added 
to standard binoculars expressly for the pur¬ 
pose of scanning. Four scanning devices which 
have been designed and tested appear to war¬ 
rant further study and development. Carefully 
controlled tests are needed to determine the 
extent, if any, by which they increase the effi¬ 
ciency of lookouts in picking up targets, in 
comparison with hand-held binoculars. The rela¬ 
tive effectiveness of monocular and binocular 
scanners of the same type should be determined 
after a headrest and eye guards to aid in lo¬ 
cating the eye have been added and maximum 
comfort provided to make the use of the monoc¬ 
ular type as effective as possible. Antivibration 
mounts should be used with binoculars in scan¬ 
ning devices on shipboard, and experiments 
should be undertaken with stabilized scanners. 
One of the most promising models should be 
provided with azimuth and elevation indicators 
in the field of view and with remote indicators 
actuated by selsyn linkages. Consideration 
should also be given to connecting a scanning 
device directly to the fire-control system. The 
importance of the lookout problem justifies ac¬ 
tive work along all these lines. 



SUMMARY 


21 


An antiglare shutter, intended to protect the 
dark adaptation of the pilot of a night fighter 
aircraft in the presence of blinding flashes 
which might be discharged by a pursued 
bomber, was developed by the Eastman Kodak 
Company as part of the program on night 
vision devices under Project AC-26. The use 
of binoculars greatly increases the likelihood 
that dark adaptation will be destroyed by any 
flash, because of the greatly increased amount 
of light which enters the eye from any point 
source. The shutter curtain is made of black 
nylon cloth with pure gum rubber cords ce¬ 
mented along the edges to insure quick opening. 
The driving force for closing the shutter over 
the apertures of the binocular is provided by 
an electrically fired charge of tetrazene, to 
which potassium perchlorate is added to oxidize 
the residue and prevent fouling the cylinder. 
The firing charge is loaded in small primer cups 
in a magazine which holds ten charges and is 
advanced automatically by a ratchet mecha¬ 
nism. The shutter is capable of closing the bin¬ 
ocular apertures completely in about 0.0015 sec. 
A delay mechanism holds the shutter closed for 
0.3 sec, after which it opens and is ready to 
close again in response to another signal. Stan¬ 
ford University developed a photoelectric con¬ 
trol circuit which triggered a Thyratron tube 
so as to discharge a condenser through the 
primary of a transformer. The resulting surge 
in the secondary fired the tetrazene cap. The 
amplifier was designed to respond to a light 
pulse of 0.01 footcandle, which is about the in¬ 
tensity of the full moon. It was particularly 
free from microphonic effects. Tests of the am¬ 
plifier in the P-61 aircraft showed excellent 
performance, but time did not permit testing 
the complete shutter and amplifier as a unit. 

Rapid processing equipment for periscope 
photography was developed, under Project 
NS-242, by the Eastman Kodak Company. It 
is frequently desirable to take a photograph 
through the periscope which is exposed for the 
minimum possible time. If this photograph 
could be processed within a minute or less, it 
would be possible for several officers to examine 
and discuss the photograph, and to base tactical 
decisions on it, thus diminishing the need for 
visual examination of targets, and thereby re¬ 


ducing the total exposure of the periscope. A 
special back was developed for the standard 
35-mm periscope camera Mark 1, which per¬ 
mitted the removal of short lengths of exposed 
film for transfer to a light-tight cassette which 
contained a knife for cutting the film, and was 
provided with light locks through which solu¬ 
tions could pass when the cassette was trans¬ 
ferred to a processing tank. The film was held 
on a metal frame inside the cassette. After de¬ 
velopment and brief washing, the film was re¬ 
moved in its frame and examined in a special 
viewer which enlarged the image 3X by means 
of a concave mirror, while illuminated with a 
fluorescent lamp and ground glass. Both eyes 
can be used simultaneously with this viewer, 
which is convenient to operate. Special film and 
special developer were provided, which made it 
possible to view the negative in about 40 sec. 
Even under these conditions, resolution was 
comparable with that ordinarily given by 
Super-XX film. Tests on a submarine in the 
Pacific showed that the performance of the 
equipment was satisfactory and that the use 
of rapidly processed negatives was tactically 
desirable. Five units, each consisting of three 
processing cassettes, eighteen film holders, four 
processing tanks, and a viewer, were delivered 
to the Bureau of Ships for further tests. 

Tests of resolution in periscope photography 
were made at New London on the USS Pilotfish, 
and also through a periscope mounted perma¬ 
nently in the optical shop at New London. A 
distant resolution chart, similar to those used 
for aerial photography (see Chapter 2), was 
mounted at a distance of more than half a mile. 
Through the 1.4-in. periscope the 50-mm cam¬ 
era resolved only about 16 lines per mm with¬ 
out filter. Chromatic aberration and curvature 
of field were shown to be the principal aberra¬ 
tions that limited resolution. A yellow filter 
helped considerably. A camera lens to correct 
the curvature of field is described in Chapter 10. 
A 72-in. focus glass-fluorite collimator with a 
point source at the focus was designed and con¬ 
structed at Harvard for testing periscope per¬ 
formance on a quantitative basis over the whole 
field, and at a wide variety of focal settings. 
Resolution targets could be introduced for test¬ 
ing photographic performance. This collimator 




22 


SUMMARY 


has been transferred to the National Bureau 
of Standards. It seems clear that marked im¬ 
provement in periscope photography can be 
realized if the optical systems are improved. 
Only a first step was taken by the Yerkes Ob¬ 
servatory in designing glass-fluorite elements 
to replace one or both of the present erector 
doublets (see Chapter 10). 

A two-star navigating device has been devel¬ 
oped to provide the observer with a direct dis¬ 
play of his position at any moment. The instru¬ 
ment is set in advance so that, if it were located 
with its base level at any selected target, the 
intersection of two astigmatized star images 
would be at the center of a reticle. If the inter¬ 
section of the star-image lines is held at the 
center of the reticle when the instrument is at 


any location other than at the target, the dis¬ 
tance and direction of a spherical bubble from 
the center of the reticle indicate directly the 
distance and direction to the target. The reticle 
might even show a faintly illuminated map. If 
the instrument were stabilized, so that the base 
were maintained always level, then the inter¬ 
section of the two star-image lines would indi¬ 
cate directly the location of the instrument on 
the surface of the earth, without the need for 
guiding, except approximately in azimuth. In 
either case, some averaging would be required 
to eliminate the effects of linear accelerations 
due to the aircraft. A preliminary model shows 
that the general design is practical, but that 
considerable further development will be re¬ 
quired. 



Chapter 1 

EQUIPMENT FOR AERIAL PHOTOGRAPHY 

By James G. Baker and J. S. Chandler a 


11 INTRODUCTION 

T he need for resolving the finest possible 
detail on aerial photographs taken for re¬ 
connaissance purposes over enemy territory 
was recognized early in the war. In October 
1941, the Army Air Forces requested the Na¬ 
tional Defense Research Committee [NDRC] 
to undertake, as part of Project AC-29, the 
development of high-resolution lenses. This in¬ 
cluded not only a request for the development 
of lenses of long focal length but also lenses of 
relatively short focal length with wide angular 
coverage. The greater part of these develop¬ 
ments was carried out at Harvard University 
under Contract OEMsr-474, but two Schmidt 
cameras were developed at the Mount Wilson 
Observatory under Contract OEMsr-101, and 
one lens for night photography was developed 
under Contract OEMsr-160 at the University 
of Rochester. The laboratory testing of aerial 
lenses, both standard types and NDRC proto¬ 
types, was conducted at Mount Wilson under 
Contract OEMsr-101 and at the Eastman Kodak 
Company under Contract OEMsr-392. Early in 
1945, the facilities at Harvard were greatly 
expanded to permit carrying out the extensive 
flight testing program described in Chapter 2, 
as well as to make it possible to complete 
prototype models of several important lenses 
in the shortest possible time. 

It is generally agreed that for purposes of 
aerial photography a lens system should be 
designed at the optimum aperture to produce 
images which are unvignetted, within the Ray¬ 
leigh limit, and located on an undistorted focal 
plane flat to the corners of the adopted film 


a The material for this chapter has been compiled 
primarily by Dr. Baker of the Harvard College Observ¬ 
atory. Dr. Chandler of the Eastman Kodak Company 
has compiled the section on Antivibration Mounts for 
Aerial Cameras (see Section 1.6) with the exception of 
the short articles on the “Center of Gravity Mount” (see 
Section 1.6.6) and “Rotating Prism Unit” (see Section 
1.6.8). 


size. However, practical considerations, as well 
as limitations imposed by nature and by pres¬ 
ent technology on image quality at a given lens 
speed and coverage, cause such drastic depar¬ 
tures from the ideal that many differences of 
opinion have arisen as to the characteristics of 
the best compromise. 

A perfect image within the meaning of dif¬ 
fraction optics is rather easily understood, in 
so far as applications to aerial photography are 
concerned. It is certain that, with due allow¬ 
ance for statistical variations, a photographic 
emulsion will react in a predictable fashion to 
a perfect optical image falling on its surface. 
Resolution patterns of any adopted kind at any 
given contrast can then be used to arrive at a 
quantitative number representing the resolving 
power of the emulsion under such conditions. 

By restricting aperture, focal length, angular 
coverage, and range of color correction, the 
optical designer could for a given type of lens 
system produce images unvignetted over the 
field and within the Rayleigh limit. Various lens 
systems would be judged for quality thereafter 
on how far the several frontiers have been 
pushed for a given light intensity on the emul¬ 
sion, dependent on /-number and efficiency. 
Moreover, systems might differ widely in econ¬ 
omy, ease of manufacture, utility, and general 
bulk. 

Lens designers in general have been forced 
by competition and by practical considerations 
to work on more ambitious lens systems, with 
aberrations quite appreciably outside the Ray¬ 
leigh limit. Herein enters a host of considera¬ 
tions governing the character and magnitude 
of the inevitable residual errors. Practicality 
relative to mass production of aerial lenses 
again demands that the simplest, most easily 
made lens system be adopted, within the ap¬ 
propriation and facilities that can be allotted 
to achieve a certain result. 

Military needs are so inherently wasteful 
that for specialized lenses the designer is given 


23 



24 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


a degree of freedom quite far beyond ordinary 
commercial practice. Economy, in so far as 
applied to aerial cameras, means little if the 
cost of the reconnaissance aircraft is far in 
excess of the cost of the camera it carries. Ends 
can be achieved by making up complex systems 
out of keeping with compromises based on 
salability to the public. However, unnecessarily 
complicated systems are undesirable since their 
mass production would prove an unwelcome 
drain on the usually inadequate optical facili¬ 
ties of a country. 

On the other hand, military needs are also 
exacting. The designer soon finds himself in in¬ 
creasing throes of image aberrations with opti¬ 
cal systems of increasing complexity, each more 
difficult and tedious to construct than its prede¬ 
cessor. 

Standard lens systems are found to yield 
images of surprising size far off axis. The 
image of a point source in the focal plane is 
often as large as 0.4 mm, taking account of all 
light. The same lens, tested on the bench at 
high contrast, will often yield resolution of the 
order of 50 lines per mm on fine grain plates 
and visually might yield 150 lines per mm. 
What is to guide the designer? 

Various persons have been heard to say that 
almost any reasonably good lens can take high- 
quality aerial pictures. Others have said that in 
practice it is not the lens that needs improve¬ 
ment but the conditions under which the lens 
is used. The latter problem is discussed else¬ 
where in this volume, but for present purposes 
has no material bearing on anything but the 
wisest limit of quality of the lens consistent 
with practicability. Because of such individual 
statements and beliefs, a great number of lab¬ 
oratory and aerial tests have been carried 
out in many laboratories and several countries. 

The Americans have adopted almost without 
exception the testing of lenses at high contrast 
with a 3-line test pattern. The English and 
Canadians 1 have found that for purposes of 
aerial photography, low-contrast 2-line pat¬ 
terns and annuli are adequate and more mean¬ 
ingful than the high-contrast patterns. Vari¬ 
ous observers have found that the vast major¬ 
ity of ground objects lie in a brightness range 
of about 1/0.10 on a first-rate photographic day. 


In the presence of enhanced atmospheric haze, 
or in shadows of clouds or buildings, this con¬ 
trast ratio drops very materially. The English 
and Canadians have concluded that the inher¬ 
ent contrast of aerial photography is low, and 
therefore that to test the effective performance 
of lens-film combinations for aerial photog¬ 
raphy realistically, a contrast of about 1:0.5 is 
desirable. The Canadians have also concluded 
that for aerial photography low-contrast test 
targets are more meaningful, both from the air 
and in the laboratory. 

Most probably, the complete story is multi¬ 
dimensional and cannot be determined by any 
single choice of high or low contrast. The Brit¬ 
ish choice more nearly fits the average facts, 
but also tends to conceal important improve¬ 
ments in peak performance of lenses and 
camera mounts. The American choice results in 
improvements that may be too expensive and 
elaborate for the average benefits gained. More¬ 
over, the Americans run the risk of concluding 
that a lens system is better than it is, because 
of high-contrast work. The British and Canadi¬ 
ans run the risk of falling behind American re¬ 
sults on first-class photographic days. 

Because of the completely independent Ger¬ 
man view presented, it seems of interest to re¬ 
produce at this point abstracts in translation of 
Development Problems in Aerial Cameras by 
H. C. Wohlrab. 2 

There is reason to believe that the failure of enemy 
[i.e., non-German] photographic apparatus to exceed 
picture sizes of 13x13 cm. or 18x24 cm. respectively, is 
due not least to the lack of suitable high-power lenses. 
This appears to be confirmed by the circumstances that 
British cameras have frequently been captured equipped 
with German lenses dating back to the last war. 

The demand for suitable long-focus lenses of course 
increases with still greater flying height, but develop¬ 
ment in this direction appears to be limited by the 
attainable precision of manufacture, since the lens is 
required to maintain the resolving power of a shorter 
focus lens, only the flying height and not the image 
scale being different. The difficulty becomes apparent 
already in the aerial production of 75 cm. lenses and 
increases incommensurably with further increase of 
the focal length. 

The principal difficulty lies in the manufacture of 
the requisite optical glass. The production of glass for 
optical lenses is not a mechanized quantitatively con¬ 
trollable process. While out of a pot-full of optical 
glass many small pieces suitable for the manufacture 



LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


25 


of lenses can be produced, pieces of large size with the 
required absolutely uniform refraction and dispersion 
over the whole mass are few and far between. Apart 
from the fact that such large lenses are disproportion¬ 
ately costly, they are also very wasteful of materials 
and labor, since production of any considerable number 
of lenses of the required uniform refractive power 
from a single charge of glass is excessively difficult, 
while the slightest deviation from the prescribed stand¬ 
ard involves complete recalculation of the correspond¬ 
ing lens. 

The aperture of the lens is limited by the same con¬ 
siderations of manufacturing technique as the focal 
length. Although on the one hand the largest possible 
aperture is desirable, to secure short exposure times 
even in bad weather, this can lead to lens dimensions 
difficult to handle in large scale production, as already 
explained in the discussion of focal length. In addition, 
it is even theoretically difficult to obtain adequate 
resolution of detail with a picture size of 30x30 cm., 
long focus and simultaneously large aperture. Lenses 
fulfilling such simultaneous requirements would have 
to be of a focal length between 50 and 75 cm., with an 
aperture of 1:5 to 1:6, and their production would 
therefore represent a technical feat of considerable 
magnitude. 

In regard to enemy [i.e., non-German] aerial photog¬ 
raphy equipment, it must be noted that for many years 
now, almost no further development has taken place. 
There is thus an appreciable divergence between the 
efficiencies of the apparatus used by either side in the 
war. 

Comparison of the efficiencies of aerial cameras pro¬ 
ceeds from the picture size, picture angle, and quality 
of the lens. Reverting to the definitions stated at the 
beginning of the present paper, the picture size in 
relation to the picture angle determines the scale, while 
the picture angle at fixed focal length is a measure of 
the ground area covered by the picture. In either case, 
the quality of the lens determines the amount of detail 
attainable in the picture. The quality of the lens in this 
context is represented by its resolving power and con¬ 
trast level. For obvious reasons, such data cannot be 
given for the German lenses, but it may be stated 
generally that the lenses used in German aerial cameras 
are superior to those in enemy [i.e., non-German] 
apparatus, both in resolving power and in contrast 
level, and their performance is also better in respect 
to picture size and picture angle. 

It is of interest that this report points out 
not only requirements on resolving power but 
also on contrast level. The importance of the 
effect of the lens itself on contrast of fine detail, 
quite apart from either target or landscape 
contrast of gross detail, seems to have been as¬ 
signed a secondary role in American reports 
and conferences on aerial photography. It may 


now be stated, in agreement with the appar¬ 
ently established German view, that resolution 
and small detail contrast give a dual complexion 
to the problem of aerial photography. The text 
below will attempt to discuss these subjects in 
the light of the still very inadequate available 
data. It is hoped that before long the uncer¬ 
tainties and numerical estimates will be re¬ 
placed by more logically determined and con¬ 
trolled test data. 

12 FACTORS GOVERNING LENS DESIGNS 
FOR AERIAL PHOTOGRAPHY 

Emulsion Considerations. A brief discussion 
of the properties of the photographic emulsion 
is presented here because of the light it throws 
on the overall problem. Let us begin by out¬ 
lining a desirable experimental procedure of 
some length based on quantitative and qualita¬ 
tive experiences. Let us suppose that we have 
an apochromatic microscope objective of ex¬ 
treme quality that forms a 20:1 reduction at a 
convergence angle in image space equivalent to 
the convergence of the average aerial camera, 
approximately //6. Let us prepare a chart of 
either 3- or 2-line patterns, as the observer 
may prefer, of sizes descending, by, at most, 
the sixth root of two. Let the illumination be of 
sunlight quality, filtered by minus-blue filter 
as in the average aerial camera, and let us sup¬ 
pose that the illumination produces a surface 
brightness uniform over the surface of the 
chart, except for the reflectivity factors of the 
lines. Let us also incorporate into the chart at 
least two large photometric areas, one of which 
agrees exactly with the bright lines, and an¬ 
other with the background. Let us assign an 
absolute surface brightness, and color tempera¬ 
ture or its equivalent, to the large brighter 
photometric area, in order that we may analyze 
the speed of a lens system in absolute terms. 
Let us prepare charts covering a range of con¬ 
trasts, but with only the background varied, 
and the bright lines held constant. 

The image thrown by the designated perfect 
microscope objective onto the emulsion will 
therefore be a perfect chart of small dimensions 
and of dependable, or at least defined, absolute 



26 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


surface brightness. The microscope is to focus 
critically on the surface of the emulsion, the 
criterion being maximum resolution. 

An effect that has appeared more than once 
should be pointed out here for further study. 
The density or effective speed of the emulsion 
for the finest resolution patterns appears lower 
than would be expected on the basis of uniform 
surface illumination. It is possible that the 
effect arises from improper laboratory setup, 
but several high-resolution emulsions have 
shown a complete disappearance of the bright 
line pattern before the resolution limit is 
reached, i.e., a longer exposure would have 
shown the finer patterns, clearly resolved. 

A series of exposures in uniform logarithmic 
steps are now to be made, with the various 
contrast targets. A uniform development pro¬ 
cedure can be adopted for coarse- and fine-grain 
development, respectively, in a dual investiga¬ 
tion. Development can be as recommended in 
practice for the several films used, in accord¬ 
ance with any improved procedures. 

The end result will be a chart for each type 
of emulsion in a two-dimensional manifold con¬ 
taining the characteristic curves as derived by 
microphotometry as a function of resolution, 
for various contrasts and absolute exposure 
times. This chart can then be considered a com¬ 
plete description of the emulsion as used in 
aerial photography in heated and dehumidified 
cameras. It can be supposed with great cer¬ 
tainty that a perfect aerial lens at //6 would 
give identical results. Because of statistical 
variations in the character of the emulsion, 
even under well-controlled development, it 
would be necessary to repeat the experiment 
enough times to show with certainty that the 
emulsion has been fairly represented on an ab¬ 
solute basis. 

It is, perhaps, unnecessary in this present 
outline to add experiments on the effect of haze 
on the emulsion. In so far as the emulsion is 
concerned, the controlled variation of log ex¬ 
posure and contrast together cover all possible 
behaviors. In turn, the chart that defines the 
emulsion becomes an excellent medium for 
studying the effects of haze in the air. For 
standard development at constant optimum log 
absolute exposure of the brighter photometric 


area (or the density equivalent), one can plot 
resolution versus contrast as a single curve of 
fundamental importance for our following con¬ 
siderations. We shall think of such a curve 
as defining the absolute properties of a “per¬ 
fect” //6 lens-plus-film combination. See Fig¬ 
ure 1. 

In a more detailed analysis, the higher color 
temperature of atmospheric haze would bear 
close scrutiny. It is evident that, to a close first 
approximation, haze reduction of contrast can 
be discussed relative to macroscopic and micro¬ 
scopic detail alike, in terms of the extra density 
of the darker photometric area, where the total 
exposure of the brighter photometric area leads 
to a chosen fixed density on an absolute ex¬ 
posure basis. 

Lens-Film Considerations. A lens test accord¬ 
ing to our outline will now consist of testing 
aerial lenses in parallel light with a mirror-type 
collimator with a focal length at least twenty 
times that of the aerial lens being tested. A 
complete repetition is made of the photographic 
test, analogous in every respect to the original 
emulsion test. Log absolute exposures are again 
varied in the same steps, with the same targets 
as used in the first experiment, preferably un¬ 
touched. A new variable is now added in the 
form of focal setting for the aerial camera. 
Because of possible inherent differences in ab¬ 
solute brightness between the original micro¬ 
scope setup and the mirror test, the log expo¬ 
sures of the two tests will be equalized by de¬ 
terminations of the density of the macroscopic 
bright area on the test charts. Thus, again we 
have at each part of the field and at each focal 
setting a chart describing the lens-film behav¬ 
ior. By tying the before and after charts to¬ 
gether at equivalent densities of the brighter 
and large photometric areas, the same coordi¬ 
nate framework can fit both cases and serve 
as a well-rounded lens test. For simplicity, 
again the observed resolution can be plotted 
against log absolute exposure to show film-plus- 
perfect-lens, and film-plus-imperfect-lens. Note 
the fundamental character of equivalent sur¬ 
face brightness of macroscopic areas as a 
means for measuring loss of light from bright 
lines, caused by poor lens correction. It is our 
thesis here that the exposure should not favor 



RESOLVING POWER IN LINES PER MM RESOLVING POWER IN LINES PER MM 


LENS DESIGNS FOR AERIAL PHOTOGRAPHY 



Figure 1 . Photographic properties of Super-XX and Panatomic-X Aerial film. (Courtesy of Eastman 
Kodak Company.) 


DENSITY DENSITY 












28 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the barely resolved patterns, but should be 
based only on the photometric areas. 

It will be evident that any light lying outside 
the dimensions of a given resolution line of a 
given pattern can be regarded as wasted light. 
The light around and between the lines can be 
regarded as not only wasteful but as an inter¬ 
ference with the results. In so far as the emul¬ 
sion is concerned, aberration light results in a 
loss of contrast as a function of resolution in 
lines per millimeter (what we shall term micro¬ 
scopic contrast). Thus, a comparison between 
the before and after charts of an imperfect lens 
would show that the loss of microscopic con¬ 
trast has ended up with a loss of resolution, as 
defined entirely by the emulsion property at the 
various altered contrasts. In a target already 
of low contrast, the lens aberration lowers the 
microscopic contrast more and more as the 
limit of resolution is approached, resulting 
therefore in a still lower resolution reading, 
but at a progressively slower rate where poorer 
lens-film performance is shown. 

Let us suppose that we have a resolution- 
contrast curve at optimum absolute exposure 
for the film-plus-perfect-lens combination. 
Then, for a film-plus-imperfect-lens test, where 
the optimum absolute exposure for the brighter 
photometric area as measured by macroscopic 
emulsion density has been realized, we will 
make a reading of resolution obtained. Using 
this reading, we can obtain a target contrast 
from the film-plus-perfect-lens curve of the 
original experiment. Hereinafter, we shall call 
this final contrast the microscopic equivalent 
target contrast. In a film-plus-imperfect-lens 
test we can then compare the known target con¬ 
trast with the observed microscopic equivalent 
target contrast. Thus, the single reading of 
resolution by parametric methods will lead to 
the dual evaluation of resolution and loss of 
contrast of microscopic detail. Finally, in haze 
studies, we can speak of macroscopic equivalent 
target contrast. A comparison of macroscopic 
and microscopic equivalent target contrasts 
with the actual target ground contrast will 
present the entire story. 

It is believed that lens tests for performance 
at all field angles should be based on the one 
exposure level that brings the density of the 


brighter photometric area at a mean field angle 
to the value found optimum for maximum 
resolution in the perfect lens-film tests. The 
density of the same photometric area will then 
be larger or smaller, in general, at other field 
angles, leading to a loss of resolution, even in 
the perfect lens-film combination. The images 
of the imperfect lens-film combination will cause 
a further decrease in resolution and microscopic 
contrast. Vignetting and the cosine fourth law 
will not only show up readily, but their com¬ 
bined effects on resolution plus lens aberration 
will then give a truer picture of what the lens 
will do in the air. 

The subject of determining an optimum over¬ 
all exposure to favor the performance of an 
existing lens is, however, a perfectly practical 
problem and is a phase of the subject that can 
best be described later under the actual aerial 
tests conducted. (See Chapter 2.) 

Examination of the curves in Figure 1 will 
bear out the above considerations. On Super- 
XX aerial emulsion, for optimum exposure 
(emulsion property) with a perfect lens, a 
1/0.001 target contrast yields 50 lines per mm. 
A 1/0.5 target contrast yields 26 lines per mm. 
It will be understood that numbers are used 
here to put the discussion on a specific basis, 
but that the actual absolute determinations are 
as yet unavailable. For Panatomic-X aero film, 
the corresponding figures are 70 and 32 lines 
per mm. It is evident that Pan-X suffers more 
from loss of target contrast than Super-XX, 
although it remains systematically the better 
film. In turn, if a lens can resolve only 26 lines 
per mm on Super-XX because of its own con¬ 
tribution to the loss of microscopic contrast due 
to aberration, the same lens will resolve only 32 
lines at most on Pan-X, and probably slightly 
less, because the effect of aberration on micro¬ 
scopic contrast with decreasing line-pattern 
image size worsens correspondingly. 

Note that macroscopic contrast of lens-film 
must be held constant in these considerations 
and equivalent to the film tests that precede, 
and that we are trying to see the final image as 
the emulsion sees it. Arbitrary adjustment of 
laboratory exposure time for optimum resolu¬ 
tion at each field angle, particularly at high 
contrast, will lead to test results at variance 



LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


29 


with conditions in the air, and is not a measure 
of performance in a single photograph. 

Improvement of the lens design or figuring 
the aberring system above would show a more 
marked improvement on Pan-X than on Super- 
XX at high contrast. Such is the result that 
might be found in American laboratories with 
their high-contrast targets. But the aberring 
lens has reduced the contrast in the imaged 
target to about 1:0.5 in so far as the emulsion 
is concerned, that is, the equivalent target con¬ 
trast is 1:0.5, although the known target con¬ 
trast is 1:0.001. 

Let us suppose now that we are repeating the 
results at low-target contrast, 1:0.5. The per¬ 
fect lens will yield 26 lines per mm on Super- 
XX and 32 lines per mm on Pan-X. Now if we 
expose the imperfect lens so that the macro¬ 
scopic photometric area corresponding to the 
brighter line areas reaches the same density 
(for on-axis image) as obtained with the per¬ 
fect lens, we find that perhaps one-third of the 
light has left the effective bright lines and en¬ 
tered or filled up the dark lines between. If we 
expose to the target of contrast 1/0.5, we find 
that light leaving the bright lines fills up the 
dark lines and that a corresponding portion of 
the dark line illumination enters the bright 
lines. When this is worked out, we find that the 
contrast is not far from 1:1. Because the length 
of the resolution lines is restricted in propor¬ 
tion to the line width, the residual slight differ¬ 
ence of contrast is lost in the grain, and the 
image appears unresolved. 

We must remember, however, that this con¬ 
clusion is based on phenomena in the neighbor¬ 
hood of a resolution of 26 lines per mm on the 
high-contrast target. The distribution of light 
in the image becomes less important for grosser 
patterns at a rather rapid rate. Thus, the test 
of the imperfect lens on the 1:0.5 target would 
actually show resolution of the order of 16 
lines per mm, which from the emulsion point of 
view would correspond to an equivalent target 
contrast ratio of 1:0.74. It is evident that if 
lens-test curves of the kind described were pre¬ 
pared, one would have the entire story at his 
finger tips and would be able to discuss what 
kind and size of aberration to permit in the 
aberring images (compare Section 1.4.5). 


The deterioration of quality in the aerial 
photograph caused by the imperfect lens is 
therefore of dual character. There is first a loss 
of resolution which is more spectacular at high 
levels of contrast on Pan-X film than at low. 
Second, there is a loss of contrast in the micro¬ 
scopic image which may not be apparent at all 
in the macroscopic picture. A microphotometer 
tracing of the successively smaller patterns will 
show a steeper rate of decline for the film-plus- 
imperfect-lens toward the ultimate reduced 
limit of resolution, even though at macroscopic 
levels microphotometer tracings of successively 
finer resolution patterns near the coarse end of 
the series for film-plus-perfect-lens and film- 
plus-imperfect-lens can be superimposed. In¬ 
deed, the stepping up of the photograph by con¬ 
trasty development and contrasty printing will 
serve to recover some of the lost contrast in 
the microscopic scale but will not recover the 
resolution. Moreover, tonal values will be de¬ 
stroyed in the process. Macroscopic areas will 
be too contrasty and unnatural, relative to the 
partially recovered microscopic tonal values. 
Contrast loss on macroscopic areas due to haze 
will be far less than accompanying micro¬ 
scopic losses. 

Effect of Haze . Discussion of the behavior of 
the aerial photograph is still not complete. The 
aerial haze, which on good days represents a 
determinable loss of contrast on both macro¬ 
scopic and microscopic areas, can be analyzed 
from the point of view of the emulsion. One 
needs the dimensions of the emulsion chart, log 
exposure, and resolution versus contrast. For 
purposes of discussion we may consider that 
aerial haze plus ground high light (analogous 
to our previously described brighter photo¬ 
metric area) will lead to a fixed density of the 
photographed brighter photometric area. 

Addition of haze to an aerial photograph 
represents also a strong variation of contrast 
with color. Thus, green and red ground targets 
of identical ground contrast will photograph 
from the air as a lower green contrast than red. 
Whether haze, as an integrated function with 
the net exposure depressed, can be added as a 
mean value to contrast reduction irrespective 
of color is debatable. Most probably, the center 
of gravity of microscopic contrast lies toward 




30 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the red resolution versus contrast curves and 
reacts only sluggishly to green haze losses. 

Under these conditions, a perfect lens ex¬ 
posing on ground targets of contrast 1/0.001 in 



Figure 2. The high-contrast Cobb chart and the 
Canadian annuli. 


the American fashion would in the absence of 
haze attain a resolution on Super-XX of 50 
lines per mm and on Pan-X 70 lines per mm. 
If we consider that haze represents an addition 
of 50 per cent (common at 30,000 ft) of the 
high light area to both high and low areas, with 
haze plus high light constant (and to both 
macroscopic and microscopic ground detail), 
we find a loss of contrast to 1/0.33, correspond¬ 


ing to a maximum possible emulsion resolution 
of 34 lines per mm on Super-XX and 43 lines 
per mm on Pan-X. An imperfect lens under the 
same condition as described in the above para¬ 
graphs would in the absence of haze resolve 26 
lines per mm on Super-XX and 32 lines per mm 
on Pan-X. With 50 per cent haze added, we find 
a resultant microscopic or equivalent target 
contrast of 1/0.60, corresponding to an emul¬ 
sion resolution of 23 lines per mm on Super-XX 
and 27 lines per mm on Pan-X. 

Carrying through the same thought with re¬ 
spect to the low-contrast targets of the English 
and Canadians (see Figure 2), we find that the 
perfect lens, which on the 1/0.5 target in the 
absence of haze would have resolved 26 lines 
per mm on Super-XX and 32 lines per mm on 
Pan-X, now has its equivalent target contrast 
reduced to 1/0.67, with 20 lines per mm on 
Super-XX and 23 lines per mm on Pan-X. The 
imperfect lens, which has in the absence of 
haze a possible resolution of say 16 lines per 
mm on Super-XX and 17 lines per mm on 
Pan-X, now has its equivalent target contrast 
reduced to 1/0.8, and has a corresponding reso¬ 
lution on Super-XX of 14 lines per mm and on 
Pan-X of about 14 lines per mm also. 

It is evident that in the presence of haze the 
perfect lens at all contrasts suffers more in pro¬ 
portion than the imperfect lens, but that it re¬ 
mains systematically superior in both resolu¬ 
tion and contrast on a microscopic scale. Again, 
development can recover some of the loss of 
contrast at a cost of distortion of the tonal 
values of macroscopic areas. In the presence of 
vibration, the resultant pictures therefore may 
not be unsimilar in resolution between the good 
and not-so-good lens, but the general tone of 
the perfect lens picture will be better at all 
levels of brightness than that of the pictures 
given by the imperfect lens. In other words, 
even if vibration is found to limit resolution to 
a fixed value, it would still be worth while to 
improve the lens design for truer contrast of 
microscopic detail. 

The addition of aerial haze to an already im¬ 
perfect image therefore produces a much 
smaller relative change in contrast and resolu¬ 
tion than that suffered by the perfect lens. The 
various phenomena are analogous to the plan- 





LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


31 


ing-off effect of a rough board with many 
bumps of various magnitude down to a common 
denominator and general level of resolution. 

On the toe of the characteristic curve, the 
situation is much more involved. Resolution 
has fallen off to a low level because of the emul¬ 
sion itself. Contrast in the negative is inher¬ 
ently low. Aerial haze over macroscopic areas, 
as in cloud shadows, will tend to distort the 
tonal values of the picture even on the straight- 
line portion of the curve. The difference be¬ 
tween perfect and imperfect lenses will become 
even less noticeable under these conditions, 
simply because low levels of resolution are in¬ 
volved, and even the poor lens has images of 
considerably better quality than the emulsion. 

On a good photographic day with little haze 
beyond the inherent scattering of the atmos¬ 
phere, and particularly at low altitude, pictures 
made with the perfect lens will take a jump in 
quality far beyond that obtainable with the im¬ 
perfect lens. 

The situation can be summed up in the fol¬ 
lowing tentative table, where the tabulated 
resolutions must be taken as representative 
but not unalterable values. 


Table 1. Resolution and contrast effects of per¬ 
fect and imperfect lenses. 


Macroscopic 

ground 

target 

Super-XX 
lines /mm 

Pan-X 
lines /mm 

Ground 

target 

contrast 

Haze 

Equivalent target 
or microscopic 
image contrast 

High contrast 

Perfect lens 

50 

70 

1/0.001 

none 

1/0.001 


34 

43 

1/0.001 

50% 

1/0.33 

Imperfect lens 

26 

32 

1/0.001 

none 

1/0.50 


23 

27 

1/0.001 

50% 

1/0.60 

Low contrast 

Perfect lens 

26 

32 

1/0.50 

none 

1/0.50 


20 

23 

1/0.50 

50% 

1/0.67 

Imperfect lens 

16 

17 

1/0.50 

none 

1/0.74 


14 

14 

1/0.50 

50% 

1/0.80 


These considerations so far are based upon 
the single comparison of perfect to imperfect 
image quality. No lens is likely to be perfect 
over the entire field of view. Many lenses are of 
extremely high quality over some part of their 
field of view, but inevitable field curvatures, 


color aberrations, astigmatism, etc., tend to in¬ 
troduce errors. Most of these errors from the 
emulsion point of view can be considered a loss 
of microscopic contrast with correspondingly 
reduced resolution. 

One must remember that the effective loss of 
contrast, contrary to haze, is a decided function 
of resolving power itself. It is absolutely neces¬ 
sary to have exact laboratory data before 
proper discussion of the relationships can be 
carried through, but such complete data are 
still lacking. All the above discussions are evi¬ 
dently based on inadequate data. We have 
neglected log exposure entirely, and have con¬ 
fined ourselves to the best resolution at opti¬ 
mum exposure. In practice, too heavy an ex¬ 
posure at a given contrast will cause an addi¬ 
tional loss of resolution, as will too light an 
exposure. Moreover, the loss of light from the 
small bright lines of the imperfect image, if we 
are to maintain the point of view of the emul¬ 
sion, will mean that for consistent results, equiv¬ 
alent microscopic target contrast should take 
into account the loss of exposure. Thus, no 
single resolution versus contrast curve will 
suffice in a prolonged investigation. 

Departures from optimum resolution at any 
given contrast will in turn join the planing-off 
process, and tend to affect the good lens more 
than the bad. If analogy of all effects is made 
to a mountain, leveling of the peak will pro¬ 
ceed quickly at first but thereafter at a rapidly 
decreasing rate. 

Considerations Involved in Lens Designs for 
Aerial Photography. Considering lens-design, 
it would seem that for some time to come, while 
using present emulsions, perfect realization of 
the Rayleigh limit is not vital nor possibly even 
desirable. For example, if Super-XX under 
ideal conditions at 1/0.001 resolves only 50 
lines per mm, the single-line width is 0.010 mm. 
Allowing for some spread in the emulsion, one 
would suppose that if all the light lay within a 
circle of 0.006 mm, and had a central peak, the 
maximum resolution of the emulsion would be 
essentially realized. At //6, this circle corre¬ 
sponds to about the diameter of the first dark 
ring of the diffraction pattern, which in turn 
has a central peak of illumination lying within 
less than 0.003 mm half-light. Thus, the exact 







32 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Rayleigh limit would attain no more than 50 
lines per mm on Super-XX at f/6, and would 
have less depth of focus at this peak level of 
resolution than would a slightly imperfect lens 
with a 0.006-mm circle of confusion of half- 
light. In addition, a contrast ratio of 1/0.1 pro¬ 
duces an emulsion loss of resolution to only 46 
lines per mm for Super-XX and 58 lines per mm 
for Pan-X at //6, which in turn permits a 
greater departure from the Rayleigh limit with¬ 
out marked loss of either resolution or contrast, 
if properly done, and which as a minor correc¬ 
tion permits even a slight enlargement of the 
0.003-mm core to perhaps 0.004 mm (if the dif¬ 
fraction image with its rings can for turbid 
emulsion purposes be replaced by a smoothed 
profile). These figures can all be established on 
an experimental basis by use of varying 
amounts of spherical aberration and departure 
from the Rayleigh limit. It would seem, how¬ 
ever, that the emulsion’s initially slow loss of 
resolution with contrast should be used to en¬ 
large tolerances and depth of focus, since no 
evident loss of quality can be noted in the 
photograph. 

In broad outline it would seem clear that it 
is desirable to draw rather heavily on the Ray¬ 
leigh limit, with perhaps only 50 per cent of the 
light left inside the first dark ring. Such slight 
imperfection will result in the desired in¬ 
creased depth of focus without loss of contrast 
or resolution beyond strictly tolerable amounts 
for highest quality results, and is certainly de¬ 
sirable, if such correction leads to more uni¬ 
form results over the field of view. 

The goal, then, for the optimum aerial lens 
design, should be to get all images over the field 
to present the same slight aberring appearance, 
preferably of residual symmetry such as zonal 
spherical aberration, overlooking aberration 
residuals varying at high powers of the aper¬ 
ture, after proper balancing has been accom¬ 
plished to bring as much peaked light into the 
0.004-mm smoothed circle as necessary on a 
uniform basis. 

If we permit aberrations and therefore loss 
of contrast beyond the tolerances discussed 
above, it would seem desirable and important 
that the image retain at all times a central 
peak with as much light as possible within 


0.004 mm and the rest spreading outwards at as 
high a rate of spread (low-surface brightness) 
as possible. If the aberration gets larger still, 
the 0.004 mm will increase in turn to keep pace 
with the known loss of resolving power of the 
emulsion at the lowered contrast. This art of 
balancing should not be overlooked. A factor of 
2 in contrast, or 20 per cent in resolution might 
be the reward for a simpler, well designed in¬ 
strument of economical manufacture. 

If one adjusts an optical instrument visually, 
by trial and error, it is evident that he should 
not strive for maximum visual resolution and 
contrast, which would imply that the peak reso¬ 
lution lies within the Rayleigh limit at a re¬ 
duced effective aperture, but instead that more 
zones be brought into the image up to the point 
where the maximum concentration of light is 
obtained within the permitted smoothed 0.004- 
mm circle. Since the effective circle at f/11 is 
about 0.007 mm for complete satisfaction of the 
Rayleigh limit, another way of looking at the 
optimum correction for Super-XX is to get an 
effective f/11 entirely within the Rayleigh 
limit, and for the rest of the light up to the 
aperture used, say //6, to fill up the 0.007-mm 
circle and outer rings by departure from the 
Rayleigh limit, and thereby cause some loss of 
contrast within that circle. 

As the aperture increases from f/11 to f/6, 
the original f/11 cross section of the image be¬ 
comes narrowed down into a much smaller cen¬ 
tral disk at f/6 but with a bright first-order 
ring containing much of the light. Thus, the 
smoothed profile of the image still approxi¬ 
mates the f/11 distribution with more total 
light, but no greater percentage of light in the 
now more numerous outer rings. The turbidity 
of the emulsion will soon dissipate the diffrac¬ 
tion character of the image into the smoothed 
profile described above. The total circle of con¬ 
fusion containing all light might be as large as 
0.4 mm. The law of decrease of intensity with 
distance from the core is much more important 
than the diameter of the circle of confusion. 

The poorer image is aided still more by the 
fact that the faintly outlying halo lies mostly on 
the toe of the characteristic curve and there¬ 
fore can be of higher intensity for a mean in¬ 
crease in density for subjects of average bright- 



LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


33 


ness. Overexposure with such a lens will find the 
flare moving up onto the straight-line portion 
of the characteristic curve and filling up the 
resolution lines at a rapid rate. An overexposed 
picture with an inferior lens therefore seems 
muddy or washed out (low contrast at high 
density). Underexposure with a poor lens will 
first of all result in lowered emulsion resolu¬ 
tion, but might result in an overall improve¬ 
ment caused by the wings of the aberring 
image lying entirely on the toe of the charac¬ 
teristic curve. For an inferior lens, there will 
be, accordingly, an optimum exposure for maxi¬ 
mum resolution that differs from the perfect 
lens-emulsion results. In an aerial picture with 
such a lens, high lights will therefore appear 
muddy and low lights underexposed and unre¬ 
solved. However, a lens designed in accordance 
with the considerations above will at all levels of 
contrast and exposure produce a more truly 
resolved overall picture of relatively undistorted 
tone values. 

The Situation in Practice. These arguments 
emphasize once more the difference between the 
British and the American points of view on 
contrast. The British designer would conclude 
that he could permit more aberration to the 
core of the image because the average low-con¬ 
trast ground images would result in inherently 
average low emulsion resolution. This, in turn, 
would permit a growth in the 0.004-mm toler¬ 
ance to even 0.016 mm without great effect on 
the net resolution at low contrast 1/0.5. On the 
other hand, a really good photographic day and 
good subject contrast would catch such a lens 
short and result in no great improvement in the 
pictures. 

The American lens designer would adhere to 
a more rigorous tolerance at the cost of more 
elaborate and expensive lens designs and pro¬ 
duction difficulties, without producing any ma¬ 
terial improvement on average days or targets, 
but giving full return on good targets and good 
photographic days, and at low altitudes. Again 
it should be emphasized that maximum care 
must be given to ideal balance of aberrations. 

Demands for wider angular coverage and 
faster lens speeds will result in a planing down 
of future American results toward the more 
sober level adopted by the British. It is signifi¬ 


cant that the Germans preferred larger angu¬ 
lar coverage to better image quality. 

The commercial lenses used in World War II 
by both British and American reconnaissance 
squadrons are of comparable quality but fall 
well below the standards discussed here as 
basis for further work. Prototype lenses in both 
countries by a number of companies demon¬ 
strate clear improvement in the direction rec¬ 
ommended here. With several exceptions, such 
improved lenses did not reach production in 
time to be of real value to military photog¬ 
raphy. 

If compromises are made over the field of 
view, they should be on the basis of area and 
equalization of departure from the Rayleigh 
limit. The aberring light will, in general, be¬ 
cause of the nature of the assignment, severely 
crowd the 0.004-mm tolerance on the central 
core of the image, and in many parts of the field 
it will greatly exceed the tolerance, because of 
the nature of the variation of aberrations. 


121 Lens Aberrations and Their Effects 
on Resolution 

Table 2 presents a list of the important aber¬ 
rations of rotationally symmetrical lens sys¬ 
tems as employed in aerial photography. Opti¬ 
cal problems are almost always so specialized 
that no single discussion can present recom- 

Table 2. Aberrations in the order of importance 

for resolution and contrast in aerial photography. 

Type of error 

1. Unsymmetrical errors of the principal rays, in¬ 
dependent of aperture, in the general sense, come under 
the heading of lateral color. There are five chief errors 
of decreasing importance. 

a. Chromatic difference of magnification, primary 
spectrum. 

b. Chromatic difference of magnification, second¬ 
ary spectrum. 

c. Chromatic difference of distortion, primary 
spectrum. 

d. Chromatic shift of effective stop for corrected 
pencils. 

e. Chromatic difference of distortion, secondary 
spectrum. 

2. Symmetrical errors of focus, linear with aperture 
(apart from diffraction considerations). 

a. Error in mean monochromatic focal setting. 




34 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Table 2—( Continued ) 

b. Astigmatism of all orders (includes curvature 
of field). 

c. Primary longitudinal color. 

d. Secondary longitudinal color (secondary spec¬ 
trum) . 

e. Variation of astigmatism with color. 

3. Unsymmetrical coma-like errors, varying as the 
square of the aperture. 

a. Primary coma (departure from the sine 
theorem). 

b. Oblique coma. 

c. Variation of primary coma with color. 

4. Symmetrical errors of zones, varying as the cube 
of the aperture. 

a. Primary spherical aberration. 

b. Oblique spherical aberration. 

c. Chromatic difference of spherical aberration. 

5. Unsymmetrical coma-like errors, varying as the 
fourth power of the aperture. 

a. Secondary coma (higher order departure from 
the sine condition). 

6. Symmetrical errors of zones, varying as the fifth 
power of the aperture. 

a. Secondary (zonal) spherical aberration. 

7. Residual errors varying with sixth and higher 
orders of aperture. 

Vignetting. Of great importance for aerial photog¬ 
raphy. This should be considered an aberration and 
minimized, even at expense of central aperture, with 
compromise depending on the problem. Includes con¬ 
sideration of cosine fourth law. 

Silhouetting. Of importance because of diffraction effects 
and lowered efficiency, without compensation in depth of 
focus. Typical of louvre shutters and mirror systems. 
Distortion. Importance ranges from vital to no im¬ 
portance, depending on the problem. 

Double or Multiple Images. In some lens designs, there 
exist in off-axis star images separate nuclei of defini¬ 
tion, each individually representing a Rayleigh limit 
or peak of light intensity. Interferometer fringe tests 
would show two separated areas on the entrance pupil 
lying within the Rayleigh limit separately, and not 
necessarily in the same phase, or wave front. The most 
common case occurs with corrected pencils of skew 
character, arising from above and below the meridian 
plane. Out-of-focus photographs suffer particularly. 
Flare. Internal reflections that throw fogging light onto 
the emulsion. Worst case is when ghost image is nearly 
in focus with true image. Aggravated by many air-glass 
surfaces. Reduced by surface coatings. 

Scattered Light. Light scattered either directly or in¬ 
directly onto the emulsion from soiled surfaces or im¬ 
properly darkened lens edges, cells, or lens barrel. 
Matter of good design and care of instrument. 
Efficiency. Maximum transmission of useful light in 
proper filter range is desired. Total glass absorption 
should be considered at time of design. Greatly im¬ 
proved by surface coating that transmits light of 
proper color. 

Centering. Lens centering is a matter for careful work¬ 
manship. Aberration caused by poor centering may 
appear as coma-like flare in center of field, and astig¬ 
matism and field curvature in the outer part of the 
field, varying with orientation around the axis. 
Analogous to points 2 and 3. 


mendations that might cover all cases. How¬ 
ever, by having a thorough knowledge of the 
underlying arguments the reader can always 
determine for himself what to do for a given 
problem. 

The table lists symmetrical errors in the 
sense of image symmetry around the chief ray. 
In general, symmetrical errors are more desir¬ 
able if measurements are to be performed, and 
should be at least of the third order if present. 
Unsymmetrical errors should be of at least the 
fourth power in the aperture for good results. 
Departures from the Rayleigh limit as recom¬ 
mended previously will then be in the sense of 
introducing third-order aberrations to balance 
the fifth and higher orders as required by the 
above considerations. 

A typical instance is given by what the Brit¬ 
ish term the Ross spherical correction. This 
consists of overcorrecting the rim rays at //6 
(or other comparable maximum aperture) in 
order to improve the diffraction correction at 
f/S and particularly //11. This correction re¬ 
sults in “clean” images at //11, with the ordi¬ 
nary //11 depth of focus increased by the zonal 
aberration still within the Rayleigh limit at 
that aperture, in contrasty but not perfect 
images at //8, and in good resolution at re¬ 
duced contrast of the images at //6. At every 
aperture the new type of correction would be 
superior photographically in performance and 
focal tolerance over the ordinary rim-ray cor¬ 
rection. 

Unsymmetrical Errors of the Principal 
Rays, Independent of Aperture 

Lateral Color. The most important aberra¬ 
tion in an aerial camera of large coverage is 
lateral color. At every point in the field the 
color dispersion laterally around the chief ray 
should be minimized and kept well within the 
0.004-mm circle. No considerations of depth of 
focus enter, and hence the lateral color should 
be reduced to the barest minimum. The lateral 
spread of the principal rays with color is dele¬ 
terious because the emulsion is almost uni¬ 
formly illuminated within its own sensitivity 
curve in cross product with the filter and with 
the character of the illumination. The aberra¬ 
tion is a blurring and outright destruction of 




LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


35 


tangential lines, which we speak of as tangen¬ 
tial resolution. For best results this aberration, 
together with the monochromatic aberrations 
discussed below, demands that the core of the 
image as a whole be formed within a 0.004-mm 
circle even to the corner of the picture. There is 
no question of should he. This correction is a 
must. 

Since the position of the chief ray is defined 
not only by the magnification but also by the 
distortion of third and higher orders, the chief 
consideration at any field angle is the primary 
spectrum. It may be necessary to balance re¬ 
siduals in one part of the field of view (chro¬ 
matic difference of magnification) against the 
corner (chromatic difference of distortion) 
error to reduce the net result overall to a satis¬ 
factory minimum. Even though this balancing 
may result in a nearly complete elimination of 
primary lateral spectrum of the principal ray, 
there may still remain a secondary spectrum 
of the principal rays, increasing linearly with 
field angle (and residually, even as the cube), 
and therefore worst in the corners. This aber¬ 
ration is serious in many telephoto systems. If 
not eliminated, at least the minimum or maxi¬ 
mum distance of the Gaussian point plotted 
against wavelength should lie in the effective 
spectral center of the filter and film combina¬ 
tion. For minus-blue filter and Super-XX, this 
wavelength of best lateral color correction 
should lie in the neighborhood of sodium light. 
For color photography the error should be min¬ 
imized even more to allow for the greater spec¬ 
tral range, and the point of best correction 
should move into the green. Many lenses that 
give reduced tangential resolution are actually 
afflicted with lateral color instead of astigma¬ 
tism. Since lateral color is independent of stop 
opening, vignetting that permits retention of 
some image quality relative to tangential astig¬ 
matism is of no help. (See section on Vignet¬ 
ting , also in Section 1.2.1.) 

It should be noted that lateral color is de¬ 
fined always by the lateral displacement be¬ 
tween the best image-forming pencils in respec¬ 
tive colors. Thus, the image core may contain 
a most symmetrical principal ray that defines 
a stop position for each wavelength. Displace¬ 
ment of the stop with color should be taken into 


account at each field angle. The problem is 
simply one of comparing blue-image quality 
and position with red at each field angle. Lat¬ 
eral color computations should also be referred 
to the best mean focus over the field, where the 
film is to lie, rather than to heights in the vari¬ 
ous optical focal planes displaced longitudinally 
with color, and in selecting this mean focal 
plane one should consider zonal errors of aper¬ 
ture and field curvature. 

Symmetrical Errors of Focus 
Linear with Aperture 

The second most serious aberration from a 
resolution point of view for a highly corrected 
lens system, within the meaning of our earlier 
discussion, is departure from the best focal set¬ 
ting. Even though departures from the Ray¬ 
leigh limit are allowed in order to increase focal 
tolerance without markedly reducing resolution 
or contrast, in the air focal errors are often ten 
times or more the desired tolerance. The good 
lens will have a depth of focus photographically 
of 0.005 in. on either side of the mean. Focal 
errors of 0.050 in. have not been uncommon in 
the past under field conditions, and have aver¬ 
aged probably 0.020 in. With the introduction 
of thermostated cameras, enclosed in a vacuum 
or gas-filled, changes of focus can be reduced 
to 0.005 in. Hence, future improved lens designs 
might well consider the optimum state of cor¬ 
rection and rather restricted depth of focus. 

To have the film out of focus is quite deleteri¬ 
ous to the definition of near perfect lenses. The 
out-of-focus image under such circumstances is 
from the emulsion point of view a uniformly 
illuminated area, rather than a loss of contrast. 
Therefore, the emulsion can make no distinc¬ 
tion between areas of higher brightness and is 
obliged to photograph the blurs as such. Thus, 
a primary rule is that extremely well-corrected 
aerial lenses inherently have small depth of 
focus and must be in focus in order to give their 
best performance. Another way of looking at 
the problem is this: If a lens must be out of 
focus from uncertainties of one kind or an¬ 
other, it is better that the lens be not too highly 
corrected, if average results of high quality are 
to be obtained. Of course, the real answer is to 
focus cameras critically rather than to relax 




36 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the quality of correction any more than is de¬ 
sirable, and to overcome the causes of focal 
errors in other ways. 

The resolution patterns used at present often 
exhibit a larger depth of focus than would be 
expected geometrically and physically. Reso¬ 
lution versus focus measured with the origi¬ 
nally described “perfect” //6 microscope lens 
will show this greater-than-expected range. 
The reason is that the limiting resolution oc¬ 
curs where the dark space between bright lines 
closes up or, in other words, the bright lines 
are very much fatter than the dark ones. If 
plots are made, based on a constant minimum 
width for the dark line, nearly perfect agree¬ 
ment with observation can be obtained. The 
apparently large resolution depth of focus in 
practice is a result of the above effect, diffrac¬ 
tion depth of focus, and zonal aberrations. It 
should be pointed out that fine dark details, like 
wire shadows, are much more sensitive to focal 
changes than fine bright details, like railroad 
rails, in terms of visibility, if not of resolution. 

The aberration known as curvature of field 
is almost identical with departure from focus. 
This error can be minimized by lens design, 
although the very nature of lenses precludes 
flattening the field sufficiently at wide cover¬ 
ages. If the Services were to adopt a curved 
platen, the designer would seek to obtain his 
best resolution on a slightly curved surface 
rather than on a plane. Most generally, the 
focal surface curves forward toward the lens to 
a minimum focal setting at a mean zone of the 
field, and thereafter curves away from the lens 
system at an increasingly rapid rate. The Serv¬ 
ices might well adopt a curved platen, where 
the corner has the same focus as the center, and 
where intermediate zones are curved forward 
in a special way by perhaps 1 mm, provided the 
designers are instructed to fit this focal surface 
at full aperture as well as possible. Specialized 
lenses should be fitted still more accurately, 
preferably with their own magazines, but pos¬ 
sibly with aspheric field flatteners. 

The linear error known as astigmatism is 
more deleterious than curvature of field. A 
curved platen can be made to fit one or the 
other, or the mean, of the astigmatic focal sur¬ 
faces, but cannot fit both simultaneously. 


Therefore, it is up to the designer to minimize 
the astigmatism at every point of the field to 
meet as well as possible the requirement that 
the effective core of light fill a 0.004-mm 
smoothed circle. Preferably, the astigmatic sur¬ 
faces should coincide throughout the entire field 
of view in order to prevent spreading apart of 
the respective tolerances on depth of focus. 

Primary longitudinal color and chromatic 
difference of astigmatism are approximately 
identical aberrations and are additive at every 
part of the field. Pure primary longitudinal 
color on the axis still produces a peak to the 
intensity distribution across the image, partly 
because of diffraction, and partly because the 
film will be focused accurately for some one 
color. Colors near the focal setting will be small 
images; colors near the extremes of the spectral 
range are out of focus and present uniformly 
illuminated disks. The image integrated over 
the spectrum therefore presents a peaked dis¬ 
tribution as is desired for maximum retention 
of resolution and contrast. 

Secondary longitudinal color or secondary 
spectrum also presents a peaked resolution even 
more sharply defined than that of primary 
color. The rate of variation of focus with color 
is not linear but varies as a function of rates 
of change of dispersions. Therefore, over a 
wide range of spectrum the axial image, other¬ 
wise well corrected, will lie within the Ray¬ 
leigh limit. Out of focus colors will appear be¬ 
yond a certain color on either side of the chosen 
color of best correction. The image in any such 
color will be of uniform illumination in the 
absence of other aberrations and diffraction 
considerations. The integration over the spec¬ 
trum will produce a sharply peaked image and 
present some loss of contrast in the image. 

Because of the nature of emulsions, the loss 
of resolution and contrast caused by secondary 
spectrum is usually rather minor. Unfortu¬ 
nately the aberration cannot be minimized or 
eliminated with ordinary glass types of lens 
construction (unless otherwise poorly corrected 
systems like the Petzval lens are adopted). Im¬ 
provements can be achieved with fluorite, with 
certain other crystals, and with a few German 
glasses. Secondary spectrum is exaggerated in 
telephoto lenses. This increase, the longer 



LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


37 


wavelength range required for color photog¬ 
raphy, and the fact that the best correction of 
telephoto aerial cameras is usually in orange 
light, combine to make present telephotos un¬ 
suitable for color photography. 

At any given field angle, it is imperative that 
a lens of optimum performance have all errors 
which are linear in the aperture and also lateral 
color reduced to such an extent that the core 
of a star image lies within a 0.004-mm circle. 
Aberrations of higher order admit of circles of 
confusion depending on that order, with proper 
balance maintained at all times between the 
ability of the emulsion to resolve at the con¬ 
trast resulting from the aberration and the 
diameter of the central peak containing as 
much light as balanced aberrations will permit. 

In view of the approximate addition rule of 
reciprocals of individual resolutions caused by 
separate factors, it would appear that toler¬ 
ances on increased core aberration diameter 
with lowered resolution due to contrast losses 
should proceed at perhaps a ratio of one to 
three. Thus, if the emulsion resolution falls to 
25 lines per mm due to the loss of contrast of 
the aberring image, the core aberration con¬ 
taining as much balanced light as possible 
should be about 0.007 per mm, with contrast 
as good as possible within the restriction. 

Since errors of astigmatism of various 
orders are often extremely hard to eliminate, 
particularly if wide-aperture lenses are needed, 
it is better to soften the focus along a principal 
ray to increase the permitted depth of focus at 
maximum resolution and contrast obtainable 
than to correct too vigorously for the higher 
aberrations. It goes without saying that astig¬ 
matism should first be minimized. 

Unsymmetrical Coma-like Errors Varying 
as the Square of the Aperture 

These errors consist of three aberrations as 
listed in Table 2. Third-order or primary coma 
can almost always be eliminated by proper 
choice of lens data, but in practice, it should 
have a residual for the purpose of balancing 
any oblique or secondary coma that may be 
present. It should be emphasized that the 
oblique coma referred to under this heading is 
often just a reappearance of third-order pri¬ 


mary coma at large field angles. Consequently, 
a residual linear variation of primary coma 
over the field can be balanced against the mini¬ 
mized cubic variation with field angle of oblique 
coma, resulting in a balancing over the aper¬ 
ture effective at any field angle into the very 
corner. 

If possible, it is well to eliminate oblique 
coma altogether, since the squared variation 
is still too exacting on loss of contrast and reso¬ 
lution. In other words, comatic flare is not the 
best residual aberration for softening up the 
focus along the chief rays, although it is defi¬ 
nitely preferable to linear aberration. The 
coma should be drawn on only if absolutely nec¬ 
essary. Balancing of coma, as of other aberra¬ 
tions, should be done with the vignetting of the 
system in mind. If the vignetting is small, it is 
apparent that the type of balancing will ap¬ 
proach a fixed choice of primary coma, and the 
function of increased vignetting will be to limit 
the flare and improve the contrast of the off-axis 
images. But in aerial photography, vignetting 
should not be drawn on for reduction of aberra¬ 
tions. Rather, the central aperture should be 
diminished. 

It should be pointed out that the oblique 
coma described here is not the coma of higher 
order given by the sine theorem. The sine theo¬ 
rem applies only to linear variations of comatic 
flare with field angle and varies as progressive 
even powers in the aperture. Oblique coma is 
more nearly like application of the sine theorem 
to the principal ray at a large field angle. 

There is another type of oblique coma vary¬ 
ing as the square of the aperture and the cube 
of the field angle. The aberration is in the 
nature of a radial line without vertical height 
(independent of the skew direction in the aper¬ 
ture). Radially this aberration presents the 
peaked distribution desired, subject to the dis¬ 
cussion of the preceding paragraphs. Tangen¬ 
tially, the aberration is nonexistent or can be 
considered absorbed into the usual considera¬ 
tions of depth of focus for radial lines. 

Symmetrical Errors of Zones Varying 
as the Cube of the Aperture 

Symmetrical errors of zones that vary as the 
cube of the aperture permit of a softening of 



38 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


focus in a symmetrical fashion without drastic 
loss of contrast or microscopic resolution. On 
axis, the aberration is simply primary spheri¬ 
cal aberration that can almost always be elimi¬ 
nated, but which in practice should be used to 
balance out the fifth and higher order spheri¬ 
cal aberration within the meaning of proper 
balancing for aerial photography. (See discus¬ 
sion of Ross correction at beginning of Section 
1 . 2 . 1 .) 

Off axis, the third-order or primary spherical 
aberration frequently reappears, varying as the 
square of the field angle, and is called oblique 
spherical aberration. Oblique spherical aberra¬ 
tion actually consists of two separate aberra¬ 
tions, the first of circular distribution analo¬ 
gous to primary spherical aberration, and the 
second of figure-eight form. Oblique spherical 
aberration, astigmatism, and curvature of field 
should be always considered together for maxi¬ 
mum depth of focus at every field angle within 
the desirable tolerance. On the other hand, 
oblique spherical aberration very often is of 
large magnitude quite apart from a controlled 
residual of proper sign for softening of the 
astigmatic bundle. Indeed, since astigmatism, 
at least of the third order, is controllable, in 
practice one more often introduces astigmatism 
and curvature of field to whatever extent is 
necessary together with oblique spherical aber¬ 
ration to produce the optimum results in the 
meridional plane and in the skew direction. 
Much of the lens designer’s time is spent on the 
oblique spherical aberration and higher order 
astigmatism problems. Because of the inherent 
difficulties in satisfying the Petzval sum, any 
residual oblique spherical aberration should 
be, and usually is, in the overcorrected direc¬ 
tion. 

Unsymmetrical Coma-like Errors, Varying 
as the Fourth Power of the Aperture 

Unsymmetrical coma-like errors are rather 
ugly visually, but fortunately affect the photo¬ 
graphic results to an unimportant degree. Pri¬ 
mary coma can be drawn on to a limited extent 
to balance the core of the higher order coma, 
to the amount considered optimum. The usual 
observer will exclaim at the large extent of 
higher order coma and may consider the lens 
bad, even though improved results have been 


achieved in the lower order aberrations. 

Symmetrical Errors of Zones Varying 
as the Fifth Power of the Aperture 

These errors are commonly considered the 
limit to the possible speed of a given lens de¬ 
sign. To a large extent, useful lenses can be ob¬ 
tained in the presence of large amounts of this 
aberration, but in practice designers are un¬ 
able to withstand criticism of axial images hav¬ 
ing a large flare of low surface brightness sur¬ 
rounding the peak of the image. Many lens de¬ 
signs would profit immensely if the barrel could 
be shortened for the improvement of astigmatic 
variations of higher order and for improved 
flatness of effective field. The usual consequence 
of a shortened barrel at the expense of in¬ 
creased lens curvatures is large fifth-order ab¬ 
erration (which is identical with zonal aberra¬ 
tion, after proper third-order balancing has 
been achieved). 

Residual Errors Varying as the Sixth and 
Higher Powers of the Aperture 

By now it is clear how aberrations affect the 
image quality photographically. Aberrations of 
sixth and higher order may present large 
circles of confusion and some loss of contrast 
but have no very serious effect on resolution. 
These aberrations are most usually controlled 
by adapting the aperture to the type of design. 
The lens designer becomes somewhat of a 
painter, hardening the focus at one point and 
softening it at another. The process would be 
of considerably greater interest if automatic 
computing machines could be used to take the 
drudgery out of the numerous routine compu¬ 
tations involved. Often, even a minor variation 
requires recomputation of an entire bundle of 
rays through a system. The Army and Navy 
might well consider that their sponsorship of 
automatic methods would be of material im¬ 
portance to the ultimate improvement of their 
apparatus, even though at present it may seem 
like an unassociated development. 

Vignetting 

The effect of vignetting is first of all to cause 
undesirable variations in negative densities and 
thereby to make procurement of good prints a 
more difficult matter. More important still, the 



LENS DESIGNS FOR AERIAL PHOTOGRAPHY 


39 


optimum exposure on axis adjusted for maxi¬ 
mum resolution on the emulsion may mean that 
in the corner the illumination is down on the 
toe of the characteristic curve, with resultant 
loss in resolution and contrast. Dodging of 
prints to recover some density and contrast in 
the corners distorts the overall tonal values 
and, of course, cannot recover resolution not 
already in the negative. The wisest course is 
for the designer of aerial lenses to consider 
vignetting as an aberration and to equalize the 
exposure over the picture size by sacrificing 
some attractive but relatively unimportant cen¬ 
tral definition. 

In average military practice, in the presence 
of uncertain lighting conditions and rule-of- 
thumb methods, it has proved necessary to 
adopt an average overexposure in order to pre¬ 
vent complete loss of numerous underexposed 
pictures. Lenses with little vignetting would 
tend to reduce such losses since lighter pictures 
might be used. It is hoped that future develop¬ 
ments in automatic iris control, or preferably 
in shutter-speed control, will reduce the uncer¬ 
tainties and permit an average optimum ex¬ 
posure. If overexposure is necessary, however, 
it is evident from all the preceding discussions 
that the good lens design with “clean” images 
will give much sharper and more contrasty 
dense negatives than will poorer lenses with re¬ 
duced resolution and “muddy” images. The 
common practice of developing aerial film to 
a high gamma is both unnecessary and unde¬ 
sirable for photographs made with a really 
good lens. The proper gamma should be pri¬ 
marily for compensating whatever haze was 
present at time of exposure, plus a bit more to 
allow for accumulated effects of residual aber¬ 
rations and for scattered light. Tonal values in 
areas of under-illumination will inevitably still 
be distorted, but at least the best possible re¬ 
sults will have been obtained. If the negative is 
exposed longer to bring up areas of low illumi¬ 
nation, the overexposed high lights will tend 
to retain their proper contrast and the maxi¬ 
mum resolution permitted by the emulsion at 
the density involved. 

Distortion 

Reconnaissance with long-focus cameras 
seems to permit rather considerable distortion 


without marked effect. Only when mosaics are 
pieced together or when photographs are used 
for mapping purposes will distortion be of real 
importance. The subject is so familiar that dis¬ 
cussion can be minimized here. From the lens 
point of view, it is possible in many types of 
symmetrical designs to keep the distortion at 
exceedingly small values, measured in microns 
of displacement. The designer should base his 
judgment on proper balance of all requirements 
of the problem assigned. 

Discussion 

Considerations of large-scale high-quality 
photographs demand that quality be considered 
by area rather than by a high peak on axis and 
progressive deterioration radially to the corner. 
Designers would be much freer in their judg¬ 
ment if government laboratories would appre¬ 
ciate the points involved and would formulate 
their specifications accordingly. The practice of 
the British, the Canadians, and the Eastman 
Kodak Company workers in weighing the per¬ 
formance of lenses by area or “number of re¬ 
solvable units” should be adopted in equivalent 
form by everyone. 

It would be fair to the designer if the picture 
size to be covered could be definitely specified. 
For example, requests are often received to de¬ 
sign for only the included circle in the square 
picture frame. It would obviously be unfair for 
corner areas to be included in final tests of such 
lenses. Designers themselves should also be 
forewarned that problems of limited scope al¬ 
most always grow into more rigorous require¬ 
ments in the end. A designer who does design 
for the included circle or restricted aperture 
will almost always find his lens replaced by 
some lens of equal performance over a large 
area or of higher speed, and even by lenses of 
lower quality with good corners or somewhat 
higher speed. 

It is important to consider the problems and 
requirements of the Services carefully for max¬ 
imum economy, production, and early delivery. 
A doubling of tolerances on distortion, or a 
slightly reduced aperture, might permit a very 
great increase in production ease and economy. 
It is significant that many German designers 
try to obtain the utmost from rather simple 
combinations. The Zeiss Telikon telephoto and 



40 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the Topogon are good examples of thrift in lens 
design. 

13 LENSES AND ASSOCIATED EQUIPMENT 

The great complexity and variety of the 
problems encountered in the course of World 
War II prohibit the writing of a completely co¬ 
herent account of the developments in lens de¬ 
sign throughout the United States and in other 
countries. It seems best to confine ourselves to 
group classifications of equipment, and then to 
proceed on the basis of individual topics. 

131 Lenses Developed for Day 

Photography 

Wide-Angle Photography (Harvard) 

Contract OEMsr-474 with Harvard Univer¬ 
sity was entered into on May 1, 1942. Up to 
that point Harvard had started an optical shop 
for the construction of several types of aerial 
lenses under purchase orders with the Photo¬ 
graphic Laboratory at Wright Field. One of 
these lenses with associated equipment com¬ 
prised a system for wide-angle photography 3 
that appears to be of promise and might be use¬ 
ful when developed to a practical state. NDRC 
undertook to carry on the wide-angle work in 
September 1942, even though more important 
items were given precedence in order of ur¬ 
gency and utility. 

The lens design selected for the Harvard 
wide-angle system was similar to the spher¬ 
ically symmetrical scheme proposed by T. Sut¬ 
ton in 1859, but was brought to a high state 
of perfection. In the final form adopted, the 
lens consisted of a spherical crown lens with 
central stop and filter, cemented within a spher¬ 
ical shell of flint glass, also with a completely 
spherical outer surface. The choice of glass 
types and radii combined to produce a well- 
corrected image over the entire sphere at speeds 
of //2.8 and //3.5, according to the model. 

Selection of a spherically symmetrical lens 
system introduced many practical problems. 
The field of best definition was spherical and 
concentric with the lens elements. In order to 
photograph over angular fields as large as 120 
degrees total coverage, it proved necessary to 


resort to emulsion-coated glass shells of special 
manufacture. 

In conjunction with the picture-taking lens, 
it proved necessary to design and construct a 
projection lens capable of projecting the spher¬ 
ically curved negative onto an enlarged, flat, 
undistorted print. Up to the time NDRC be¬ 
came responsible for continuation of the pro¬ 
gram, Harvard had delivered to the Air Corps 
one complete system, comprising taking lenses 
of 4-in. focal length at //2.8 (there were two 
separate lens systems, one with red and one 



Figure 3. Front and rear views of wide-angle 
camera with //2.8 spherical lens. 


with infrared filters cemented between the cen¬ 
tral hemispherical elements), a camera with 
rotating ball shutter, a projection lens enlarg¬ 
ing to 40x40 print size, and an enlarging cabi¬ 
net for housing a fluorescent illuminator and 
the projection lens. 

Figures 3 and 4 show views of the pre-NDRC 
apparatus. It is to be noted that the camera 



LENSES AND ASSOCIATED EQUIPMENT 


41 


shutter started from rest each time, swept with 
a narrow slit across the emulsion, and came to 
rest against a shock absorber after traveling 
through a 180-degree arc. A shutter speed of 



Figure 4. Projector for use with 120-degree 
wide-angle camera. 


% oo sec was obtained with an efficiency of 
90 per cent. Figure 5 shows a cross-sectional 
view of the projection lens. 

Further NDRC developmental work culmi¬ 
nated before the end of the war in an //8.5 
wide-angle lens of 5.950-in. focal length (see 
Figure 6). Much work was accomplished on a 
double ball with differential gearing, shutter, 
and camera arrangement to provide continuous 
operation and constant shutter speed over the 
120-degree adopted field. Figure 7 shows the 
scheme of the proposed prototype. Between 
successive phases of the differential rotation, a 
slow-moving mechanical shutter operated elec- 
tromagnetically against spring action through 
a commutator, was to serve as the capping 
shutter. The electric relay system was designed 
to prevent multiple exposures on the same shell 
from whatever cause. 

Although automatic reloading was contem¬ 


plated at length, no progress was actually made, 
partly owing to the low priority of the wide- 
angle program and partly to the inherent diffi¬ 
culties. 

As a result of discussion of this development 
with a number of Army officers, and experience 
gained over several years of thought on the 
problem, this system for wide-angle photog¬ 
raphy may be reduced to the following conclu¬ 
sions. 

Conclusions. The performance of a spheri¬ 
cally symmetrical lens at //3.5 is nearly con¬ 
stant over any assigned field of view, which for 
practical purposes may be chosen to be 120 de¬ 
grees. It is probable that a projection lens can 
be designed to convert shell negatives into sin¬ 
gle flat undistorted prints with enlargement 
1.5 X> without great loss of resolution. 

Owing to the very high lens-film resolution 
(70 lines per mm on Super-XX) of this type 
of lens system at all contrasts, and to the pos¬ 
sibility of using fine-grain films at //3.5 with 
a shutter of high efficiency, most probable aerial 
resolution of fully-developed equipment is 30 
or more lines per mm over the entire field of 
view. Projection of the negative will produce 
much lower linear resolution in the outermost 
parts of the print, relative to the central linear 



definition, if fair comparison is to be made 
with standard wide-angle equipment. Because 
of the constant density of shell negative and of 

























































42 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the practical projection of such negatives, it is 
certain that the final prints will constitute a 
marked improvement over those made with flat 


to various difficulties encountered with the air¬ 
plane and equipment. The use of infrared emul¬ 
sion was attended with the difficulties of heavy 



Figure 6. The 5.950-in. f/ 3.5 wide-angle lens in cell. 



field lenses of comparable speed, and certainly 
over those of comparable coverage. 

The outstanding problem raised by the pro¬ 
posed wide-angle method is lack of convenience. 
On the other hand, the procedure is best fitted 
to one or a few exposures permitted over a 
large area under battle conditions. 

The wide-angle devices introduced by Har¬ 
vard should be re-examined and developed into 
a practical working unit for 120-degree cover¬ 
age and intended for a limited number of ex¬ 
posures per flight. Special attention should be 
given to the convenience and safety of the 
operator. It is believed that use of the camera 
from a completely darkened photographic com¬ 
partment in the airplane would lead to con¬ 
venience and ease of handling when in the air. 

Table 3 lists the equipment for wide-angle 
photography developed primarily for the Army 
Air Forces. 

Testing. In the summer and fall of 1942, 
aerial tests were carried out on items 1, 3, 
and 4, followed by laboratory printing making 
use of items 5 and 6. The tests showed promise 
but were not in themselves successful, owing 


background fog and storage problems. Several 
shell negatives were made from altitudes of 
2,500, 20,000, and 30,000 ft. None was in suffi¬ 
ciently good focus to be spectacular for quality, 

Table 3. Equipment for wide-angle photography. 


Pre-NDRC 


1. 4-in. //2.8 wide-angle taking lens with infrared 
filter. 

2. 4-in. //2.86 wide-angle taking lens with red filter. 

3. A complete camera with changing bag. 

4. One dozen glass photographic shells. 

5. One projection lens for wide-angle printing. 

6. Enlarger cabinet and fluorescent illuminator. 

Under NDRC 


7. 5.950-in. //3.5 wide-angle taking lens with red 
filter. 

8. Wide-angle camera, unfinished. 

9. Seventy-two glass shells of large size for lens in 
item 7. 

10. Improvements in equipment of items 2, 3, and 5. 


but they were striking for their covering power. 

Improvements made in the equipment in 1944 
and 1945 under NDRC were never tested in the 
air, although plans to that effect were in prog- 





















































LENSES AND ASSOCIATED EQUIPMENT 


43 


ress. Among these improvements were rema¬ 
chining of the camera, substitution of item 2 
in a more suitable cell, and more careful focus¬ 
ing. Ground exposures of panoramic character 
gave extremely sharp pictures and confirmed 
expectations. 

Prints made in the laboratory in 1942 were 
striking for their size and coverage, but unfor¬ 
tunately there was lack of sharpness in the 
negative due to the inadequacy of focus. It ap¬ 
peared from the laboratory printing that the 
projection lens itself was of suitable quality, 


Mount Wilson Findings . 4 (1) The best focal 
setting with wavelength from 5,800 A to 7,200 A 
lies in a range of 0.04 mm, with minimum focus 
at 6,600 A. (2) All errors of asymmetry, coma, 
lateral color, and distortion were found to be 
negligibly small, indicating good workmanship. 
The focal surface was found to be spherically 
concentric with the lens surfaces within the 
measuring accuracy of 0.01 mm. (3) Astig¬ 
matism was found to be absent, although a 
slight difference in resolution in tangential and 
radial directions existed, owing to obliquity of 


Figure 7. 


A schematic view of the double-ball wide-angle camera. 



but that the enlarging equipment was too cum¬ 
bersome and needed redesign along more prac¬ 
tical lines. 

Item 7 was completed in April 1945, and 
delivered to the Mount Wilson Observatory for 
testing which made use of the Mount Wilson- 
NDRC optical bench, and which was conducted 
both visually and photographically at a number 
of contrasts. Further tests were conducted at 
the Eastman Kodak Company in the fall of 
1945. 


the aperture and loss of diffraction resolution 
far off axis. (4) Spherical aberration at //3.5 
was found to be negligibly small. 

Resolving power was measured visually and 
photographically. All photographic resolving 
powers were found to be higher than 90 per 
cent of the reciprocal sum of the reciprocals of 
the resolving power of the lens visually and 
of the film alone. Maximum visual resolution at 
1/0.005 contrast was found to be 400 lines per 
mm with little scattered light around the image. 


































44 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Radial resolutions held to this value over the re¬ 
quired total field of 120 degrees, but tangential 
resolution, in agreement with computation, due 



200 

150 

100 

80 

70 

60 

50 


TANGENTIAL 


c =1.96 

MICROFILE 
c = 0.30 



T RADIAL J 

c= 1.96 

*1 

• — 

- 

- 

TANGENTIAL"* 

' 

- 


SUPER-XX 

- 

RADIAL 

- - A IQ 

- 

TANGENTIAL''-* 

c- 

0° 

30° 60° 



FIELD ANGLE 



Figure 8. Resolution tests on the 5.950-in. //3.5 
wide-angle lens. 


to the reduced elliptical aperture fell by a 
cosine law to approximately 200 lines per mm 
at 60 degrees off axis. Other visual measures 
at a contrast of 1/0.5 showed nearly constant 
radial resolution of 300 lines per mm over the 
field, and tangential resolution falling to 180 
lines per mm at 60 degrees off axis. 

Figure 8 shows the Mount Wilson photo¬ 
graphic results, where the contrast c refers to 
the logarithm of the ratio of surface brightness 
of the test target. Figure 9 shows the depend¬ 
ence of resolving power on focal setting at 
contrast of 1/0.5 for Microfile and 1/0.65 for 
Super-XX. The curves showing resolution ver¬ 
sus focal setting are in excellent agreement 


with the assumption of constant turbidity of 
the emulsion, but are in complete disagreement 
with the assumption that the law of reciprocals 
holds. Therefore, the comparatively great depth 
of focus indicated does not mean in this case 
that the lens suffers from zonal aberration but 
that the resolved coarser patterns out of focus 
consist of fat, exposed lines separated by thin, 
unexposed spaces of the emulsion. 


The 40-in., //5, 9x9 Distortionless 
Telephoto Lens (Harvard) 5 

In 1942 Harvard delivered to the Army Air 
Corps an //5 telephoto lens of 40 in. focal length 
for the K-22 camera. During 1941 and part of 
1942, under direct purchase orders with the 
Army Air Corps, there had been designed and 
developed three telephoto lenses of similar char¬ 
acter. The first of these was the //5 lens cor¬ 
rected for a minus-blue filter, mounted in an 
aluminum cone that fitted the experimental 
K-22 camera, also just delivered to the Air 



Figure 9. Dependence of resolving power on 
focal setting at low contrast. 


Corps by the Fairchild Camera and Instrument 
Corporation. 

The second telephoto, partially completed by 
Harvard at the time of the NDRC contract, 
















LENSES AND ASSOCIATED EQUIPMENT 


45 


was a 40-in. //5 for 9x9 picture size, but rede¬ 
signed for best color correction with an infra¬ 
red filter. The third telephoto was a 60-in. //6 
for 9x9 picture size. The optical parts for this 
lens were completed by Harvard prior to the 
NDRC contract, but delivery was held up for 
lack of a mounting. 


Final Model. The final production model of 
the //5 40-in. telephoto with minus-blue filter 
is shown in the assembly drawing of Figure 10, 
and the first prototype in Figure 11. It is to 
be noted that seven elements were found to be 
necessary to achieve the necessary optical cor¬ 
rections at //5, including the correction of 



Figure 10. Cutaway view of the 40-in. //5 telephoto. 


Improvements and completion of advanced 
models were placed on the new NDRC program 
as of the highest urgency. In addition, aerial 
testing carried on at Wright Field in Septem¬ 
ber 1942, and again in February 1943, estab¬ 
lished that in spite of a 40-in. focal length the 
//5 telephoto with minus-blue filter was capa¬ 
ble of yielding aerial pictures of very sharp 
linear resolution. 

Aerial tests also established clearly that the 
aluminum mounted telephoto was subject to 
changes of focus in the air due to thermal 
gradients and great range of temperature, and 
to ground-distance and air-density changes with 
altitude. The efforts of the laboratory at Har¬ 
vard were concentrated during 1943 and 1944 
on means for making the camera system com¬ 
pletely compensated for focal changes from any 
cause. 

Production contracts were concluded in May 
1943, between the Air Forces and the Perkin- 
Elmer Corporation, for 100 units of the 40-in. 
//5 telephoto lens. A contract for 12 units was 
also made with Harvard. Because of the ex¬ 
pected long wait for production of the requisite 
optical glass, time was taken in the Harvard 
laboratory to undertake construction of three 
more experimental telephotos, each of succes¬ 
sively greater improvement. 


distortion. If distortion had been left badly un¬ 
corrected, fewer elements would have sufficed. 
The filter was located in the converging beam 
behind the rear element, and the corresponding 
small optical effects at //5 were taken into ac¬ 
count in the lens formula. 

The //5 optical system and mounting were 



Figure 11. The 40-in. f/5 telephoto for 9x9. 

well integrated into a unit fully compensated 
to prevent focal shifts. The mounting contains 
many points of engineering that contribute to 
production ease, production control, economy of 
manufacture, and to performance in the air. 
One of the novel features of the system is the 



















































46 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


unit for automatic focusing with altitude. Cal¬ 
culations had shown the probable change of 
focus with altitude to amount to many times 
the detectable depth of focus in the air with the 
40-in. lens. It seemed worth while to attempt 
elimination of this variation in focus in order 
to provide automatic use of the unattended 
telephoto in fighter aircraft. The optical con¬ 
struction of the //5 telephoto was well adapted 
to accomplishing differential focal shifts by 
means of longitudinal movements of the rear 
element. Extended computations proved that 
the aberrations of the system were insensitive 
to the location of the rear element, out of pro¬ 
portion to the effect of its movement on the 
focal position. After some careful considera¬ 
tions and experiments the automatic focusing 
unit shown in Figure 12, which maintains a 
constant focal plane by moving the rear ele¬ 
ment, was designed and constructed. 



Figure 12. The automatic focusing unit and 
inner cell assembly. (Courtesy of Perkin-Elmer 
Corporation.) 

The change of focus with altitude comprises 
two effects, one of ground distance from the 
camera and the other air-density changes. Both 
of these are additive and together produce the 
curves shown in Figure 13. In order to elim¬ 
inate this type of variation with focus as effec¬ 


tively as possible, it was decided to fit the curve 
with two straight lines, each representing a 
constant spring rate of a sylphon bellows ar¬ 
rangement. The rear element is suspended in 
effect between ten springs and ten bellows. At 
altitudes up to 15,000 ft, six nitrogen-filled 
bellows under pressure push against the springs 
and four others, partially evacuated, retard this 
action. At 15,000 ft, the four partially evacu¬ 
ated bellows move free of the suspension against 
a stop and are thereafter inoperative. There¬ 
fore, above 15,000 ft the spring rate is mate¬ 
rially reduced for the system, resulting in a 
much slower compensation with pressure. Fig¬ 
ure 14 shows how well the system so designed 
and applied worked in practice. In view of the 
long industrial experience with bellows, there 
seems to be little likelihood of failure in use 
from ordinary causes. 

Figure 15 shows the results of cold-chamber 
tests made in 1943 on an early unthermostated 
model. The thermal gradient caused the focus 
to shorten in spite of the shrinking of the steel 
lens barrel, which in the equilibrium state pre¬ 
dominates. Note that between the third and 
fifth hours, the focus moved rapidly away from 
the lens. 

The 40-in. //5 lens was thermostated by 
means of soil-heater cable wound in two paral¬ 
lel circuits about the camera in such a way 
that all solenoid effects were eliminated. The 
heating system was capable of delivering about 
100 w at 24 v. To conserve heat and to reduce 
thermal gradients, the outside of the camera 
was insulated with asbestos and Micarta tub¬ 
ing. The latter served to protect the camera 
against shock and handling, and to provide a 
streamline exterior. Figure 16 shows further 
cold-chamber tests with a thermostated unit. 

Although most commercial aerial lenses, both 
in the United States and abroad, were mounted 
in brass or aluminum for ease of production, 
freedom from corrosion, and in the case of 
aluminum, for lightness, the production Har¬ 
vard 40-in. //5 lens, like the German Telikon, 
was mounted in rustproof steel. It was felt that 
the long time-stability of steel, its resistance 
to rough treatment, and its close mating with 
glass on thermal expansion were highly desir¬ 
able for a lens system of high precision. The 



LENSES AND ASSOCIATED EQUIPMENT 


47 



Figure 13. Dependence of focus on air density and ground distance, before and after compensation. 


use of steel, however, added very materially to 
the cost of the unit and multiplied the difficul¬ 
ties of mass production. 

In spite of the high specific gravity of steel, 
careful engineering of parts reduced the weight 
of the complete telephoto system to a reason¬ 
able figure. The completed lens system, apart 
from camera body and magazine, weighed ap¬ 
proximately 88 lb. For maintenance of shape 
with age, all steel parts were subjected to deep 
freezing before final machining. 


.080 

'.077 

.073 

.070 

.066 

.062 

.059 

.055 

.051 

1.047 


5 5 


0 . 

0 
0 
0 
0 

V) 0 

So 

n 

z °- 
- 0.044 
[7 0.040 
f 0.036 
</> 0.033 
u, 0.029 
o 0-025 
0.022 
0.018 
0.014 
0.010 
0.007 
0.003 
0 
70 


-1---1- 

T-1--1- -1 

40,000 FT 

. 

30,000 FT 


25,000 FT VX.OOOFT 

20,000 FT. 


15,000 

•f' 

- 10,000 FT 

© = NEW VALUES'CALCULATED ! 

J' 

0 = BELLOWS UNIT A 

/''5000 ft 

■ 





60 


50 40 30 20 

VACUUM-G OF MERCURY 


10 


Figure 14. Observed and calculated bellows 
movement. 


The spacer rings shown in the cells for the 
last four elements were used in production for 
control of image quality of the system. Differ¬ 
ential correction formulas were used that took 
into account random variations in melts of glass 
and lens thicknesses. Optical tolerances on 
radius variation were held to such a minimum, 
without marked increase in production diffi- 


















48 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


culty, that all radii were considered to be con¬ 
stant. Indices of refraction were measured by 
the Perkin-Elmer Corporation on a five-place 
refractometer. To facilitate such work, each 
glass blank as delivered contained a boss that 
could be sliced off, one face polished, and the 


B-29’s for use in the Pacific area. No reports 
have been received concerning the lens units in 
service. 

Of the several telephoto lenses submitted as 
prototypes, only the 40-in. //5 with minus-blue 
filter was produced. The infrared 40-in. //5 


MON OCT 23,1944 TUES OCT 24 WED OCT 25 THURS OCT 26 FRl OCT27 SAT OCT 28 SUN OCT 29 
12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 4 8 12 



Figure 16. Cold-chamber observations on thermostated telephoto. 


index of refraction measured. The extra opera¬ 
tions proved to be very economical from the 
point of view of assembly. By means of the dif¬ 
ferential correction formulas and production 
control, the later production units rarely had 
to be disassembled for readjustment. 

Production Results. During the period of de¬ 
velopment of the lens, several other test flights 
were made at altitudes up to 30,000 ft. The 
clear pictures made time after time convinced 
the Air Forces of the utility of the lens in spite 
of its inherent complexity. Throughout 1944 
and 1945 there was constant pressure exerted 
to get the units into production. Delivery actu¬ 
ally began in the spring of 1945. By the end of 
World War II in August, the Perkin-Elmer 
Corporation had reached the unexpected rate 
of 30 units a month. It was quite possible, had 
the emergency continued, that a production 
rate of 100 units a month could have been 
reached. The requirements for this rate, as 
expressed by the Air Forces, took form in a new 
contract for another 200 units, and near the 
end of World War II discussions were being 
held on procurement of at least 200 more units 
during 1946. 

Installation of the 40-in. //5 was made in 


telephoto, delivered in the winter of 1943, was 
tested successfully, but was never contemplated 
for production. 5a A 60-in. //6 telephoto 5b was 
delivered in the winter of 1943-44 also. The 
60-in. was mounted in a U-shaped camera 
(see Figure 17) measuring only 16x26x31 1 /<> 
inches, small enough to be used in the nose of 
a P-38 plane. Aerial tests of this lens during 



Figure 17. The 60-in. f/6 U-shaped telephoto. 










LENSES AND ASSOCIATED EQUIPMENT 


August 1944 over Dayton, Ohio, proved suc¬ 
cessful. The lens was later flown over Berlin 
at least once. 

During the fall of 1944, steps were taken to 
procure one dozen units of the 60-in. //6 for 
9x9. Since the design of the optical system was 
unsuitable for mass production, Harvard pro- 


War II prevented completion of the two proto¬ 
type units under way. 

The optical system was to be mounted in an 
internal tube supported in such a way as to 
minimize flexure with the weight distribution 
as planned. By use of the principle of a ball 
inside of a cylinder at the front end, it would 



FOCAL 
PLANE—► 

9X 18 
PICTURE 
SIZE 


vided a reworked design during March and 
April, 1945. Subsequently, the entire project 
was canceled, owing to cost difficulties in part, 
and to the restricted ground coverage. 

The 60-in., //5, 9x18 Telephoto Lens 
(Harvard) 50 

The lessons gained from the several years of 
experience on telephoto systems led Harvard to 
propose a 60-in., //5, 9x18 camera. Although, in 
principle, this lens was a scaling up of the 
earlier design, new computations were made 
for the purpose of improving the vignetting of 
the earlier system. In addition it was proposed 
to mount the lens system in a vacuum to com¬ 
bat the rigors of service conditions. Figure 18 
shows the proposed system. The end of World 


be quite difficult for external distorting forces 
to be imparted to the sensitive end of the sys¬ 
tem. The ring and outside contact cylinder were 
both to be sufficiently heavy to withstand any 
possible blow that might still be transmitted 
through the thick external rubber cushion. It 
was felt that the elastic limit of the steel 
mounting tube could never be exceeded by ordi¬ 
nary forces, short of outright destruction. 

The front of the telephoto system was to be 
enclosed by a heavy plane-parallel window of 
BSC-2 glass. The rear of the system was to be 
formed by the rear element itself, purposely 
thickened to withstand the atmospheric pres¬ 
sure. The filter, again mounted in the converg¬ 
ing beam for interchangeability, lay outside the 
vacuum. 












































50 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


The tube supporting the atmosphere was to 
be lined on the inside with aluminum foil, op¬ 
posed across the vacuum by foil covering the 
outside of the inner optical system. The mount¬ 
ing was therefore to resemble a vacuum flask. 
Calculations indicated that the probable slight 
heat loss could be restored by a thermostated 
50-w heating circuit next to the optical tube. 

Although the proposed mounting undoubt¬ 
edly is more expensive and more difficult than 
ordinary designs, it was thought that the lens 
would only be used in precision reconnaissance 
and it was anticipated that production require¬ 
ments would amount at most to perhaps a dozen 
units. Moreover, the cost of such a precision 
reconnaissance camera would still be far less 
than the cost of an F-13 photographic airplane. 

At the end of World War II some of the 
optical elements had been finished, but glass 
had not yet been received for approximately 
half of the elements. The size of the elements 
and lack of heavy-duty machinery caused the 
optical work to proceed relatively slowly. 

The 60-in. //5 lens represents at this time 
the most advanced development of the NDRC 
lenses. The lessons taught by the extensive 
aerial camera testing during World War II 
indicate that lenses of great focal length prop¬ 
erly made and used are capable of yielding an 
improvement in angular resolution which is in 
keeping with their focal length, and that linear 
resolution on the picture can reach at least 
20 lines per mm on average days. It is therefore 
recommended that the two 60-in. prototypes 
be completed whenever practicable, and that 
the necessary careful attention to detail be 
given at all stages of the work. Particular 
attention should be given to “tight” but not 
“jammed” fits of lenses to their cells. Such fits 
can best be accomplished by reasonable toler¬ 
ances on machine work followed by careful 
shimming. When properly rpounted, the lens 
should resist turning in its cell but nevertheless 
be capable of being turned by some force. The 
Germans included the same requirement in 
their wartime lenses. 

The 100-in., //10, 9x18 Figure-4 Anastigmat 
(Harvard) 6 

In October 1942, it was decided that a 100- 


in. focal length lens should be added to the 
Harvard NDRC program. The only company 
manufacturing large disks of optical glass in 
this country at that time was the Bausch and 
Lomb Optical Company. The yield on large 
glass was so scanty and in such conflict with de¬ 
mands for optical glass from other consumers 
that it was decided not to exceed 10 in. di¬ 
ameter, which accordingly made //10 the fast¬ 
est possible lens speed. 

Figure 19 shows the proposed lens system in 
the form of a “Figure-4” mount, a type pecu¬ 
liarly suited to this aperture and lens design. 
The folded design conserves space as well as 



Figure 19. The 100-in. //10 anastigmat. 


possible for the purpose of aerial photography. 
Such a design unfortunately resembles the 
structure of a seismograph, in that the two mir¬ 
rors multiply image vibrations. 

The optical design was completed in June 
1943. The lenses were made during the course 
of that year. The project was assigned very low 
priority on repeated occasions and conse¬ 
quently, during months at a time, went un¬ 
touched. In 1945, however, interest reawak¬ 
ened, in view of the needs in the field for long- 
focus cameras. The lens was finished in the 
spring of 1945 and delivered to the Mount Wil¬ 
son Observatory for testing. 

A direct contract was made between the 
Army Air Forces and Mount Wilson for the 
design and fabrication of the camera itself, in¬ 
cluding a fast shutter for a 9x18 picture size. 
Although this work was not accomplished un¬ 
der NDRC, the mounting was designed to carry 







































LENSES AND ASSOCIATED EQUIPMENT 


51 


the NDRC lens. In cooperation with Harvard, 
the mounting was so planned that a larger aper¬ 
ture lens of identical back focus could be substi¬ 
tuted at any time for the //10 lens. (See Fig¬ 
ure 20.) 

Design work was begun at Harvard on an //8 
apochromatic lens (see text that follows) for 
the same mounting. 7 The end of the war pre¬ 
vented completion of the design. Enough work 
had been accomplished, however, to point to the 
probable success of the endeavor. It was hoped 
that substitution of the //8 aperture and the 
longer spectral range in sharp focus would 
combine to make shorter exposures possible, 
thus minimizing ground motion and vibration. 



Figure 20. The 100-in. //10 “Figure-4” anastig- 
mat. 


The 100-in. lens was tested both at Mount 
Wilson and at Eastman Kodak, under Contracts 
OEMsr-101 and OEMsr-392 respectively. The 
tests at Mount Wilson were carried out on the 
special optical bench. The overall length of the 
100-in. lens was such that the nodal slide carry¬ 
ing the 100-in. lens barrel had to be mounted 
on a separate stand. Measurements of flatness 
of field and color and residual aberrations were 
carried out visually. Resolving powers at sev¬ 
eral contrasts were determined both visually 
and photographically. 

Mount Wilson Results . 4a 1. Astigmatism and 
field curvature, measured with respect to the 
best axial focus, are negligibly small for field 


angles below 4 degrees (about 7 in. off axis), 
but increase at larger angles to a maximum 
value in the corner of —0.35 mm radially and 
— 1.1 mm tangentially. The variation is due to 
higher order astigmatism and varies at least as 
the fourth power of the field angle. Conse¬ 
quently, the effects of the aberration on resolu¬ 
tion are felt only in the corners of the pic¬ 
tures. 

2. Distortion is sufficiently small for any 
practical purpose to which the 100-in. lens 
might be put. The ray-tracing results on the 
design indicate that distortion in the corners 
amounts to only 0.012 mm pincushion type. The 
observed value appears to be 0.070 mm barrel 
type. Such small variations at such a long focal 
length are to be expected, unless special care 
is used during the final adjustment of the lens. 
Measurement is no less difficult. 

3. Chromatic aberration. Lateral color is 
stated to be barely noticeable, but too small to 
be measured. Design figures indicate 0.013 mm 
in the corner with blue light nearer the axis 
than red. This residual design aberration can 
be reduced to less than one tenth of its present 
value by differential correction. 

Longitudinal color is characteristic second¬ 
ary spectrum, partially concealed by the Ray¬ 
leigh limit depth of focus, with minimum focus 
at 6,350 A. The correction is optimum for use 
with orange filter, as incorporated in the 
mounting near the film. The chromatic differ¬ 
ence of spherical aberration seems negligible. 
Although, visually, the secondary spectrum is 
very marked, photographically the effect is 
small. 

4. Spherical aberration is veiled in the proto¬ 
type lens by effects of inhomogeneity in one or 
more of the glass elements, or possibly by er¬ 
rors in surface figure of uneven character. De¬ 
sign figures show that the zonal aberration is 
within the Rayleigh limit. The zonal aberra¬ 
tion is therefore nearly of optimum character 
in accordance with principles set down in the 
first part of this chapter. 

5. The Mount Wilson report states that the 
resolving power (by the rule of reciprocals) in 
the center of the field is 80 per cent of the theo¬ 
retical maximum for all contrasts, and better 
than 65 per cent of the theoretical maximum 







52 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


for all field angles into the very corner of a 
9x18 picture size. 

Tests on two Pyrex optical flats indicate that, 
when edge-supported on felt in a suspended po¬ 
sition as used, the mirrors are flat over the area 
used to at least one-half wave together, and 
over most of the area to better than one-quarter 
wave together. In view of the small area of the 
mirrors used by a beam converging to any 
single point image over the field, the flatness 
would appear to be adequate. For further 
studies of the 100-in. lens, see Section 1.4. 

Apochromatic Aerial Lenses 8 

Under Contract OEMsr-474 Harvard Uni¬ 
versity was requested to initiate investigations 
involving the use of synthetic fluorite for the 
purpose of removing secondary spectrum from 
aerial camera lenses. Calculations were begun 
in the fall of 1942 for the purpose of testing 
the performance of fluorite as an optical ma¬ 
terial. Successful results obtained with several 
telescope apochromatic objectives of long focal 
length showed that it would be feasible to use 
fluorite for the purpose specified. 

Calculations were completed in the spring of 
1943 on two types of apochromatic //8 aerial 
lenses of 36 in. focal length. The first of these 
is shown in Figure 21. The “short barrel” form 
was disappointing as to design, and was made 
up as an interim lens. It proved difficult to 
flatten the field without leaving excessive off- 
axis coma, owing to the low index of refraction 
of the materials used, especially fluorite. 

The lens was flight-tested at Wright Field on 
several occasions, on Super-XX and on color 
film. In spite of the large comatic flare, the lens 
gave fairly good aerial pictures. “Even with less 
than one-third of the light in the effective image, 
the pictures seemed sharp to the eye.” The 
aberration curves from the lens design show 
that at the side of the picture the total spread 
of rays from outward oblique coma amounted to 
0.3 mm. The fact that the pictures were deemed 
fairly good is consistent with the discussions in 
Section 1.1. 

Shortly afterwards, in April 1943, a more 
satisfactory fluorite apochromatic design was 
finished. During May 1943, a “long barrel” //8 
apochromat of 36 in. focal length was made up 


and delivered to Wright Field for aerial tests. 
In the laboratory at Harvard, visual tests 
showed the lens to be of high quality, especially 
for its freedom from color. The predominant 
residual aberrations were rapidly increasing 
astigmatism and vignetting in the corners of 
the 9x9 picture area. Rather than decrease the 
quality of the larger area of the picture, the 
performance in the corners was sacrificed. The 
final lens was deemed to cover satisfactorily the 
9-in. included circle. It is apparent that the lens 
system could not be used for 9x18 coverage 
without further and more complicated design 
work. 

During the summer of 1943, a second model 
was completed according to the same design. As 
in the first case, this model was found to yield 



Figure 21. The 36-in. //8 fluorite apochromatic 
aerial lens (short barrel). 

sharp images over a 9-in. circle. Figure 22 
shows a cross-sectional view of the lens system. 

Cold-chamber measures in the Harvard lab¬ 
oratory showed a great change of focal posi¬ 
tion with temperature, due mostly to the single 
fluorite element. Over a range from room tem¬ 
perature to —73 C, the back focus shortened 
0.215 in. (see Figure 23), which at //8 would 
be disastrous. For such a lens, thermostating is 
obviously essential. 

Although the lens was tested with focus runs 
in the air at Wright Field, various difficulties 
prevented good aerial pictures. Laboratory 
measures, however, indicated a high level of 
performance. The lens was described in a 
Wright Field report as unsuited to military 
photography, primarily because of excessive 
camera length and change of focus with tem¬ 
perature. 































LENSES AND ASSOCIATED EQUIPMENT 


53 


Extensive testing of the 36-in. lens was car¬ 
ried out at the Mount Wilson Observatory. 9 
Figures 24, 25, and 26 show the results obtained 
on the optical bench. All tests were carried out 
with slit illumination provided by a high-pres- 



Figure 22. The 36-in. //8 fluorite apochromatic 
aerial lens (long barrel). 


sure mercury lamp with yellow filter transmit¬ 
ting wavelengths longer than 4,800 A. The 
energy distribution in the light source approxi¬ 
mated daylight quality. 

Results. 1. Color aberrations. Visual meas¬ 
urements made of longitudinal color aberration 
indicate that the secondary spectrum has been 
reduced to the sufficiently low value of 20 per 
cent of the normal curve for glass objectives, 
but not entirely eliminated. The maximum de¬ 
parture from mean focus between 5,000 and 
6,700 A on axis amounts to 0.0001 of the focal 
length, which at //8 is sufficiently good for all 
purposes of aerial photography. The chromatic 
difference of spherical aberration seems negli¬ 
gibly small. Figure 25 shows the color curves 
for three field angles. The change in correction 
with field angles seems inappreciable. 

No direct measures of lateral color have been 
reported. However, inspection of the monochro¬ 
matic astigmatism at 5,450 A, in comparison 
with heterochromatic astigmatism with yellow 
filter, shows the latter to give better definition 
tangentially. Such a result cannot happen un¬ 
less the lens is well corrected for astigmatism 
in orange and red light, and is sufficiently free 
of lateral color. Design figures indicate ex¬ 
tremely small lateral color, although exact ray¬ 
tracing values to the corner of the field in sev¬ 
eral colors are not at hand. Indeed, visual in¬ 
spection in the Harvard laboratory indicated 
that residual chromatic variation of astigma¬ 
tism was prominent in this lens, although not of 
large magnitude. The error arises from the 


large front air space, required for the correc¬ 
tion of other more important aberrations. 

2. Astigmatism and curvature of field. These 
are inappreciable over an 8-in. diameter circle. 
Outside this circle, however, the rapid growth 
of fifth-order astigmatism produces progres¬ 
sive forward curvature, especially of tangential 
lines. The vignetting present maintains the 
resolution but in itself must be considered an 
aberration and disadvantageous. The 80X pho¬ 
tomicrographs of star images given in the 
Mount Wilson report 9a show that the concen¬ 
tration of light into point images on a flat focal 



Figure 23. Cold-chamber observations on focus 
of fluorite apochromat. 


plane has been very well achieved over an 8-in. 
diameter circle, but that the corners of the pic¬ 
ture fall to ordinary levels of resolution. 

3. Distortion. Over the 9x9 picture size, dis- 

















































54 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


tortion was found to amount to less than 0.005 
mm over most of the field, increasing to about 
0.020 mm pincushion type in the corner. These 
minute values are in good agreement with cal¬ 
culations presented in the Harvard report. 8 

4. Resolution. Figures 24 and 26 show the 
results of extremely well determined visual and 
photographic resolution tests made at several 



Figure 24. Dependence of resolving power on 
field angle. 


contrasts by Mount Wilson and described in 
their report. 9 It is evident that the lens is well 
corrected in the center of the field, but that the 
visual quality at the center is concealed on 
Super-XX by the film itself. The depth of focus 
versus resolution curves for Super-XX and Mi¬ 
crofile indicate that almost all of the effective 


light of the //8 aperture lies within the Ray¬ 
leigh limit in the center of the field. 

According to the Mount Wilson conclusions, 
the resolving power reached 96 per cent of the 
theoretical maximum in the center of the field, 
and was better than 75 per cent of the theo¬ 
retical maximum for a field 8 in. in diameter. 

Aerial Tests. In addition to the unfruitful 
tests made in the air at Wright Field, the sec¬ 
ond long-barrel model was tested at length in 
1945 under the Harvard contract. 10 The diffi¬ 
culties encountered at Wright Field were par¬ 
tially overcome by revision of the mounting and 
by careful thermostating of the entire mount. 
Focusing runs at 10,000 ft over Orange, Massa¬ 
chusetts, resulted in an average maximum reso¬ 
lution of 32 lines per mm across the line of 
flight. These tests were made with a K-22 
camera and yellow filter, and a shutter speed 
of y 350 sec on Pan-X. In spite of the high state 
of correction for color, the maximum resolution 
figures for the fluorite lens did not reach the 
best results achieved with the 40-in. //5 tele¬ 
photo lens. One must conclude that the //5 
monochromatic resolution was enough of an 
advantage to overcome the superior color cor¬ 
rection of the //8 fluorite lens, and particularly 
that the 40-in. lens was better mounted for 
aerial purposes. The fluorite lens undoubtedly 
would be much superior to the 40-in. for color 
photography in view of its high correction over 
an extended spectral range and in view of the 
disadvantage of telephoto designs for even 
normal color correction. 

The 48-in., f/8, 9x9 Apochromat . 8a In order 
to overcome both change of focus and length, 
the Harvard laboratory planned a 48-in. U- 
shape camera with thermostated heating sys¬ 
tem. Although assigned to low priority on the 
program, the optical parts for this lens, includ¬ 
ing two Pyrex mirrors of suitable size and 
shape, were completed. No mounting was ever 
constructed. The picture size of 9x9 was re¬ 
tained, which in further work should be in¬ 
creased to 9x18 with great emphasis on low 
vignetting. 

Discussion. Apochromatic systems will ulti¬ 
mately be required for perfected aerial pho¬ 
tography. It is probable that at the present time 
the loss of contrast due to secondary spectrum 








LENSES AND ASSOCIATED EQUIPMENT 


55 


is less important than vibration and other limit¬ 
ing factors. For color photography and for 
lenses of very large aperture, however, apo- 
chromatic systems will be of great value. Such 
a lens would be of about 10 in. aperture, used 
either for color work or for night photography 
without filter. 

The Harvard 36-in., //8, 9x18 Wide- 
Angle Telephoto 11 

One of the most important problems under¬ 
taken by Section 16.1 of NDRC in the last 12 
months of World War II was a 36-in. focal 
length //8 telephoto for the K-18 (9x18) 


as prepared for production. Only two types of 
glass were used, distributed among five ele¬ 
ments. The design was much more suited to 
mass production than the 40-in. //5 automatic 
lens, and consequently was felt to be a lens of 
general utility, like the British 36-in. for 7x9. 
Limitation of the aperture to //8 resulted in 
improved resolution and reduced vignetting. 
Indeed, the first prototype had no appreciable 
vignetting. The second prototype had vignet¬ 
ting which, while less than in the standard 24- 
in. lens, was not as small as desirable. 

Discussion. The wide-angle telephoto has the 
largest coverage at 36 in. focal length yet used 



FOCAL SETTING IN MM 


Figure 25. Astigmatism, distortion, and color curves. 


camera (see Figure 27). In the field, the dual 
need appeared for large-scale contact printing 
and compact, light equipment. It was felt that 
a replacement telephoto lens for the K-18 Tes- 
sar would answer the problem adequately, even 
though the resolution achieved might not be 
superior or perhaps quite as good as that al¬ 
ready obtained with the standard 24-in. lenses. 

Design . The design of the 36-in. telephoto 
was worked out at the Harvard laboratory in 
the summer of 1944 and a prototype was de¬ 
livered in December of that year. Visual lab¬ 
oratory tests at Harvard indicated that fur¬ 
ther improvement might be made. An improved 
design was worked out and a second prototype 
was made and delivered to Wright Field in 
April 1945. 

Figure 28 shows the second prototype design 


in the air. The 36-in. telephoto was designed to 
fulfill a sudden need, and its design was re¬ 
stricted by the need to adapt it to the K-18 
camera. With the appearance of new types of 
optical materials and under peacetime condi¬ 
tions, there is every reason to believe that con¬ 
siderable improvement can be made in both the 
speed and covering power of the 36-in. lens. It 
is believed that future efforts should compro¬ 
mise neither field nor vignetting, but should 
concentrate on improved resolution and in¬ 
creased speed, in accordance with the principles 
outlined in the introduction. (See Section 1.1.) 

Tests. Tests made in the air on the first pro¬ 
totype at Midland, Texas, in February 1945, 
proved that photographs made with the lens 
were comparable with the linear resolution 
given by the standard 24-in. lens, and therefore 















56 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


possessed somewhat superior angular and 
ground resolution. Laboratory measures indi¬ 
cated still greater superiority, even on a linear 
basis. The use of the between-the-lens shutter, 



-0.5 0 +0.5 

FOCAL SETTING IN MM 


Figure 26. Dependence of resolving power on 
contrast and depth of focus. 

which gave an exposure of y 150 sec, limited the 
aerial performance of the 36-in. lens in tests 
to date. 

Laboratory tests were carried out at the 
Eastman Kodak Company late in 1945. Wedge 
patterns and resolution measurements are re¬ 
produced in Section 1.4. 

Uncompleted Lens Designs at Harvard 

On the Harvard program for 1946 there were 
several long focal length lenses some of which 
were only partially designed or constructed at 
the end of the war. These will be described 
briefly. 


The 100-in., f/8, 9x18 Apochromat . 7a A 100- 
in. //8 apochromat, for 9x18 picture size, was 
investigated to serve as a replacement lens of 
faster speed for the 100-in., //10, 9x18 focal 
length lens for the “Figure-4” camera. The 
apochromatic correction was not to be obtained 
by means of fluorite but by use of hyperchro- 
matic combinations of ordinary crown and flint 
glasses. Calculations presented in report form 
show that by making use of heavy flint glass for 
positive elements, one can obtain not only a 
great contribution to the Petzval sum for bet¬ 
ter flatness of field, but can also eliminate 
almost entirely the troublesome secondary spec¬ 
trum. No final design has been submitted, but it 
is recommended that such a lens be added to 
any continuation of a program in aerial pho¬ 
tography intended for extreme altitudes. 

The 36-in., f/8, 9x18 Anastigmat. 7h An un¬ 
completed lens design of great promise was a 
36-in. focal length //8 anastigmat, modeled 
along the lines of the 6-element Planar. In spite 
of the use of ordinary glass types, it proved pos¬ 
sible to obtain an extremely flat field of high 
correction. No final design has been submit¬ 
ted, but it appears from the Harvard report 
that the completed lens should give results 
greatly superior to those obtained with the 36- 



Figure 27. The 36-in. f/8 telephoto in the K-18. 

in., f/8 telephoto lens. The anastigmatic form 
is intended to provide a 9x18 picture almost 
free of vignetting. The use of six elements 
makes possible unusually good correction for 







LENSES AND ASSOCIATED EQUIPMENT 


57 


oblique spherical aberration. Owing to the sym¬ 
metry of the lens, exceptional correction for un- 
symmetrical errors is possible, relative to tele¬ 
photo designs. It is recommended strongly that 
this type of lens or its equivalent be pursued to 
the limit with improved optical materials in 



Figure 28. The 36-in. //8 telephoto for 9x18. 


order that the Army Air Forces may have at 
their disposal a lens optimum in all respects, 
except perhaps focal length. Of all the designs 
submitted during the war, this lens holds forth 
best promise of meeting the stringent ultimate 
requirements outlined in the introduction. (See 
Section 1.1.) It is felt that //8 is too slow and 
that effort should be made to increase the speed 
to //5. 




Figure 29. A preliminary design for a 12-in. 
//5 for 9x9. 


The 12-in., f/5 , 9x9 Anastigmat. 7c This de¬ 
sign was planned to explore the possibilities of 
providing a lens of improved definition for the 
standard 12-in. camera with 9x9 picture size. 
Figure 29 shows a cross section of the design in 
its present unfinished state. The design is prob¬ 


ably much more elaborate than is necessary to 
achieve the desired result. The lens described, 
however, shows promise of an extremely flat 
field free from astigmatism, and in addition 
a high state of correction for oblique aberra¬ 
tion. The symmetry of the design is very favor¬ 
able for excellent correction of the unsym- 
metrical aberrations, provided that some at¬ 
tempt is made to eliminate the oblique coma in¬ 
troduced by the first doublet. 


Lenses Developed for Night 
Photography 

Most of the night photographic work carried 
on in the war made use of flash bombs of in¬ 
creasing brilliance set off at the proper altitude, 
and of capping shutters activated by photo¬ 
electric pickup of the early light from the 
bomb. In the latter respect, difficulty was ex¬ 
perienced with exposures which were tripped 
and fogged by enemy searchlights. 

A number of lens forms were developed by 
NDRC for improvement in night photography. 
Standard equipment developed outside NDRC 
included mass production of a 7-in. //2.5 lens 
for 5x5, used with a focal plane shutter in the 
K-24 camera, and a 12-in. //2.5 lens for 9x9, 
used in the K-19 camera. An 8-in. focal length 
//1.5 lens designed at Eastman for the Air 
Forces, quite apart from NDRC, was produced 
in limited numbers by the Canadians and the 
British. Little use was made of this lens by the 
Army Air Forces, owing to a trend toward 
greater focal lengths. 

The 6-in., //1 Curved-Field Lens 12 
(U. of Rochester) 

One of the most promising lenses was a 6-in. 
f/1 lens covering a 40-degree field, which was 
developed at the University of Rochester. Ex¬ 
ceptional optical correction was made possible 
by designing the lens for a curved focal surface, 
nearly spherical, and equal in radius to about 
0.8 of the focal length. Although the curved 
field introduced mechanical problems, experi¬ 
ments made at the Institute of Optics of the 
University of Rochester under Contract 
OEMsr-160 proved that ordinary roll film could 























































58 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


be stretched into the curved focal surface back¬ 
ing-cup by means of applied air pressure up 
to 20 psi. It was discovered that film so 
stretched regains its shape and can be contact- 
printed in the usual way. Consequently, in spite 
of the curved field, a means has been found to 
provide very material improvement in night 
photography. It is highly recommended that 
future work exploit these possibilities to the 
full, particularly along lines of slightly moder¬ 
ated focal curvature for wide angular coverage. 

Figure 30 shows a cross-sectional view of the 
lens design. The form is based essentially on an 



Discussion. The 6-in. f/1 night lens should be 
completed at the earliest opportunity and tested 
at length. It is very probable that high-altitude 
night photography, particularly with flash 
units, will require lenses of this speed and 
quality. Since roll film can be used with the 
apparatus, no objection on the basis of maxi¬ 
mum convenience should be allowed to stand in 
the way of an important forward step in night 
resolution and coverage. 

The 7-in., //2.5, 5x5 Lenses for 
Night Photography 

Several lens types of slightly differing speci¬ 
fications were designed and constructed at 
Polaroid and Harvard to supplement and pos¬ 
sibly to improve the 7-in. //2.5 standard Aero- 
Ektar for the K-24 camera. In 1942, production 
of the Ektar lenses was behind the large Army 
schedules and behind the large production of 
camera units. At the request of the Army, 
NDRC undertook to provide some supplemen¬ 
tary lenses for the same purpose and to meet 
the production requirements temporarily until 


Figure 30. The 6-in. f/1 night lens with curved 
field. 

enlarged microscope objective, corrected for 
astigmatism. The use of all positive elements, 
especially of ordinary glass pairs for achro¬ 
matism, produces the large curvature of field 
but at the same time removes astigmatism and 
gains in net power. The rear positive elements 
tend to decrease both the zonal aberration of 
the front triplet and the secondary spectrum of 
the front achromatic triplet system. Both cor¬ 
rections are exceedingly good, and more than 
adequate for the purpose. 

Figure 31 shows about a 100 X enlargement 
of a star image, photographed at several field 
angles inside and outside focus and at the best 
focal setting, made with a 50-mm lens that was 
actually constructed as a pilot model. Over at 
least a 30-degree total field the images are small 
enough to get peak resolution from Tri-X night 
film at the contrasts likely to be encountered 
at night. Under the best conditions there is no 
reason why the night pictures should not be as 
sharp as day pictures, especially if shorter ex¬ 
posures or sweep mounts can be used. 


f/1 LENS FOCAL LENGTH = 50 MM X*5461A 
05443 INSIDE FOCUS 

20° 15° 10° 5° ON AXIS 5° 10° 15° 20° 



MICRONS 

Figure 31. Photomicrographs of star tests of 
the f/1 night lens with curved film. 

the standard lenses appeared. In the interest 
of quick design and production, tolerances on 
performance were therefore relaxed. 

None of the lens types for the K-24 camera 
developed under the Polaroid and Harvard con- 
















LENSES AND ASSOCIATED EQUIPMENT 


59 


tracts were actually put into production by the 
Army, owing mostly to later rapid strides in 
production of the standard lens. 

The 7-in., f/3, 5x5 Plastic Lens (Polaroid) , 13 
The design of this lens is shown in Figure 32. 
The lens is capable of high axial performance, 
and has a flat field in the neighborhood of the 



Figure 32. The 7-in. //3 plastic lens for 5x5. 


axis. Most probably, off-axis performance suf¬ 
fers from excessive astigmatism. All lens sur¬ 
faces are spherical in this design, and all ele¬ 
ments are of plastic. The off-axis astigmatism 
seems indicated also by resolution figures from 



190.5 MM = EFL 


Figure 33. The 7:5-in. //2.8 plastic lens for 5x5. 


The 7.5-in., f/2.8, 5x5 Plastic Lens (Polar¬ 
oid). 1 ^ The lens form is shown in Figure 33. 
This design has been developed by Polaroid as a 
means of countering the troublesome low in¬ 
dices of the plastic elements. It makes use of a 
DBC-1 nearly equiconvex positive element be¬ 
hind the stop. The negative rear element lens 
is intermediate between a field-flattener and a 
proper member of the lens barrel. In the posi¬ 
tion where it is placed, good use can be made of 
the element for optical corrections. 

Three models of the //2.8 lens system were 
made and delivered. After several adjustments, 
always necessary in a new design, the //2.8 lens 
was found to resolve 50 lines per mm on the 
axis, and 30 lines per mm 2.5 in. off axis. 

Extensive tests of the //2.8 Polaroid lens in 
comparison with the Aero-Ektar lens were car¬ 
ried out at Mount Wilson. In general the Polar¬ 
oid lens seems better than the Aero-Ektar near 
the optical axis, but in the outer part of the field 
its aberrations, chiefly tangential astigmatism, 
increase more rapidly than the Aero-Ektar 
residual errors. The differences in part are a 
choice of balancing of errors. If the Ektar lens 
were stopped down to //2.8 and redesigned for 
flatter central field at the expense of the corner 
performance, then its performance in the cen¬ 
tral region would be at least as good as that of 
the Polaroid //2.8 lens. On the other hand, it is 
doubtful whether the Polaroid lens could be bal- 



Figure 34. The 7-in. //2.5 plastic lens for 5x5. 


the Polaroid tests. The lens resolved 28 lines at anced for improvement of the corner images 
the center of the field, 13 lines at half field, and without decreasing the quality of the central 
3 lines at full field at 46 degrees total. region considerably below that of the Aero- 












































60 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Ektar lens, particularly if the aperture were 
increased to //2.5. The greater symmetry of the 
Ektar lens, and particularly the coincidence of 
the radial and tangential surfaces over most of 
the field, gives it a strong advantage for uni¬ 
form good performance. 

The 7-in., f/2.5, 5x5 Lens (Harvard-Polar- 
oid). 1 * In 1942, an early plastic design, as shown 
in Figure 34, was made up and tested at Wright 
Field. The front two surfaces and also the front 
surface of the field-flattener are aspheric. The 
lens is corrected accurately for spherical aber¬ 
ration at every zone. In addition, since Polaroid 
plastics were used, the secondary spectrum is 
approximately one-half of the usual amount. 
The design incorporates into a Schmidt-like 
lens system, with only two air-plastic surfaces, 
an aspheric field-flattener so shaped that the 
mean focal surface is flat over the entire field. 
The Harvard report states that the astigmatic 
difference is about 0.7 mm in the very corner, 
with the tangential curvature about 0.5 mm to¬ 
ward the lens, and the sagittal curvature about 
0.2 mm away from the lens. Decrease of the 
astigmatic difference toward the axis follows 
the fourth power of the field angle. Vignetting 
is very slight. Distortion is small. 

The prototype lens made in 1942 was experi¬ 
mental both in the sense of plastic molding and 
of aspheric work. The best resolution given by 
the lens on axis was 20 lines per mm, although 
in principle the resolution should have been 
limited only by diffraction. No satisfactory 
aerial pictures were obtained with the first 
model, and the project was dropped. 

Discussion. Of the plastic lenses only the 
//2.8 seems worth further consideration. The 
subject of plastic lenses has been viewed in so 
many ways that one cannot here make a dis¬ 
tinction between plastic and glass. Should plas¬ 
tic lenses be required, however, it would appear 
from the above that the //2.8 deserves further 
attention. 

The 7-in., f/2.5, 5x5 Lenses (Harvard) . 14a A 
series of 7-in. //2.5 lenses was designed and 
constructed by the Harvard laboratory, for use 
with the K-24 camera, during 1942 and early 
1943. The first three models (A, B, and C) were 
constructed of ophthalmic glass for the purpose 
of maximum production, and for this purpose 


were designed with the shallowest possible 
curves for adequate performance. A fourth 
model (D) of optical glass was designed and 
made up during the winter of 1942 to 43. This 
design came nearer to production than any of 
the other //2.5 lenses. Indeed, glass was ordered 
for 1,500 units, but later the order was reduced 
to 88 units, made up at Harvard. 

Figure 35 shows the lens forms of design D. 
The third and fourth are modeled on the Biotar 
type, which forms the basis of many modern 
lens forms. The Biotar design, although capable 
of great speed, is afflicted by higher order astig- 



Figure 35. The 7-in. f/2.5 night lens for 5x5, 
model D. 


matism and oblique spherical aberration of con¬ 
siderable amount. The Ektar 7-in. lens is re¬ 
lated to the Biotar form, except that the use of 
rare-earth glass has made it possible to use 
thinner elements around the stop, which in 
turn reduces the higher order astigmatism dif¬ 
ference. Oblique spherical aberration in the 
Ektar is large and a major factor in limiting 
the performance of the lens. Curvature of field 
of the Ektar is also large and a definite limita¬ 
tion on its performance. 

Like the Polaroid lenses, the Harvard f/2.5 
lenses are afflicted with higher order tangential 
astigmatism as a consequence, partly, of the 
Army request for the shallowest possible 
curves for maximum production. The Harvard 
report 14 states that the oblique spherical aber¬ 
ration of models C and D is smaller than in 
the case of the Aero-Ektar but that the astig¬ 
matic difference is larger. The mean focal sur- 





















LENSES AND ASSOCIATED EQUIPMENT 


61 


face of the Harvard lens is flatter than the 
mean focal surface of the Ektar, but on this 
surface the optical resolution is inferior. The 
reduced oblique spherical aberration actually 
tends to accentuate the astigmatism, with the 
net result somewhat lowered resolution. The 
shallow curves result in a central image of 
great purity in Harvard models C and D. 

The Harvard lenses model C and D present 
a novel form of construction in that the rear 
element consists of a high-index flint glass. The 
use of a highly dispersive flint glass for a posi¬ 
tive element introduces a considerable amount 
of color which is overcome by a hyperchromatic 
doublet after the stop. The high index of the 
flint positive element, in spite of the color- 
correcting flint negative element, produces a 
net gain in the Petzval sum. Since the Petzval 
sum of the lens system is still fairly large, the 
overall result is very shallow curves and an ex¬ 
cellent central correction. 

One other Harvard //2.5 lens design was sub¬ 
mitted, but by this time no further production 
of //2.5 lenses was contemplated. This lens de¬ 
sign attempted to overcome some of the aberra¬ 
tions of the earlier shallow-curve models, but 
requires further study. Heavy-index glasses 
were used for the purpose of .reducing the 
hitherto too large Petzval sum, but at a cost of 
rather complicated construction. It is probable 
from the performance figures that further de¬ 
sign work might convert the lens into a well 
corrected system. The use of high-index flint 
glass for long focal length lenses is attended 
by large absorption. Most probably, better per¬ 
formance can be achieved from an overall point 
of view by means of the high-index white 
glasses now available and by close attention to 
optimum image structure. 

Figure 36 reproduces this lens form. Note 
that the greater symmetry around the stop is 
likely, although not sure, to lead to better cor¬ 
rection at large field angles. The unsymmetri- 
cal doublet at the front of the barrel leads to 
comatic flare off axis, but, as described in the 
introduction, the resulting performance is 
likely to be better than if only astigmatism 
were retained. The standard Tessar makes sim¬ 
ilar substitution and achieves good perform¬ 
ance with large coverage. 


No laboratory tests are available on the vari¬ 
ous Harvard //2.5 lenses except those made at 
Wright Field. Briefly, for model D, tangential 
resolution at high contrast starts at 53 lines per 
mm on axis, decreasing gradually to 10 lines 
per mm, and then increasing once more to 24 
lines per mm. The radial resolution, owing to a 
slight error in centering, begins at 42 lines per 



mm on axis in the model tested, falls slowly to 
about 20 lines per mm at 22.5 degrees off axis, 
and then decreases very rapidly. At the usual 
low contrast of night photography, such resolu¬ 
tion is probably adequate for general use, but 
not good enough for future specialized work. 

Discussion . Although the Rochester f/1 lens 
seems the most likely to provide spectacular im¬ 
provement in future night photography, it 
seems well worth while to continue develop¬ 
ment of faster and better conventional lenses. 
It is believed that none of the //2.5 lenses de¬ 
scribed above has exhausted all possibilities, 
particularly should new optical materials be¬ 
come available. 

As a goal in the near future, the Air Forces 
might well request bids on an //2 lens of image 
quality at least as good as that obtained at pres¬ 
ent from the standard //2.5 Ektar. Specifica¬ 
tions should be written around an effective f/2 
speed in order that superior image quality may 
not be obtained at the expense of transmission 
losses. For some time to come, it would appear 
that a 12-in. focal length f/2 lens, with 9x9 cov¬ 
erage, is a worth-while goal. The use of a field- 
flattener should not be ruled out, provided the 
field-flattener is more than 1 in. away from the 





















62 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


film. The use of a curved backing-plate, how¬ 
ever, would be preferable. 


1 ' 3 ' 3 Lenses for Specialized Purposes 
(Harvard) 

Telephoto Scout Camera, 48-in., //8, 
3 1 4 ) x4% Lens™ 

In 1945, need developed in the Services for 
some means of recording by hand in the air 
camera objects which are of interest as seen 
through binoculars. This suggested the develop¬ 
ment of a combined binocular-camera arrange¬ 
ment, boresighted to record on 3%x4% picture 
size approximately the field of view of the 
binocular. In the model planned by Harvard, 
a 48-in. focal length f/8 telephoto lens was to 
be used. The field was to be somewhat smaller 
than that of the binocular in order to keep the 
overall size within reasonable limits. It was 
felt that the recording of as much detail as 
could be seen on selected objects with 10x50 
binoculars might be achieved with a 48-in. lens 
and that this was much more important than 
covering the whole field. 

Figure 37 shows a cross-sectional view of the 
proposed scout camera. No detailed ray-tracing 
calculations are available other than computa¬ 
tions showing that the total inward radial co- 
matic flare in the corner of the picture amounts 
to about 0.1 mm. In view of the f/8 speed, it is 
probable that a comatic extension of this slight 
magnitude will cause no great deterioration in 
resolution or contrast. 

The lens form is modeled after the //6.3 
Zeiss-Telikon by substitution of the American 
glasses most nearly similar. The 4-element tele¬ 
photo lens of this form is extremely sensitive 
to variations in curves, indices, and lens thick¬ 
nesses, more so than one would suppose from 
the speed rating. Consequently, adaptation of 
the Telikon lens to 1945 American glasses re¬ 
quired almost as much redesign as if started 
from the beginning. 

The lateral color has been reduced to second¬ 
ary spectrum in the lateral color, with mini¬ 
mum magnification at the D line. Field flatness 
and astigmatism are optimum for the size of 
picture adopted. Vignetting is small. It is evi¬ 


dent from the design figures of the Harvard 
report that the resolution of the scout camera 
over the field can be expected to be 20 or more 
lines per mm in the air, provided exposure 
times as short as % 50 sec can be used. 

Recommendations. The 48-in. focal length 



scout camera may be too large for general util¬ 
ity. It would seem worth while to design a 24- 
in. model for the same picture size, in order 
that the full field (7 degrees) of the binoculars 
be covered. The Telikon lens form, when 
worked to its limit of performance, should be 
capable of providing such an angular coverage 
with sharp resolution and good contrast. 

Composite f/8 Triplet Lenses for the 
K-18 Camera™ 

For use with the aerial testing program un¬ 
der the Harvard contract, a lens design for a 
Cooke triplet of small angular field and high 
quality was developed. This design was to be 
made up in 48-, 24-, 12-, and 6-in. focal lengths. 
All of these lenses were to be mounted in a 
single rigid assembly and used with a single 
focal plane shutter and A-7, 9x18 magazine. 
Each lens was to be of essentially perfect qual¬ 
ity across the chosen area of film assigned to it. 
It was believed that such a test-lens assembly 






































LENSES AND ASSOCIATED EQUIPMENT 


63 


could lead to exact evaluation of the influence 
of haze on resolution in the air, as well as the 
practical dependence of resolution on focal 
length. Difficulties of proper focal setting for 
each focal length were to be overcome by ther- 
mostating and by focus runs in the air. 

Figure 38 shows a schematic view of the con¬ 
templated system. In order to retain the ther- 


printing equipment at 1.5X enlargement with 
condenser illumination should be designed and 
constructed. The end of World War II pre¬ 
vented much progress along these lines. Later 
work has indicated that the standard printer 
yields a resolution of 60 lines/mm on contact 
printing of an extremely fine, high contrast 
target. 


:=f 


LL 


—id 


Figure 38. An //6 test system. 


mostated heat, the front of the assembly was 
to be covered by a plane-parallel window of 
optical quality. 

The triplet design adopted is optimum for 
good axial correction and small angular field of 
high quality. The relatively long barrel length 
leads to large tangential astigmatism far off 
axis, but for the present problem the higher 
order field errors can be ignored. Exact ray¬ 
tracing curves are provided on axis and in the 
corner of the restricted field. These curves are 
of nearly identical shape and are very close to 
the Rayleigh limit. It was planned that any 
residual flare of spherical aberration in the 48- 
in. //6 lens would be figured out of the lens. 

Lens for Projection Printing 7 * 

The flight test program at Harvard under 
Contract OEMsr-474 indicated a need for bet¬ 
ter printing of high-resolution negatives. Ex¬ 
periments made with contact point-source 
printing and contact diffuse printing indicated 
that the best negatives lose a considerable 
fraction of their resolution and contrast in the 
printing process, especially on the standard 
Army printer. It was concluded that projection 


Because of the resolution possible with pro¬ 
jection lenses, especially at one-to-one ratio, 
or at small enlargement, it would seem that 
much of the negative resolution can be pre¬ 
served in the print. The Harvard plan was to 
enlarge by 1.5X in order to remove most of the 
tendency of the printing paper to limit resolu¬ 
tion. The plan should be pursued in further 
work. 

36-in., //11, 9x18 Apochromatic Telephoto 815 

Following the completion of the 36-in. //8 
telephoto for the K-18 camera, tests at Wright 
Field on color film proved that the telephoto 
with its correction at D was not suited for 
color work. Calculations were begun at Har¬ 
vard with the view to designing an apochro¬ 
matic 36-in. lens, making use of either barium 
fluoride or calcium fluoride. The calculations 
were only provisional. Paraxial color calcula¬ 
tions indicated that a high degree of color cor¬ 
rection can be obtained, even in a telephoto 
construction. The Harvard report states that 
an apochromatic telephoto at //11 can be de¬ 
signed when needed. This design would follow 
along the lines of the 4-element Telikon, and 



























64 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


would use two elements of one or the other 
crystal. In order to protect the leading crystal 
it would probably be necessary to enclose the 
entire lens system by means of a front win¬ 
dow. 

12-in., //4.5, 9x18 Anastigmat for the 
K-18 Camera 7 * 

The need for maximum coverage and scale 
led to an Army request for a 12-in. lens of su¬ 
perior resolution for the K-18 camera. A low 
priority on an already overcrowded program 
permitted only exploratory calculations to be 
made at the Harvard laboratory. Computations 
made on the performance of the wide-angle 
Metrogon and Hasselkus lenses made prospects 
for improved performance at //4.5 seem very 
poor indeed, in spite of the reduced angular 
coverage. No lens design was submitted by 
NDRC, although further work along these lines 
should be continued. In 1945, Bausch and Lomb 
delivered to the Army a 12-in. Metrogon rede¬ 
signed for the smaller angular field. Conse¬ 
quently, the need of the Air Forces for wide- 
angle photographs of this scale was largely 
relieved. 

24-in., //3.5, 9x18 Lens for 
Night Photography 711 

Exploratory calculations were made at Har¬ 
vard on possible lens designs for the above spec¬ 
ification. It seemed from the calculations and 
from consideration of the problem that second¬ 
ary spectrum would prove to be a rather seri¬ 
ous limiting factor in the performance. The use 
of a lens at night without filter and of such 
aperture serves to accentuate the secondary 
spectrum trouble, which reduces contrast and 
peak resolution. The 24-in., //3.5, 9x18 lens was 
thought to crowd to the practical limit what 
can be accomplished with lenses not corrected 
for secondary spectrum, particularly for ap¬ 
plications where suitable filters cannot be used. 
A discussion is given in OSRD Report No. 
6028 7 on the effect of aperture and speed com¬ 
promises in lenses for night photography, and 
the relationships to final resolution. 

Calculations made during the course of the 


investigation led to the proposal of the apo- 
chromatic 100-in. lens design described earlier 
in this chapter. However, the speed of this 
apochromatic arrangement cannot be extended 
much beyond //8, and would not suffice for 
night photography. 


13 4 Wide-Field Designs of High Speed 

A number of interesting designs are reported 
under Contract OEMsr-1078 by the Yerkes Ob¬ 
servatory. 15 Although only a few might con¬ 
ceivably be used in aerial photography, it is 
important to bring these systems to the atten¬ 
tion of those who are interested in very fast 
systems. There is insufficient space here to 
cover more than the underlying principles of 
these designs. 

In May 1944, a paper was published by 
Maksutov 10 in which he describes the theory 
and presents computations of various lens-mir¬ 
ror systems making use of achromatic menisci 
to be used as correcting lenses for mirror ar¬ 
rangements. These menisci, in general, pro¬ 
duce aplanatic, color-corrected mirror systems 
with speeds of the order of f/1 or slower, and 
fields of a moderate number of degrees, limited 
for practical purposes by curvature, and op¬ 
tically limited by astigmatism. In particular, 
one of Maksutov’s systems made use of a 
meniscus with concentric surfaces, concentric 
also with a spherical mirror. Such a system 
operates equally well at all field angles. Its 
speed is limited only by considerations of zonal 
aberration, and longitudinal color. 

Independently of Maksutov, the Yerkes 
workers developed a large variety of optical 
systems based on the fundamental idea of con¬ 
centric surfaces. Such systems have the great 
virtue from the point of view of the designer 
that they require only axial ray-tracing and 
design; but they are necessarily always afflicted 
with curved field concentric around the same 
point. 

The Yerkes report presents a large number 
of variations of this principle, including de¬ 
partures from symmetry, mostly for the pur¬ 
pose of color correction, both for all-air sys¬ 
tems, and for air-glass systems. In the latter 



LABORATORY TESTING 


65 


case, the film rests against glass in the focal 
surface. 

One of the most interesting of the systems is 
presented in Figure 39. This scheme can be 
used for high-speed wide-angle photography. 
It should be pointed out, however, that if curva¬ 
ture of field of the extent inherent in such sys¬ 
tems (generally of radius equal to the focal 



Figure 39. A concentric mirror-meniscus sys¬ 
tem. 


. length itself) be allowed for lens-type systems, 
the latter can compete on rather favorable 
terms at speeds of f/1 or slower. Therefore, it 
would seem expedient to recommend that mir¬ 
ror combinations be considered only where the 
speed exceeds f/1, and where the angular field 
does not exclude an important part of the aper¬ 
ture. 

One of the most common troubles experi¬ 
enced with mirror systems is the blocking out 
of a considerable part of the light by the pres¬ 
ence of a secondary mirror or by the photo¬ 
graphic film. Very often, in the presence of 
residual aberrations, the important central 
peak of light (see discussions in the introduc¬ 
tion, Section 1.1) is eliminated, leaving only 
the aberrations. It is evident that such systems 
cannot yield very sharp resolution. Conse¬ 
quently, it is important that in all systems 
where the central core of the image is blocked 
away, the residual aberrations be very small in¬ 
deed. One cannot use effectively a blurred image 
with a hole in the center. Out-of-focus pictures 
are still more unsatisfactory. 


14 LABORATORY TESTING 

141 Introduction 

The Mount Wilson Observatory (Contract 
OEMsr-101) and the Eastman Kodak Company 
(Contract OEMsr-392) carried out lens tests 
on both NDRC prototypes and other lenses of 
American and English manufacture, in the 
laboratory and in the air. 

In addition, many laboratory and flight tests 
were carried out at Wright Field and again at 
Harvard (Contract OEMsr-474). Contact was 
maintained throughout World War II with 
other testing laboratories, such as the Royal 
Aircraft Establishment at Farnborough, Eng¬ 
land, and the National Research Council at 
Ottawa, Canada. Great benefits were derived 
from these several contacts, and particularly 
from Wright Field, where many years of ex¬ 
perience prior to World War II opened the way 
for the extensive wartime testing there, and 
elsewhere. 

The present section will limit discussion to 
laboratory testing as conducted under NDRC. 


Table 4. Lens tests at Mount Wilson. 


No. 

Focal 
length 
Name (in.) 

Speed 

Cover¬ 

age 

Maker 

1 . 

Cooke Aviar 

20 

//5.6 

7x8 

Taylor, Taylor 

2. 

Ross survey 

20 

//6.3 

7x8 

and Hobson 
Ross 

3. 

Pentac 

8 

// 2.9 

5x5 

Dallmeyer 

4. 

Aero-Ektar 

7 

// 2.5 

5x5 

Eastman 

5. 

Aero-Ektar 

24 

//6.0 

9x18 

Eastman 

6. 

Ross Astro 

28 

//7.0 

9x9 

J. W. Fecker 

7. 

Plastic 

8 

//3.0 

5x5 

T. S. Curtis 

8. 

Telestigmat 

40 

//8.0 

9x9 

Bausch and 

9. 

Apochromat 

36 

//8.0 

9x9 

Lomb 

Harvard 

10. 

Aerostigmat 

12 

//5.0 

9x9 

Eastman 

11. 

Plastic 

7.5 

//2.8 

5x5 

Polaroid 

12. 

Telestigmat, 

special 

40 

// 8.0 

9x9 

Bausch and 
Lomb 

13. 

Anastigmat 

100 

//10.0 

9x18 

Harvard 

14. 

Wide-angle 

5.95 

//3.5 

120° 

Harvard 


Considerations of flight testing will be deferred 
to Chapter 2, except where laboratory testing 
attempts to duplicate the conditions in the 
air. 














66 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Testing at Mount Wilson 

Observatory 4 "' 9b ’ 17 ’ 18 ' 19 

During World War II the lenses listed in 
Table 4 were tested at Mount Wilson. The large 
range of focal lengths required that careful 


is that of a continuous spectrum with super¬ 
posed mercury lines. With a yellow filter, the 
spectral distribution approximates that of sun¬ 
light through the same filter. 

In order to provide for a uniformly illumi¬ 
nated slit in either vertical or horizontal direc- 



Figure 40. Principal parts of the Mount Wilson optical bench. 


attention be given to details of the optical bench 
and auxiliary equipment. 

Test equipment at Mount Wilson consisted of 
a specially constructed optical bench shown in 
Figure 40. Lenses as large as the 100-in. //10 
anastigmat were accommodated on the nodal 
slide. Early in the testing program (lenses No. 
1 to 7), the light source used was a small pin¬ 
hole at a distance of 100 ft (No. 1 to 3) and 
225 ft (No. 4 to 7), where the path was doubled 
by means of a large plane mirror. Illumination 
was provided by mercury arc, a headlight bulb, 
and a monochromator as needed. 

For the later tests, a General Electric H6 
high-pressure mercury lamp was added to the 
equipment for improving the illumination. The 
light distribution from this high-intensity lamp 


tion, the following setup was made. Light pro¬ 
ceeded vertically upwards from the capillary 
of the mercury lamp to a 45-degree mirror, and 
was there directed horizontally to the slit. Ordi¬ 
nary condenser lenses were used to form the 
image of the capillary on the slit. Rotation of 
the light source caused its image through the 
tilted mirror to rotate also. Thus, either a hori¬ 
zontal or a vertical slit could be used as needed. 
Finally, a rotating square prism was used for 
the purpose of moving the image of the light 
source back and forth on the slit to obtain uni¬ 
form illumination. 

Early in the Mount Wilson testing program, 
use was made of photomicrography for the de¬ 
termination of the image characteristics of the 
lens alone. Later, the testing procedures found 



LABORATORY TESTING 


67 


most informative were adopted and carried 
through systematically. Photomicrography was 
replaced by visual measures of resolution and 
by direct photography in the focal plane on 
both Super-XX and Microfile film. 

Test of a 40-in., // 8 Bausch and Lomb 
Telephoto Lens (Improved Prototype) 40 

The many lens tests made at Mount Wilson 
cannot all be fully described, but it is expedi¬ 
ent to reproduce the results of a typical test. 
Figures 41 to 44 contain most of the pertinent 
information, and require only a few additional 
comments. 



Figure 41. Astigmatism and distortion of 
Bausch and Lomb 40-in. telephoto lens (improved 
prototype). 


have his judgment swayed by superlative visual 
resolution. Moreover, as described in the intro¬ 
duction (see Section 1.1), the best visual image 
is not identical with the best photographic 
image in regard to optimum distribution of 
light in the core of the image. 

The testing results on the improved Bausch 
and Lomb //8 telephoto lead to the following 
conclusions. 

Astigmatism and Field Curvature. The re¬ 
sults are shown in Figure 41. The asymmetry 
of the curves is due to imperfect internal colli- 
mation. The astigmatic images at large field 


£ w o> 

O o WAVELENGTH § 

O o o 



FIELD ANGLE 



ZONE RADIUS 


The data on visual resolving power requires 
careful interpretation. Filter differences, and 
the phenomenal ability of the eye to distinguish 
resolution at low contrast can be very mislead¬ 
ing. For example, if one sets up an ordinary 
condenser lens to form the image of a standard 
3-line target, it is entirely possible to make 
readings of even 100 lines per mm, even though 
almost no light falls in the fine lines observed, 
and even though the chromatic and spherical 
aberrations are several millimeters in diame¬ 
ter. 

It is probable that the more closely the lens 
approaches perfect correction, the more safely 
the visual observations can be used as a basis 
for evaluation. Visual measures are useful, but 
they provide only a necessary but not a suffi¬ 
cient condition. The designer cannot afford to 


0.999f 
I.OOOf 
1.001 f 
1.002 f 
1.003 f 
1.004 f 
1.005 f 
1.006 f 


Figure 42. Color curve and spherical aberra¬ 
tion of Bausch and Lomb 40-in. telephoto lens 
(improved prototype). 

angles were very asymmetrical and the accu¬ 
racy of the measures of the astigmatic foci was 
therefore comparatively low. Astigmatism and 
field curvature are relatively small up to about 


o p ? p p o 
















RESOLVING POWER IN LINES PER MM 


68 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 



1 CONTRAST 2 3 


Figure 43. Visual resolving power versus contrast. 





Figure 44. Photographic resolving power ver¬ 
sus contrast. 


5 degrees from the axis (3.5 in. from the center 
of the film). Their variation with wavelength 
between 5,000 and 6,700 A is not large. But the 
results of resolving-power tests with infrared 
film show that both astigmatism and field cur¬ 
vature increase very rapidly in the infrared 
region. 

Distortion. The distortion (see Figure 41) 
has been measured with monochromatic light, 
since lateral chromatic aberration was notice¬ 
able. The distortion, which varies approxi¬ 
mately with the third power of the field angle, 
is unusually large. 

Chromatic Aberration. The color curve (see 
Figure 42) has its flat part at about 6,600 A. 
Obviously, the use of panchromatic film with a 
light red filter transmitting above 5,800 A is 
desirable, but the flat part of the color curve 
is at a wavelength somewhat longer than that 
which would give the best results even with 
this combination. 

The chromatic difference of magnification is 
noticeable but not excessive. As a measure of 
the chromatic difference of magnification, the 
length of the spectrum between 5,000 and 
6,500 A has been plotted in Figure 42, positive 
values indicating that the red image was at a 
larger distance from the center than the blue 
image. 

Spherical Aberration. The spherical aberra- 
















LABORATORY TESTING 


69 


tion (see Figure 42) shows considerable chro¬ 
matic variation of a kind which suggests 
strongly the use of a light red filter. The spheri¬ 
cal aberration seems to be smallest for wave¬ 
lengths somewhat too far to the red for the use 
of panchromatic film. 

Resolving Power . The targets were illumi¬ 
nated with tungsten “daylight” lamps. Visual 
resolving powers for the center of the field are 
shown in Figure 43. They were determined both 
with a yellow filter transmitting above 5,200 A 
and with a red filter transmitting above 
5,800 A. There was a distinct difference between 
the best focal setting for high and low contrast; 
the focal setting for low contrast was smaller 
by 0.6 mm with the yellow filter and by 0.2 mm 
with the red filter. The results for each focal 
setting have therefore been plotted separately. 
As might be expected from the state of the 
chromatic corrections, the resolving power with 
red filter is better than with yellow filter. 

Photographic resolving powers were deter¬ 
mined on Super-XX 35 mm film with yellow 
filter and with red filter. Development was for 
10 min in D-19 at 68 F. Since the color curve 
and the chromatic variation of the spherical 
aberration suggested good performance in the 
infrared, resolving powers were determined 
also on 35-mm Infrared Aero Panchromatic 
film (using the same development) with a dark 
red filter transmitting above 6,700 A. No focal 
difference between high and low contrast was 
found for the photographic resolving power. 
The results for high contrast (2.15) are 
plotted in the figures; the results for low con¬ 
trast are somewhat less complete, some of the 
extrafocal settings at large field angles giving 
low resolving powers beyond the limit of the 
targets. 

For the best focus in the center of the field, 
the dependence of resolving power on contrast 
and on field angle is shown in Figure 44. The 
best performance is obtained on Super-XX film 
with red filter. The resolving power with this 
combination for high-contrast targets in the 
center of the field is about 85 per cent of the 
theoretical maximum. The performance on in¬ 
frared film in the center of the field is almost 
equally good, but the resolving power decreases 
rapidly with increasing distance from the cen¬ 


ter. The pronounced asymmetry of the curves 
for resolving power vs field angle, due to im¬ 
perfect internal collimation, makes it difficult 
to assess properly the performance for larger 
field angles. 

The objective is definitely much improved 
compared with the Bausch and Lomb Telestig- 
mat //8 tested earlier, 18a and in a better state 
of collimation would probably be one of the best 
long focus objectives. 

General Conclusions from the 
Mount Wilson Tests 

The NDRC lenses proved to be on the whole 
an improvement over commercial types, partly 
because of a better color correction and partly 
because the lenses were designed for precision 
use. The improved commercial designs tested 
would, however, definitely take aerial pictures 
of high quality. It is believed that the NDRC 
lenses also represent a considerable improve¬ 
ment in the air from a practical point of view, 
since they are mounted in steel and are there¬ 
fore relatively insensitive to thermal changes. 
The 36-in. fluorite apochromat was later ther- 
mostated in order to remove change of focus 
caused by the fluorite itself. The improvements 
achieved in the NDRC lenses were at the ex¬ 
pense of more rigorous tolerances, and were 
unsuited to mass production. However, for pre¬ 
cision reconnaissance these lenses are to be 
recommended. 

It is believed that thorough tests of micro¬ 
scopic contrasts would show that the NDRC 
lenses are superior to most commercial lenses 
and, in accordance with the principles outlined 
in the introduction (see Section 1.1), would 
yield truer tonal values at all levels of exposure 
than most commercial lenses. The wartime ex¬ 
perimental lenses manufactured by several com¬ 
panies represent also a definite improvement 
over the standard lenses. 

1 - 4 - 3 Testing at the Eastman Kodak 
Company 20 

A very considerable number of laboratory 
tests were made at the Eastman Kodak Com- 




70 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


pany under Contract OEMsr-392. Table 5 lists 
the lenses tested on a comparative basis. 

The Eastman tests represent a lengthy con¬ 
trolled series employing the greatest range of 
variables yet attempted. In addition, composite 
results are presented by actual area of coverage 
instead of by field angle. Consequently, it is 
important to reproduce as much of the large 
volume of data here as feasible. 


Table 5. Lens tests at Eastman Kodak Company. 


No. 

Name 

Focal 

length 

(in.) 

Speed 

Cover¬ 

age 

Maker 

1 . 

Wide-angle 

5.95 

//3.5 

120° 

Harvard 

2. 

Plastic 

7.5 

//2.8 

5x5 

Polaroid 

3. 

6-H-62 

24.0 

//6.0 

9x18 

Eastman 

4. 

8-J-35 

24.0 

//8.0 

3x3 

Eastman 

5. 

56-M-56 

24.0 

// 5.6 

9x9 

Eastman 

6. 

Apochromat 

36.0 

//8.0 

9x9 

Harvard 

7. 

Wide-angle 

telephoto 

36.0 

//8.0 

9x18 

Harvard 

8. 

8-T-87 

36.0 

//8.0 

9x18 

Eastman 

9. 

Telephoto 

40.0 

//5.0 

9x9 

Harvard 

10. 

Anastigmat 

100.0 

//10.0 

9x18 

Harvard 


Image-Energy Distribution Measurements 

This testing procedure is known as the wedge 
method for measuring the distribution of light 
across the image formed by a lens of a distant 
collimated line-source. As originated and used 
at Eastman, an illuminated slit 0.002 in. wide 
by 2 in. long is imaged with the lens under test 
at the object plane of a microscope. This image 
is in turn reimaged by the microscope on a 
colloidal carbon wedge, orientated with its slope 
parallel to the line image. Directly behind and 
in contact with this wedge is placed a fast neg¬ 
ative panchromatic film. The wedge pattern 
so obtained is printed on Kodalith orthochro- 
matic film and processed in a Kodalith devel¬ 
oper for two minutes. The positive image is 
then printed on A-3 paper, which is processed 
in D-72 (2/1) for 45 sec. The contour of the 
wedge pattern is, then, a plot of log energy 
distribution against distance from the image 
measured along a line that is mutually perpen¬ 
dicular to the optical axis and to the line image. 

Figures 45 to 55 reproduced from the East¬ 
man report 20 show some of the energy-distribu¬ 
tion patterns for the ten lenses, tested under a 


variety of circumstances. It is of interest to 
comment on unusual features of the tests. 

The vertical scale is sufficiently constant be¬ 
tween the many pictures to serve as a ready 
basis of comparison. The horizontal scale dif¬ 
fers widely, however, so that the reader must 
judge the apparent linear sharpness on the 
basis of the image size scale at the bottom of 
each picture. Thus, the sharpest line of all is 
that given by the wide-angle //3.5 lens of Fig¬ 
ure 45. The apparent doubling of the tangential 
image in Figure 45 at large off-axis distances 
indicates that the lens is not in adjustment 
after its several trips across the country. Both 
Harvard and Mount Wilson results, as well as 
the nature of the design with its spherical sym¬ 
metry, show a clean tangential image far off 
axis. A break in the cement at the center of 
the lens would not only explain the double 
image but also the low intensity of the image. 
When first assembled, there was no apparent 
difference in image quality in the radial direc¬ 
tion anywhere over the 360-degree field. 

The wedge patterns for the Polaroid 7-in. 
//2.8 lens are enlarged (see Figure 46) by a 
factor of four as compared with the patterns 
for the standard Aero-Ektar. In photographic 
performance, overlooking the large difference 
in scale, the Polaroid plastic lens should give a 
photograph of the same order of quality as the 
widely used Aero-Ektar 24-in. lens, at least in 
the center of the field. The absence of sharp 
peaks in the images formed by both lenses in 
the outer parts of the field means reduced 
resolution. 

Visual tests of the 36-in. //8 apochromat 
indicated the presence of uncorrected violet in 
the nature of a tertiary spectrum rapidly in¬ 
creasing with the shorter wavelengths. More¬ 
over, since the design makes use of only light 
crown glasses and fluorite, the lens is unusually 
transparent to ultraviolet light down to 3,500 A. 
It is not surprising, therefore, that Figure 50 
shows a sharp central peak surrounded by con¬ 
siderable out-of-focus flare, since the heaviest 
filter used transmitted the near ultraviolet. 
Figure 51, showing the same test through a 
Wratten No. 12 filter, indicates that the 
apochromatic correction of the lens has been 
well achieved. 







lOOIo 



45° 

TANGENTIAL 



i 


i 



45° 

RADIAL 


1 


60° 

TANGENTIAL 

ii, 

ill 

iU 

J. A 

it. A 

60° 

RADIAL 

-0.45 

-0.30 

A 

-0.i5 

i 

0.0 

+0.15 




DISTANCE IN MM 

FROM BEST VISUAL 

AXIAL FOCUS 


0 0.05 0.1 MM 
IMAGE SIZE 


Figure 45. Harvard College Observatory 6-in. //3.5 wide-angle lens. 


4 1 


+0.30 


100 Io 

0° lOIo 
Io 




A A 



10 ° 

TANGENTIAL 

10 ° 

RADIAL 



▲ A 





15° 

TANGENTIAL 

15° 

RADIAL 






20 ° 

TANGENTIAL 

20 ° 

RADIAL 




- 0.6 


0 0.1 0.2 MM 

IMAGE SIZE 


- 0.4 - 0.2 0.0 + 0.2 

DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 

LENS ATf/2.8 


Figure 46. Polaroid 7-in. //2.8 aerial camera lens. 



+ 0.4 


71 







100 I 


-)° 10 Io 

h 


JL 4 4 4 4 4 


10 

TANGENTIAL 

10 ° 

RADIAL 


15 

TANGENTIAL 

15° 

RADIAL 


23 

TANGENTIAL 

23° 

RADIAL 



L 

1 1 



“ 1.2 


4 4 4 4 

4 4 4 

k 4 4 4 4 


14 4 4 


- 0.6 


0.0 


+0.6 


+ 1.2 


100 lor 
0° io I 
I 


8 ° 

TANGENTIAL 

8 ° 

RADIAL 


i i i_i—1— 


DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 
0 0.2 0.4 0.6 MM LENS AT f/6 YELLOW 12 FILTER 

IMAGE SIZE 

Figure 47. Kodak 24-in. f/6 Aero-Ektar EM-170 6-H-62. 


A 

A 


i 


i i 

1 1 
I i 


i 

i 


I 


k L L 


1 


+ 1.8 


10° 

TANGENTIAL 

10° 

RADIAL 


i 


1 i 

i 


A 

i 1 


I 


I 


15 

TANGENTIAL 

15° 

RADIAL 


- 0.8 

0 0.2 0.4MM 

IMAGE SIZE 


-0.4 


1 i 


0.0 


+0.4 


i 


+ 0.8 


+ 1.2 


DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS AT f/8 

LENS AT f/8 


Figure 48. Kodak 24-in. //8 aerostigmat formula 8-J-35. 


72 







0 


I 00 lor- 


A 


10 I, 


A A 


TANGENTIAL 

5° 

RADIAL 


TANGENTIAL 

10° 

RADIAL 


TANGENTIAL 

!5° 

RADIAL 


- 1.0 


A 


A 


k i A A 
A 1 A A 


1 


A 


1 


l 


A 


i 


i 


1 


-0.5 0.0 +0.5 +1.0 H- 1.5 

DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS AT f/5.6 

LENS AT f/5.6 


0 0.2 0.4MM 
IMAGE SIZE 

Figure 49. Kodak 24-in. f/5.6 Aero-Ektar formula 56-M-56. No. EE0001-3. 


1001 , 
o° io r 0 - 

Io 

5° 

TANGENTIAL 

5° 

RADIAL 

7.5° 

TANGENTIAL 

7.5° 

RADIAL 


A 


A i 


A i 1 
A i i 


i 


A 


i 


1 1 


A 

1 


Alii 


k 


i 


10 

TANGENTIAL 

10 ° 

RADIAL 


-1.8 MM 


i 1 


- 1.2 

0 0.2 0.4 MM 
AERIAL IMAGE 


- 0.6 


I 


0.0 


1 


0.6 


DISTANCE FROM BEST VISUAL AXIAL FOCUS 


Figure 50. Harvard College Observatory 36-in. //8 daylight lens F-3. 


A 


1.2 


73 






•1001, 


0° l0L 
Ic 



k 


i 


i 


i 


5° 

TANGENTIAL 

5° 

RADIAL 





4 


4 


4 


1 


I 


I 


4 


1 


4 


7.5° 

TANGENTIAL 

7.5° 

RADIAL 


1 



1 


4 





k 


4 




10° 

TANGENTIAL 

* 4 

A 

A 

4 

4 

10° 

RADIAL 

4 4 

-1.8 MM -1.2 

1 

-0.6 

1 

0.0 

i 

0.6 

1 

1.2 


6 02' 0.4 MM DISTANCE 

AERIAL IMAGE 

FROM 

BEST VISUAL AXIAL 

FOCUS 



Figure 51. Harvard College Observatory 36-in. //8 daylight lens F-3 plus No. 12 Wratten filter. 


lOOIo 



5° 

TANGENTIAL 


▲ 

A 


5° 

RADIAL 



A 

1 

JL 




k A 
k A 

I i 


10° 

TANGENTIAL 

10 ° 

RADIAL 

15° 

TANGENTIAL 

15° 

RADIAL 


1 i i A 4 A 

A i 1 it 




i i i 

0.0 + 0.8 + 1.6 


i . i . i l i i.i-i-j 

0 0.2 0.4 0.6 0.8 1.0MM 
IMAGE SIZE 


DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 

YELLOW 12 FILTER 


k 

+ 2.4 


Figure 52. 


Harvard College Observatory 36-in. //8 wide-angle telephoto 


lens ORL-T7-Y7-002. 


74 






100 Io^ 
0 ° 101 . 

lo 


TANGENTIAL 


til** 

I i 1 1 t . 


5° 

RADIAL 


10 

TANGENTIAL 

10 ° 

RADIAL 


15 

TANGENTIAL 

15° 

RADIAL 


ill I 


I i 


i I 


Alii 


i i 


i 

i i 

i i 


k 1 i 


i 


-2,4 


“1.6 


“ 0.8 


0.0 


+ 0,8 


+ 1.6 


i 


DISTANCE IN MW FROM BEST VISUAL AXIAL FOCUS AT f/8 

YELLOW 12 FILTER 


O 0.2 0.4 0.60.8 1.0MM 
IMAGE SIZE 

Figure 53. Kodak 36-in. f/S telephoto EROOOO 8-T-87. 


lOOIo 

lOIo' 


TANGENTIAL 

5° 

RADIAL 


7.5° 

TANGENTIAL 

7.5° 

RADIAL 


TANGENTIAL 

9 W 

RADIAL 


A A I I 

kill 
A A 1 I 

111 1 
1 1 1 


I 


A 


I 


k i i A A A 

1 1 I 


-2.4 


- 1.6 


- 0.8 


0.0 


+0.8 


+ 1.6 


i i i_i— 


DISTANGE IN MM FROM BEST VISUAL AXIAL FOCUS AT f/5.0 

YELLOW 12 FILTER 


0 0.4 0.8 MM 

IMAGE SIZE 

Figure 54. Harvard College Observatory 40-in. (1016 mm) //5 distortionless telephoto Aero lens 








76 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


lOOIo 

0 10 Io . 

Io_ 


I 


I 


4° 

TANGENTIAL 

4° 

RADIAL 


I 


i 


I 


l 


I 


I 

I 


i 

1 


I 


6 ° 

TANGENTIAL 

6 ° 

RADIAL 


I 


-3.0 




1 


0.0 


1 


+ l.o 


DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 

RED 25 FILTER 

0 Q5 1.0 MM 
.MAGE SIZE 

Figure 55. Harvard College Observatory 100-in. //10 anastigmatic lens A1-R2-001. 


1 

I 

+ 2.0 


The next NDRC lens, the 36-in. //8 wide- 
angle telephoto for the K-18 camera, shows 
improvement in image quality of red over yel¬ 
low filter. The improvement in tangential con¬ 
trast in the red is due partly to the elimination 
of most of the secondary spectrum in lateral 
color rather than to reduced astigmatism. The 
lens design was based on sodium light. From 
experiments in laboratory adjustment of the 
lens, it is known that a slight alteration of 
curves would reduce the entire central 8-in. 
circle of the 9x18 picture to a much sharper 
image than shown and result in a general over¬ 
all improvement. In the prototype a compro¬ 
mise had to be made on the central air space 
between spherical aberration and astigmatism 
far off axis. A slight redesign would materially 
improve the central image without affecting the 
optimum image quality far off axis. 

The NDRC 40-in. //5 telephoto also shows 
improvement between the yellow and red filters. 
Again the elimination of residual secondary 
spectrum in lateral color in green and deep red 
results in improved tangential contrast. The 
central peak in either test comes from the 


sharply focused light centered around sodium 
at 5,893 A. 

It should be noted that the complexity of the 
7-element 40-in. //5 resulted in some variation 
in image characteristics between different 
lenses. The assembler was able to change air 
spaces over such a wide range that the image 
at any point could assume a variety of forms. 
Differences noted between production lenses 
involved centering flares, comatic residuals, 
and astigmatism near the edge of the field. In 
almost every case the image possessed a sharp 
central peak as required. It is believed that the 
tests shown here are of a typical production 
unit, assembled with greater care than is cus¬ 
tomary in the production run, but making no 
use of selected elements or mounting. The other 
lenses under test were less sensitive to produc¬ 
tion variations, owing to reduced aperture, 
focal length, and fewer elements. 

The NDRC 100-in. //10 lens, owing to the 
small lateral scale, shows less difference be¬ 
tween yellow and red filter than would be sup¬ 
posed by a visual examination of the image. 
The design was based on red light for use with 






LABORATORY TESTING 


77 


an orange filter. The visual image showed a 
clear yellowish diffraction center, surrounded 
by the inevitably large green and blue out-of¬ 
focus flare due to secondary spectrum. The 
wedge tests emphasize again the importance of 


contrast with the filling up of the lines, and the 
decrease in peak density with aberration, based 
on an absolute exposure level. In laboratory 
studies intended to aid the designer in further 
work, it would be informative to enlarge the 


1 . 


2 . 

3. 

4. 

5. 

6 . 

7. 

8 . 

9. 


10 . 


11 . 

12 . 

13. 

14. 

15. 

16. 

17. 

18. 
19. 


20 . 


21 . 

22 . 

23. 


Table 6. Laboratory testing technique for measuring lens-film resolving power of aerial lenses. 


Test objects 


Variation in line spacing between 
test object charts 
Method of measuring test object 
A D 


(A) 3-line transparency. Line length 5X line width. Space between lines 
equal to line width 

A D > 3.0 log units 

(B) Same as (A) except A D = 0.20 

(C) Cobb-type 2-line transparency. Line length 3X line width. Space 
between lines equal to line width. A D = 0.18. (Test object made by 
Kodak-Harrow.) 

(A) 0.1 log unit, 26 per cent increase 
(C) 0.04 log unit, 10 per cent increase 
Measured diffuse (pot opal) density 


Method of mounting test object 
Method of illuminating test object 
Distance from test object to lens 
Position on axis about which lens 
is rotated when field is explored 
Method of maintaining focal plane 
perpendicular to optical axis 
Field size 

Relation between astigmatic field 
and resolving-power measure¬ 
ments 

Photographic materials used 

Development 

Contrast (7) 

Processing equipment 
Test film size 

Method of keeping film flat 

Test exposure used 
Exposure timing method 
Density values produced on test 
strips 

Resolving power criterion 

Method of viewing test 
Method of making visual measure¬ 
ments 

Method of securing average re¬ 
solving power 


Test object placed over a diffuser of white leader stock 
3,000 K lamp + filter as listed in Table 7 
As listed in Table 7 
Lens nodal point 

Exposure plane orientation controlled by tangent bar 
As listed in Table 7 

Tangential resolving power measured with tangential lines. Radial re¬ 
solving power measured with radial lines 

Super-XX Aero Pan 

8 min, D-19, 68 F 

1.6 

Sensitometric developing machine 
35 mm by 12 in. 

Pressure pad forces film in contact with a flat plate containing a %-in. 
diameter hole 

Constant for all angles on Super-XX, Aero Pan 
Pendulum-type focal plane shutter 

Density giving resolving power maximum is produced at approx. 0.7 
field radius (D = 1.2 for Super-XX resolving power maximum) 
Smallest chart at which lines are just resolved using optimum viewing 
conditions 

Microscope equipped to give from 12.5 X to 100 X magnification 
A microscope with axis parallel to axis of lens bench is used to examine 
the aerial image 

The harmonic mean of the radial and tangential resolving power values 
is secured. This is then averaged over an area of the same size and shape 
as the exposure plane of the camera in which the lens being tested is to 
be placed, consideration being given to the increase of picture area with 
angle. 


a central peak of resolution and the tolerable 
presence of large amounts of secondary color. 

It is believed that further wedge tests should 
include studies of 3-line patterns at several con¬ 
trasts. Such photographs would reveal clearly 
the influence of the central peaks, the loss of 


test photographs to such a point that structural 
detail in the image can be ascertained in the 
vicinity of the central peak. It would also be 
desirable to replot the data for constant or¬ 
dinate and abscissa to serve more readily for 
visual comparisons. 






78 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Lens-Film Resolving-Power Measurements 

The lens-film resolving-power measurements 
were made on an optical bench assembly, which 
consisted essentially of a nodal slide for hold¬ 
ing the lens, a test object at distances up to 
100 ft, a 22-in. diameter 30-ft focal length 
paraboloidal mirror for collimation where 
needed, and the usual auxiliary equipment of an 
optical bench. 

Wherever the distant source was used, the 
distance and orientation of the test objects 
were varied in order to maintain flat field con¬ 
ditions off axis. Both high and low contrast 
3-line patterns and Cobb charts were used. 
Table 6 shows the complete specifications of the 
various testing procedures. 

From the ten lenses tested there is a wealth 
of data too voluminous for reproduction. For 
detailed study the reader is referred to OSRD 
Report No. 6127. 20 A typical series of tests is 
given in Figures 56 to 63 for the 40-in. //5 
telephoto lens and comparative data for Mount 
Wilson results are shown in Section 1.4.4. 


Average Lens-Film Performance 

In accord with various procedures already 
used in England and Canada for averaging the 
lens-film performance over the specified picture 
area, the Eastman data include special aver¬ 
ages obtained by integration over the square 
or rectangular field used. Figures 64 to 76 re¬ 
produce comparative data for the ten lenses 
tested. The averages were obtained by numer¬ 
ical integration over the picture area of the 
increment of area weighted according to the 
resolving power. 

The apparent forward curvature of the aver¬ 
age focus for the 5.950-in. //3.5 wide-angle lens 
shown in Figure 64 is without significance, 
since the field curvature of the emulsion is 
based on the field curvature of the lens. How¬ 
ever, the discrepancy is equal to that between 
the 5.950-in. known radius of the focal surface 
as constructed, and the 6-in. radius indicated. 
The photographic shells are curved to a radius 
of 5.950 in., and the lens was known to have the 
proper back focus within 0.002 in. 


Overall Conclusions 

It is better that the reader study the original 
data of the Mount Wilson 4 * 19 and Eastman 20 
test results before forming his own conclusions. 
However, it is possible here to isolate several 
factors of interest. 

The peak performance indicated by the East¬ 
man data was given on axis by the //3.5 wide- 
angle lens and amounted to nearly 70 lines per 
mm on Super-XX. Next in order of increasing 
/-number, among lenses showing sharp peak 
in the wedge pattern, was the 40-in. //5 with 
red filter, which gave 40 lines per mm. At //8 
the same lens yielded about 50 lines per mm. 
The Eastman experimental lens, 8-J-35, de¬ 
signed for telescopic performance over a re¬ 
stricted field with 24-in. focus at //8, gave 42 
lines per mm peak performance, going up to 
about 50 lines per mm at //16. The 36-in. f/S 
apochromat yielded a peak of 43 lines per 
mm. 

These figures indicate a slight dependence 
of peak resolution on /-number, involving there¬ 
fore film resolution-Rayleigh limit data. It 
would appear that a perfect f/S lens might 
resolve 75 lines per mm on Super-XX at high 
contrast, that a perfect //5 lens might resolve 
65, a perfect f/S lens about 58, a perfect //16 
50 lines per mm, etc., decreasing rapidly at 
lower speeds. There is no doubt some depend¬ 
ence on color region of the emulsion, but results 
are inconclusive in the test data, and the dif¬ 
ferences are small between yellow and red. 

Table 7 presents the final results of the aver¬ 
aged Eastman data. At first sight the figures 
on mean resolution are disappointing. The 
standard 24-in. Aero-Ektar, whose wedge im¬ 
ages are the coarsest of the longer focal length 
lenses tested, yields a resolution of 20 lines 
per mm on the average over the 9x18 picture 
area. The Eastman lens, 8-J-35, designed for 
peak performance at f/S over a 10-degree half 
field, yields 31 lines per mm for an infinitely 
distant object, and 36 lines per mm for an ob¬ 
ject 36 focal lengths away. It is evident that a 
slight readjustment would bring the perform¬ 
ance for an infinitely distant object up to 36 
lines per mm. This, then, may be adopted as 
the expected peak from a lens with field and 



LABORATORY TESTING 


79 


aperture restricted to near the Rayleigh limit. 

Since the lenses of large focal length intended 
for 9x9 or larger picture areas with apertures 
up to //5 average about 30 lines per mm over 
the field, it is evident that when full design 
performance in the numerous cases is realized, 
considerable effort will be required for the de¬ 
sign and production of lenses capable of per¬ 
formance, on the average, over the field of 
better than 30 lines per mm. 

Is the difference between the easily obtained 
20 lines per mm for the Aero-Ektar field aver¬ 
age, with its accompanying large depth of focus 
and insensitive production tolerances, and the 
30 lines per mm average to date of the best 
experimental aerial lenses worth while? Will 
it be possible in the future to design lenses 
whose average performance over still larger 
picture areas at greater speeds approaches the 
perfect lens-film figures of about 65 lines per 
mm mentioned above? 


For a partial answer to these questions the 
reader is referred once more to the considera¬ 
tions on contrast and resolution outlined in the 
introduction (see Section 1.1). The curves in 
Figure 1 show only a slight initial loss of res¬ 
olution for a large loss of contrast. This fact 
is consistent with the apparently small differ¬ 
ences between the resolution figures over the 
picture area for lenses with excellent wedge 
patterns and those with coarse patterns. It 
must be determined in future tests whether the 
50 per cent gain in resolution and considera¬ 
tions of overall tonal values, or true range of 
contrasts on a microscopic scale are of sufficient 
importance for the purposes of aerial reconnais¬ 
sance to justify procurement of lenses of sharp 
images instead of those with poorer images 
which can be made more easily. 

The range of values of the resolution figures 
obtained from the Cobb low-contrast chart in¬ 
dicates the insensitivity of this type of testing 



or 

LU 

CL 

c n 

UJ 

z 


a: 

LU 

5: 

o 

CL 


e> 


> 

_i 

o 

CO 

LU 

tr 


-RADIAL -TANGENTIAL 

Figure 56. 40-in. // 5 telephoto at //5, Super-XX Aero Pan tungsten + 12 filter, high-contrast 3-line test 

object. 










80 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Table 7 


03 

a 

o 


01 

u 

3 

pH 

0) 

a 

<3 


£ 

Q o 


•22 C w 

Q 5 


High-contrast 3-line 
test object 


Low-contrast 
test object 


« 

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bO 0) 

o c 



a i 

>> I 

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wide angle 

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60° half 
angle 
spherical 

00 

1.69 

49 

28 

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1.27 

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73 

7.5-in. 

plastic 

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14 

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34 

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9x18" 

44 

1.34 

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15 

44 

1.02 

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32 

44 

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1.39 

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1.49 

31 

11 

44 

1.14 

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angle 

36 F.L.f 

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1.24 

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angle 

44 

1.61 

41 

8.3 

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18 

19 

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10° half 
angle 

44 

1.56 

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9.3 

44 

1.26 

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19 

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//5.6 

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00 

1.43 

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1.00 

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1.42 

26 

13 

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1.14 

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1.46 

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12 

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1.16 

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10 

44 

1.24 

17 

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36-in. 

Apochromat 

//8 

daylight^ 
daylight + 12 

44 

28.8 F.L. 

1.33 

21 

11 

Cobb 

1.07 

12 

19 

36-in. 

Apochromat 

44 

12 

44 

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1.39 

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36-in. 

Telephoto 

44 

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00 

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44 

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36-in. 

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44 

25 

44 

44 

1.42 

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21 

8-T-87 

44 

12 

44 

44 

1.25 

18 

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44 

44 

25 

44 

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7.7 

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6 


* Resolving power, averaged over the field, 
f Focal length. 

t 3,000 K tungsten + 78AA filter. 

§ Length/width of high-contrast test object 15/1. 










25 
16 
10 
6.2 5 


25 

16 

10 

6.25 


RADIAL 


-TANGENTIAL 


Figure 57. 40-in. //5 telephoto at //5, Super-XX Aero Pan tungsten + 12 filter, low-contrast Cobb test 

object. 



cr 

UJ 

CL 

If) 

UJ 

2 

_l 


cr 

UJ 


o 

a. 


o 


o 

C!) 
UJ 

cr 


DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS THROUGH 25 FILTER 
RADIAL -TANGENTIAL 


Figure 58. 40-in. //5 telephoto at //5, Super-XX Aero Pan tungsten 25 filter, high-contrast 3-line test 

object. 


81 


RESOLVING POWER IN LINES PER MM 














o: 

UJ 

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Figure 59. 
object. 


-1.6 0.0 16 -1j6 OX) 16 -L6 0.0 1.6 

DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 

- RADIAL -TANGENTIAL 

40-in. //5 telephoto at //5, Super-XX Aero Pan tungsten + 12 filter, low-contrast Cobb test 



2 

2 

or 

UJ 

a. 

co 

UJ 

z 


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$ 

O 

CL 


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DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS AT f/5 

- RADIAL -TANGENTIAL 

Figure 60. 40-in. //5 telephoto at //8, Super-XX Aero Pan tungsten + 12 filter, high-contrast 3-line test 
object. 


82 













or 

UJ 

GL 


0 ) 

UJ 


CC 

UJ 

£ 

o 

GL 


e> 

z 

5 

o 

CO 

UJ 

oc 


-RADIAL -TANGENTIAL 

Figure 61. 40-in. //5 telephoto at //8, Super-XX Aero Pan tungsten + 12 filter, low-contrast Cobb test 

object. 



DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS AT f/5 THROUGH 12 FILTER 

-RADIAL -TANGENTIAL 

Figure 62. 40-in. //5 telephoto at //8, Super-XX Aero Pan tungsten + 25 filter, high-contrast 3-lme test 

object. 


83 















RADIAL -TANGENTIAL 


5 


tr 

LU 

CL 

U) 

LU 


-J 


a: 

LU 

5S 

o 

Q. 


o 

z 

3 

O 

(/) 

LU 

DC 


Figure 63. 40-in. //5 telephoto at //8, Super-XX Aero Pan tungsten + 25 filter, low-contrast Cobb test 

object. 



- HIGH CONTRAST 3-line TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 64. 6-in. //3 wide-angle lens, Super-XX 

Aero Pan tungsten + red filter, average resolv¬ 
ing-power. Over 60° half angle field on spherical 
exposure surface. 


40 

25 

16 

10 

625 


40 
25 
16 
10 

6.25 

-0.4 0 0.4 

DISTANCE IN MM FROM 
BEST AXIAL FOCUS 



5 

o 


_ high contrast 3-LINE test object 

_ LOW CONTRAST COBB TEST OBJECT 


Figure 65. 7.5-in. //2.8 plastic lens, Super-XX 

Aero Pan -f- yellow filter. 


84 












LOG AVERAGE RESOLVING POWER 



o 

z 


o 

<s> 


% 

a: 


DISTANCE IN MM 

FROM BEST VISUAL AXIAL FOCUS AT f/6 
THROUGH 12 FILTER 


-HIGH CONTRAST 3'LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 66. 6-H-62 EM170, Super-XX Aero Pan, 
average resolving power for 9xl8-in. picture. 



DISTANCE IN MM 

FROM BEST VISUAL AXIAL FOCUS AT f/6 
THROUGH 12 FILTER 


-HIGH CONTRAST 3'LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 67. 6-H-62 EM170, Super-XX Aero Pan, 

average resolving power for 9x9-in. picture. 



DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 


-HIGH CONTRAST 3-LINE TEST OBJECT AT oo 

_LOW CONTRAST COBB TEST OBJECT AT oo 

Figure 68. 8-J-35 at //8, Super-XX Aero Pan 

tungsten + 12 filter, average resolving power 10° 
half field. 


AVERAGE RESOLVING LINES PER MM 




















-HIGH CONTRAST TEST OBJECT AT oo 

-LOW CONTRAST 3-LINE TEST OBJECT AT oo 

AD - 0.20 

Figure 69. 24-in. //8 aerostigmat formula 8-J- 

35, Super-XX Aero Pan tungsten + 12 filter, 
average resolving power 10° half field. 



DISTANCE IN MM FROM BEST VISUAL 
AXIAL FOCUS AT f/5-6 


- HIGH CONTRAST 3'LINE TEST OBJECT AT qq 

_LOW CONTRAST COBB TEST OBJECT AT oo 


Figure 70. Kodak 24-in. //5.6 Aero-Ektar 
EE0001-3 formula 56-M-56 Super-XX Aero Pan 
tungsten + 12 filter, average resolving power 
for 9x9-in. picture. 



2 

2 

oc 


DISTANCE IN MM 

FROM BEST VISUAL AXIAL FOCUS AT f /5.6 


-HIGH CONTRAST TEST OBJECT AT OO 

-LOW CONTRAST 3-LINE TEST OBJECT AToo 

A o = n?o 

Figure 71. Kodak 24-in. //5.6 Aero-Ektar 
EE0001-3 formula 56-M-56, Super-XX Aero 
Pan tungsten + 12 filter, average resolving 
power for 9x9-in. picture. 



- HIGH CONTRAST 3-LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 72. Harvard College Observatory 36-in. 
fl 8 aerial apochromat, F-3, Super-XX Aero Pan. 


86 





















DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 


- HI6H CONTRAST 3-LINE TEST OBJECT 

- LOW CONTRAST COBB TEST OBJECT 

Figure 73. Harvard College Observatory 36-in. 
//8 wide-angle telephoto, Super-XX Aero Pan, 
average resolving power for 9x18-in. picture. 



DISTANCE IN. MM FROM BEST VISUAL AXIAL FOCUS 


- HIGH CONTRAST-3 LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 74. Kodak 36-in. // 8 telephoto 8-T-87, 
Super-XX Aero Pan, average resolving power 
for 9xl8-in. picture. 



THROUGH 12 FILTER 



-HIGH CONTRAST 3*LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 75. 40-in. // 5 telephoto at // 5, Super-XX 

Aero Pan, average resolving power for 9xl8-in. 
picture. 


-HIGH CONTRAST 3-LINE TEST OBJECT 

-LOW CONTRAST COBB TEST OBJECT 

Figure 76. Harvard College Observatory 100-in. 
//10 anastigmatic aero lens A1-R2-001, Super- 
XX Aero Pan, average resolving power for 9x18- 
in. picture. 


87 






















88 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


for the isolation of lenses with sharp images, 
although the same figures may possibly empha¬ 
size the unsatisfying nature of aerial photog¬ 
raphy. In other words, it would be very difficult 
from studies of Cobb results alone to predict 
the appearance of the wedge patterns of good 
and bad lenses with any surety beyond what is 
known from general considerations. 

Finally, the excellent angular resolutions ob¬ 
tained on the average for the lenses of large 
focal length reaffirms that an increase of focal 
length is of great importance for the final pur- 


1 ' 4 ' 4 Comparison of Lens Tests at Mount 
Wilson Observatory and the Eastman 
Kodak Company 

Three NDRC lens prototypes were tested at 
both Mount Wilson and the Eastman Kodak 
Company. For evidence on the reliability of such 
test programs, comparable data (see Figures 
77 to 81) are reproduced from the respective re¬ 
ports 4 ’ 20 for the 100-in. //10 anastigmat. The 
resolution curves for tungsten plus Wratten 
No. 12 filter from the Eastman report are not 


0.995 f I.OOOf 



O.OOOf 0.0005f 



I.OOOf 1.0005f I.OOlOf I.OOISf 



FOCAL SETTING IN MM 


Figure 77. Astigmatism, distortion, and color curve of 100-in. f/10 anastigmat (Mount Wilson). 


pose of reconnaissance, that of greater ground 
resolution from a given altitude. Thus, in lab¬ 
oratory performance even the 100-in. focal 
length lens does not begin to exhaust the pos¬ 
sibilities of angular resolution obtained by in¬ 
crease of focal length. It should be pointed out 
that the probable turbulence of the atmosphere 
is of the order of 1 sec of arc under aerial 
conditions, and that in the absence of move¬ 
ment or vibration, a focal length of even 250 in. 
in the air might be reached before picture 
sharpness would drop appreciably below 20 
lines per mm in the absence of haze. 


included, since the design was based on use 
with an orange filter. 

On axis, the peak Mount Wilson performance 
on Super-XX developed for 10 min in D-19 and 
68 F is 29 lines per mm, whereas the Eastman 
results are more than 40 lines per mm under 
very comparable conditions. In general, the 
Eastman resolution figures are higher than 
those of Mount Wilson on the same test. Similar 
discrepancies occur in tests of the other lenses. 
(See Figures 8 and 64.) 

The Mount Wilson results, in agreement with 
design data, indicate a very flat mean field 








LABORATORY TESTING 


89 


(see Figure 79) at peak resolution, varying by 
0.2 mm at most. The results of visual deter¬ 
mination of field curvature indicate good sym¬ 
metry on both sides of the optical axis, proving 
both good alignment of the elements and of 

150 

100 
90 
80 
70 
60 
50 




35 

25 


15 

10 

Figure 78. Resolution and contrast of 100-in. 

//10 (Mount Wilson). 

the testing equipment. The Eastman data show 
a backward curving field to the extent of almost 
1.5 mm, based on measurements apparently 
confined to one side of the axis. The indicated 
discrepancy is small compared to the size of 
picture and camera, and illustrates the difficulty 
of large lens work. 

It is obviously very dangerous to lump to¬ 
gether testing results at separate laboratories 
until considerable standardization has been 
achieved. Each laboratory must work toward 
the utmost self-consistency of the procedures 
chosen, and check the precision of the testing 
equipment in all respects. 



TANGENTIAL 
RADIAL 
,+TANGENTIAL . 
SUPER-XX FILM 


FIELD ANGLE IN DEGREES 


110 General Considerations Concerning; 

Lens Tests 

The complete laboratory lens test would in¬ 
clude both visual and microphotometer meas¬ 
ures of tangential and radial resolution at 
spaced focal settings at all parts of the picture 
area, making use of various targets, contrasts, 
absolute exposure levels, apertures, filters, and 
emulsions. In addition, studies would be made 
of the distribution of light in the image under 
all the above circumstances, employing both 



SUPER-XX FILM 
CONTRAST 0.30 

Figure 79. Resolving power versus focal setting 
for 100-in. lens (Mount Wilson). 

photomicrography and the wedge method. In 
practice, some limitation on these numerous 
variables must obviously be imposed. 

Type of Targets 

The investigation of the Royal Aircraft 
Establishment at Farnborough to determine 
























90 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


the reliability of different types of targets 
proved that either the Cobb 2-line test chart 
(see Figure 2) or the American 3-line chart are 
of the same order of certainty within the 
knowledge of what is being tested. The objec¬ 
tive character of Canadian annuli (see Fig¬ 
ure 2) is deserving of considerable attention. 
The same investigation established that the 
ratio of successive target sizes should be 2, or 


sent ground objects of widely varying nature, 
it would appear that present types of testing 
targets in the various laboratories may as well 
be retained. 

Each research worker should realize that his 
own test methods will ultimately produce opti¬ 
mum results on ground targets most nearly like 
his own targets, and should discount the abso¬ 
lute importance of improving results already 



DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS THROUGH 25 FILTER 


« 2 
o <r 

CL Ui 
CL 


o 

z in 


-J 2 

83 


- RADIAL 

-TANGENTIAL 

Figure 80. Resolving power versus focal setting for 100-in. lens (Eastman Kodak Company). 



DISTANCE IN MM FROM BEST VISUAL AXIAL FOCUS 


RADIAL 


cr 2 
£ 2 

O cn 
0 . uj 

O 0 - 

Z in 

83 

UJ 


Figure 81. 
Company). 


-TANGENTIAL 

Resolving power versus focal setting for 100-in. lens at low contrast (Eastman Kodak 


smaller, if repeated tests are to agree within 
the uncertainty of the photographic process. 

It would be desirable for all laboratories here 
and abroad to adopt the same testing targets 
and procedures, but during World War II no 
general agreement was reached. Since the real 
cause of the uncertainty is the impossibility of 
identifying any single measure that will repre- 


within 20 per cent of the theoretical overall 
limits. 

Contrast Factors 

The arguments presented in the introduction 
(see Section 1.1) show that contrast has a very 
decided effect on resolution. A good lens with 
respect to contrast may seem to represent only 











LABORATORY TESTING 


91 


a slight improvement over a poor lens yielding 
almost the same resolution. On the basis of 
resolution alone, therefore, one cannot predict 
that the aerial picture from one lens is likely 
to be better than that from another. It is too 
early to comment with certainty on the impor¬ 
tance of good microscopic contrast for aerial 
reconnaissance, but the following is suggested 
as a basis for further work. 

Test data should be obtained on microscopic 
contrast as a function of resolution and initial 
target contrast. The microscopic contrast should 
be determined by means of precise micropho¬ 
tometry. As described in Section 1.1, the ex¬ 
posure level should be determined by means of 
the density of the macroscopic photometric 
areas whose surface brightness should equal 
that of the bright lines. 

There are always many desirable ways of 
presenting test data. In every case some direct 
comparison should be made with similar data 
for perfect lens-film combinations. It is be¬ 
lieved that a very useful method would be to 
determine the profile of successive resolution 
patterns by means of precise microphotometry, 
standardized by characteristic curve and con¬ 
stant density of the large photometric area. 
After reduction to an intensity scale, a plot 
would be made of the equivalent microphotom¬ 
eter curve, where the ordinate is in true inten¬ 
sity units in percentages of the intensity of 
the photometric area, and where the abscissa 
is a true linear scale of lines per millimeter 
greatly enlarged, with the coarse to very fine 
patterns all on the same basis. 

Such a well-determined curve could be com¬ 
pared by superposition, or on the same plot, 
with the perfect lens-film result. The observer 
could then tell at once the loss of intensity in 
the bright lines, the gain of intensity in the 
dark lines, the contrast, and the rate of change 
of these quantities with lines per millimeter 
down to the limit of resolution. It is evident 
that the microphotometer would have to have 
excellent resolution in order to reproduce re¬ 
liable measures on the finest patterns now 
resolved at peak performance by many modern 
lens types. 

A complete lens test would then include a 
number of such curves according to focal set¬ 


ting, field angle, tangential or radial resolution, 
color, etc. Corresponding tests in the air would 
indicate haze effects, motion, etc. Finally, com¬ 
plete wedge patterns based not only on single 
line images but also on resolution patterns at 
different contrasts should accompany the mi¬ 
crophotometer data. Perhaps the whole story 
could be told by an expansion of the wedge 
method, quite independently of microphotom¬ 
etry, provided the final patterns are replotted 
to constant coordinates. 

Exposure Level 

Consideration of all the many problems in¬ 
volved in evaluating laboratory lens tests on 
an exact comparative basis leads to the conclu¬ 
sion that the absolute exposure level with re¬ 
spect to macroscopic photometric areas should 
be held constant. In general, the exposure should 
be adjusted at the level most nearly represent¬ 
ative of optimum picture quality by area. A 
lens with heavy vignetting should have its ex¬ 
posure adjusted not for the center but for a 
mean zone of the field. Likewise, in testing 
resolution the exposure should not be adjusted 
at each field angle to favor maximum resolu¬ 
tion, since a plot of such peak resolution versus 
field angle is unrepresentative of the aerial 
performance of the lens. 

Any lowered resolution caused by inadequate 
exposure due to vignetting or to washed-out 
dense images in the center of the field, when 
the average exposure is correct, is a true prop¬ 
erty of the lens and should be considered in the 
testing. It is very likely that if close attention 
is paid to such details, the laboratory differ¬ 
ences in resolution between the old style lenses 
and the newer types will be much more marked, 
and more in accord with the observed differ¬ 
ences in quality of the aerial pictures. 


14 - 6 Film Properties 

Examination of the curves in Figure 2 with 
respect to proper exposure level for maximum 
resolution shows that for Super-XX at 1/0.5 
contrast the density of macroscopic areas equal 
in brightness to the white lines should be 1.6. 
For Pan-X at the same contrast and maximum 




92 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


resolution, the density should be 0.84. Under 
these conditions Super-XX will resolve 32 lines 
per mm and Pan-X 36 lines per mm. For maxi¬ 
mum resolution at contrast 1/0.5, Super-XX is 
only 1.5X faster than Pan-X, partly because 
the optimum density for white lines is twice 
as great (1.6) for the former as (0.84) for the 
latter. 

For most favorable tone reproduction, Super- 
XX at 1/0.5 target contrast has its optimum 
exposure for maximum resolution somewhat 
above the middle of the characteristic curve, 
therefore favoring areas within cloud shadows. 
For the same location on the characteristic 
curve (density 1.6) and about the same con¬ 
trast, Pan-X resolution falls to 31 lines per 
mm, almost identical with Super-XX perform¬ 
ance, and requires 5.4 times the exposure of 
Super-XX. 

For best tone reproduction, Super-XX at 
1/0.125 target contrast has its optimum ex¬ 
posure and maximum resolution still at density 
1.6 and resolves 43 lines per mm. Pan-X now 
resolves 52 lines per mm and requires as before 
5.4 times the exposure. Super-XX has a longer 
straight line portion of the curve, almost en¬ 
tirely on the low density part of the scale, and 
is therefore superior to Pan-X for tone repro¬ 
duction at low exposure levels. Super-XX has 
a somewhat higher level of fog. 

All of the above provisional data are based 
on the high gamma of 1.9 for Super-XX, as 
compared to 1.5 for Pan-X. For aerial purposes 
with improved lenses, such gamma values are 
too high. A laboratory development gamma of 
1.3 would probably overcome the average effect 
of haze on macroscopic areas. Consequently, all 
of the above comparisons should be reviewed 
from data determined at a gamma of 1.3. 

It appears provisionally that for the same 
final range of tonal values favoring low lights 
and moderate contrast, Super-XX and Pan-X 
have comparable resolution. The fine-grain 
character of Pan-X probably cannot outweigh 
the factor of 5.4 in speed of Super-XX over 
Pan-X especially when vibration, movement, 
and exposure problems are considered. The 
whole subject needs further investigation, in¬ 
asmuch as aerial results at Bedford indicate 
significantly better results on Pan-X. It is clear 


that for daytime tests of mounts, high-contrast, 
medium-density exposures on Pan-X are best. 
For general landscape objects, particularly in 
the presence of haze, Super-XX is probably 
best. For laboratory testing both films should 
be used whenever feasible. If that is not possi¬ 
ble, then the choice should fall to Super-XX. 


15 SCHMIDT CAMERAS 

The growing popularity of the Schmidt cam¬ 
era in science and industry during the years 
immediately preceding World War II led a 
number of workers in the field of optical instru¬ 
ments to propose Schmidt cameras for military 
uses. Only a few proposals were considered for 
purposes of aerial photography, owing prima¬ 
rily to the limited angular coverage of the 
Schmidt camera, the inconvenience of curved 
film, and the great shutter difficulties. 

The Schmidt camera still represents the sim¬ 
plest solution to the problem of obtaining a 
very fast lens system covering a moderate size 
field with critical definition. Other systems like 
the Yerkes wide-angle symmetrical system excel 
in one or more points, but the Schmidt camera 
cannot be overlooked for general utility. 


Two-Mirror Schmidt Camera 
(Mount Wilson ) 21 

Two Schmidt cameras (12-in. f/1 and 24-in. 
f/2) were developed by the California Institute 
of Technology under direct Army contract in 
1941. The Mount Wilson Observatory, under 
Contract OEMsr-101, initiated in 1941 a 
Schmidt camera project for both day and night 
photography. So little was known throughout 
most of World War II regarding the ultimate 
limits of resolution in the air that it was 
deemed worth while to construct a 2-mirror 
Schmidt camera with critical definition over the 
entire field of view, and to provide this camera 
with a separate shutter. 

The 2-mirror Schmidt camera is similar in 
principle to the ordinary Schmidt camera. A 
secondary convex spherical mirror, concentric 
with the primary concave spherical mirror sur- 



SCHMIDT CAMERAS 


93 


face, forms an image on a curved focal surface 
concentric with the common center of the two 
mirror surfaces, and located just behind the 
primary mirror. The negative secondary mir¬ 
ror converts the system as a whole into a tele¬ 
photo system. 

The common Schmidt camera has a length 
about twice as great as the focal length. The 
2-mirror form is only approximately as long 
as the focal length. Indeed, the curvature of 
the focal surface about the common center of 
symmetry requires that the radius of curvature 
and the distance of the film from the correcting 
plate be equal to the focal length. 

The magnification caused by the secondary 
mirror increases the /-number of the system in 
proportion. Also, the correcting plate in a 2- 
mirror Schmidt at //2.5 has the curvature and 
form of an //1.25 ordinary Schmidt as modified 
by partial correction from the secondary mirror. 
For astronomical use over a wide spectral range, 
the color aberration of the //1.25 simple 
Schmidt plate would probably be noticeable; 
for aerial photography, however, there is still 
a wide margin of tolerance. 

Figure 82 shows a schematic view of the 
complete installation in the well of a B-17 
bomber ready for flight. The drawing presents 
graphically the optical and mechanical details 
of the 2-mirror Schmidt system. 

The louvre shutter is carried by suspension 
rods on the main framework of the camera 
mount. The shutter therefore occupies a fixed 
position in the airplane. The connection be¬ 
tween the shutter and the correcting plate is 
made light-tight by means of a flexible bel¬ 
lows. 

The following is a tabulation of the optical 
constants of the 2-mirror system. 

Table 8. Optical constants of the 2-mirror 

Schmidt camera (in.). 


Effective focal length 30.0 

Focal ratio //2.5 

Effective focal ratio //3.4 

Radius of curvature of primary mirror 27.5 

Diameter of primary mirror 16.0 

Radius of curvature of secondary mirror 19.0 

Diameter of secondary mirror 7.5 

Diameter of correcting plate 12.0 

Diameter of film 5.5 

Angular diameter of field 10.5 degrees 


The space between the back of the primary 
mirror and the film is approximately 1 in. This 
distance is stated to be too small for a focal 
plane shutter, but could have been increased 
had general sentiment in 1942 been in favor 
of focal plane shutters. Any future development 
of apparatus of this type might very well con¬ 
sider the problem anew. 

Photographic tests of resolving power on 
high-contrast targets gave the following re¬ 
sults : 


Table 9. Resolution measures on Super-XX film 
with 2-mirror Schmidt camera. 


Distance off axis 
(degrees) 

Radial 

(lines per mm) 

Tangential 
(lines per mm) 

2.75 

27 

24 

2.0 

30 

30 

1.0 

30 

27 

0.0 

30 

27 

—1.0 

27 

30 

—2.0 

30 

27 

—2.75 

30 

27 


The mean resolution would therefore indicate 
an equivalent target contrast in the neighbor¬ 
hood of the microscopic image of 1/0.4. The 
departure from the ideal resolution at //2.5, in 
the vicinity of 65 lines per mm on Super-XX 
at high contrast, can be ascribed to imperfec¬ 
tions of figure of the correcting plate and to 
flexure of the primary mirror. Had more time 
been allotted to the Schmidt camera, the figur¬ 
ing could easily have been carried to crisp 
image quality. It is probable, however, that the 
system is already of adequate quality for ex¬ 
perimental flights, especially at night with 
photoflash bombs. 

For a time it was thought possible that the 
correcting plate might be replaced by one or 
more achromatic meniscus lenses, following the 
published work of Maksutov, 10 and that the 
negative paraxial power of these lenses could 
serve to displace the focal surface far enough 
back to allow for insertion of a focal plane 
shutter. Computations made at Harvard estab¬ 
lished, however, that for a system of this kind 
the demands upon spherical correction are too 
heavy to be met by any practical form of achro¬ 
matic meniscus lens or pair of lenses. More¬ 
over, even with meniscus lenses of reduced 












94 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 



Figure 82. A schematic view of the 2-mirror Schmidt and installation 











SCHMIDT CAMERAS 


95 


power that still require figuring, the oblique 
spherical aberration introduced into the corner 
image is much larger than reasonable toler¬ 
ances. Color correction, on the other hand, was 
found to be nearly as good as that of the cor¬ 
recting plate alone. 

A description of the 2-mirror system made 
at Mount Wilson follows. 

Camera Frame 

The camera frame consists of two Meehanite 
castings, one holding the correcting plate and 
the other the primary mirror. The secondary 
mirror is very ingeniously held in position by 
twelve light rods under tension. This method 
makes use of the spherical symmetry of the sec¬ 
ondary mirror. Four spider rods near the cor¬ 
recting plate fix the center of curvature of the 
mirror relative to the correcting plate. The 
other eight rods in tension permit a swing-like 
adjustment of the secondary mirror around its 
center of curvature. Any tendency of the sec¬ 
ondary mirror itself to oscillate will not affect 
the resolution, since the primary vibration will 
keep the surface of the mirror in its own sphere. 
The longitudinal rods under tension also tend 
to minimize longitudinal vibration and adjust¬ 
ment of rod length is used to locate the sec¬ 
ondary mirror relative to the correcting 
plate. 

The mounting of the primary mirror was a 
source of difficulty. The position of the camera 
in the air requires that the mirror be edge- 
supported. In addition, the nearness of the 
film to the back of the primary mirror, neces¬ 
sitated by considerations of protecting the film 
from direct fogging by stray light around the 
secondary mirror, required the primary mirror 
to be thinner than otherwise desirable. Finally, 
the size of the photographic field requires that 
a large hole be cut in the primary. All of these 
circumstances are unfavorable for proper sup¬ 
port of the primary mirror surface, and ap¬ 
parently in practice caused flexure greater than 
desirable. The secondary mirror caused no diffi¬ 
culties of this kind. 

The main tube was made light-tight by cover 
sheets of aluminum. Gelatin filters mounted 
without glass in light brass frames fit a shoul¬ 
der in the opening in the primary mirror. 


Adjustment of the optical alignment is ac¬ 
complished chiefly by lateral movement of the 
primary mirror. The secondary mirror is lo¬ 
cated initially by mechanical measurement, 
together with adjustment of the tension rods. 
Finally, the focal surface is adjusted by push- 
pull screws until it also is concentric with the 
mirror surfaces. 

Focusing of the camera is accomplished by 
longitudinal movement of the secondary mir¬ 
ror, or by movement of the film magazine. A 
movement of the secondary mirror along the 
optical axis by the amount x produces a move¬ 
ment of the focal surface by an amount Sx. 

Antioscillation Mount 

The camera mount is designed for maximum 
protection from angular vibration, and to a 
lesser extent from translatory vibration. The 
latter is accomplished by providing Lord-type 
shock absorbers at the four support points be¬ 
tween main frame and airplane bracket. 

The camera is mounted on three octagonal 
rings in a double gimbal suspension, which at 
the same time provides for a sweep mechanism 
to overcome ground motion of the plane and for 
elimination of vibration by means of fluid 
dampers and elastic restoring forces. Briefly, 
the lowest ring carries three ball-bearing roll¬ 
ers and enables the entire camera unit to be 
crabbed about a vertical axis and clamped in 
position. A Duralumin circular ring carries the 
full load of the assembly as well as the louvre 
shutter and is connected to the air frame 
through the Lord mounts mentioned above. 

The camera rotates through small angles 
around an axis parallel to the line of flight with 
bearings in the intermediate octagonal ring. 
The camera and ring together rock in a pair of 
bearings transverse to the line of flight. These 
two gimbal axes pass through the center of 
gravity of the camera assembly. 

Damping is provided by tandem liquid-filled 
sylphon bellows, connected by an orifice of ad¬ 
justable length. Any angular change of the 
camera is accompanied by a flow of the damp¬ 
ing fluid from one sylphon to the other through 
the orifice. The spring rate of the sylphons is 
used as source of a restoring force. The damp¬ 
ing fluid recommended is isopentane. Shake- 






96 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 



Figure 83. A view of the sweep and damping mechanism. 











SCHMIDT CAMERAS 


97 


table tests proved that transmitted frequen¬ 
cies were low and that high frequencies were 
sufficiently well filtered. 

Ground Sweep Mechanism 

The camera and the middle ring of the gim- 
bal assembly must rock together about a trans¬ 
verse axis in order to compensate ground move¬ 
ment of the plane. Since the middle unit is 
connected for damping purposes with the upper 
ring, it is equally necessary that the upper ring 
share the rocking motion around the same 
transverse axis. For this reason the drive for 
the sweep mechanism consists of a connection 
of variable length between the lowest and the 
uppermost ring by means of rack and pinion 
movement. The upper ring, which is therefore 
directly power driven and which shares the 
vibrations of the plane before they are damped 
out, must in turn impart a rocking motion to 
the intermediate ring and camera to which it 
is only softly connected by way of the sylphon 
dampers. 

For drive purposes, an electromagnetic 
clamping device is provided to tie temporarily 
together the upper two rings. Once the motion 
through the acceleration stage has passed to the 
camera and middle ring, this electromagnetic 
clamp lets go and the drive is continued only 
by way of the sylphon dampers, which in the 
meantime eliminates random vibrations dur¬ 
ing the swing. For best results the exposure 
has been set to occur near the end of the swing, 
in order that a maximum amount of time be 
given for damping out any vibrations im¬ 
parted by the sweep-drive mechanism, or by 
the plane during the time the sylphon dampers 
are inactive. 

Figure 83 shows a view of the rack and 
pinion drive. The electric motor, by means of a 
mechanical linkage provided directly by a 
standard type electric windshield wiper, drives 
the pinion H through an oscillatory motion of 
140 degrees, and therefore produces a similar 
oscillation of the rack. The amplitude of swing 
is determined by the size of the pinion used. 
The period of the sweep is varied by means of 
a rheostat connected in the armature circuit of 
the driving motor. 

Figure 84 shows the timing scheme. All op¬ 


erations are linked by cams and relays in such 
a way that the sequence of operations cannot be 
repeated until the next exposure is ready at the 
proper moment. During the cycle the film and 
the shutter are wound. Variation of the period 
does not affect the sequence or position in the 
sequence of the various operations. 

Film Magazine 

Because of the nature of the camera it was 
not possible to make use of standard maga¬ 
zines. Figure 85 shows two views of the maga- 



ALTITUDE h IN THOUSANDS OF FEET 


Figure 84. Timing diagrams for the 2-mirror 
Schmidt camera. 

zine mechanism. In principle the use of the 
vacuum and film metering mechanisms is very 
similar to those of the standard magazines. 
There are a number of well engineered details 
that are attractive. One of these is the use of 
braking action by converting the electric drive 
motor into a generator on the cutoff of power 
by the relay by introducing a resistance of 1.5 



































































98 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 



Figure 85. The magazine mechanism for the 2-mirror Schmidt camera. 





SCHMIDT CAMERAS 


99 


ohms across the armature. This type of braking 
at the same time provides a shock absorbing 
action that is protective to all mechanisms. It 
is recommended that the minimum cycle of op¬ 
erations occupy about 2 sec. 

The film itself is deformed to the 30-in. radius 
of the focal surface by means of a vacuum. The 
backing sphere is made of ground glass and is 
provided with radial slots to aid in obtaining 
greater uniformity of fit between film and 
sphere. The depth of the bending is of the order 
of 0.1 in. Such a deformation requires rela¬ 
tively slight stretching of the film. The vacuum 
is needed more for positioning the film than for 
stretching purposes. It was found that a pres¬ 
sure of 5 cm of mercury was sufficient for hold¬ 
ing the film in position. 

In order to hold the vacuum it was necessary 
to provide a light steel clamping ring that is 
activated by a cam on the same shaft that 
drives the rotatory vacuum valve. 

Louvre Shutter 

A considerable amount of development work 
had already been accomplished at the California 
Institute of Technology on louvre shutters in 
connection with the two earlier Schmidt 
cameras furnished under direct contract with 
the Army. The experiences with an air-driven 
louvre of high speed were used in the design 
and construction of the louvre shutter for the 
2-mirror Schmidt camera. 

The louvre shutter has a clear aperture of 
lS 1 /^ in. and is carried below the correcting 
plate as described above. In order to provide 
for crab adjustment of the camera in azimuth, 
the bellows connection for light-tightness was 
fastened to a light aluminum ring that in turn 
rotates in a light-tight groove. Thus, the bel¬ 
lows can follow the rotation of the camera as¬ 
sembly without requiring rotation of the louvre 
shutter. 

The shutter has ten blades which rotate 
through 180 degrees on ball-bearing shafts by 
means of pinions meshing with a rack. On each 
successive exposure the direction of travel of 
the rack is reversed. 

The rack bar contains V grooves on each side 
and is guided by a series of ball bearings con¬ 
fined by additional V grooves in the stationary 


guide bars supporting the shutter. One end of 
the rack bar carries the actuating rod which 
is driven by a coiled spring in a spring housing. 
The other end of the rack bar carries the fric¬ 
tion damping rod R. 

The spring drive is so arranged that it drives 
the rack bar in either direction according to the 
cycle of the shutter. A spring detent pin falls 
behind a steel projection on the rack bar to hold 
the shutter blades fixed while the spring is 
compressed. Release of the detent, by means of 
a magnet connected to the switch and cam 
mechanism on the sweep drive, operates the 
shutter. 

The rack bar is brought to a shock-absorbed 
stop by means of a dry-friction damper. This 
damper is mechanically very simple and effec¬ 
tive. In principle, a loose sleeve fits over the 
rod that forms the end of the rack bar. Stops 
are arranged on the rack bar in such a way 
that the sleeve has a definite free range to allow 
for uninhibited movement of the shutter in 
either direction during the exposure. Near the 
end of the stroke, the stop engages the sleeve 
which in turn is made to slide with considerable 
and adjustable friction in an outer cylinder. 
Thus, the energy of the moving shutter is used 
up rather quickly and smoothly in forcing the 
sleeve through the outer cylinder. Adjustment 
of the friction is provided by tapered joints and 
screw threads. 

For varying the exposure time with the 
spring drive, a dynamic principle was used 
rather than any device that might encounter 
the disadvantages and uncertainties of adjust¬ 
ing either the friction or the tension on the 
spring. Part of the energy of the spring is 
diverted into rotation of an auxiliary flywheel. 
The design is so arranged that the retardation 
varies during the exposure in a way that im¬ 
proves shutter efficiency. The energy stored in 
the rotation of the flywheel tends to release it¬ 
self near the end of the stroke, where the 
energy from the spring drive has greatly 
diminished. 

The shutter blades are made of aluminum. 
The design is carefully engineered to prevent 
light-fogging. Each of the shutter blade shafts 
which carry the driving gears has two ball 
bearings, while the more or less unloaded shafts 



100 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


at the opposite end of each blade have a single 
ball bearing. 

Tests were made of shutter speed and effi¬ 
ciency by means of photographs of oscilloscope 
traces. The fastest setting gives an effective 
exposure of 14.5 msec, or about y 70 sec. The 
efficiency characteristic of louvre-type shutters 
is very low, in this case only 54.7 per cent. The 
total exposure time is accordingly almost twice 
the effective time and amounts to about % 0 sec. 
Efficiency improves as the exposure time in¬ 
creases, owing to the flywheel action. 

The low shutter speed for a 30-in. focal 
length puts a very heavy demand on both the 
antioscillation features of the camera and on 
the ground sweep mechanism. Assuming that 
the latter features are nearly ideal, however, it 
is evident that at the effective speed of //3.4, 
very fine grain film can be used. Aerial tests of 
the equipment would be highly desirable. 

In addition to low efficiency, the louvre shut¬ 
ter tends to cause loss of contrast in the aerial 
image. In the position of the blades on either 
side of the center of the exposure, where the 
apparent cross section of blades and openings 
are equal, diffraction smears out the resolution 
altogether. In the opening and closing phases of 
the shutter, therefore, in one direction on the 
film, the light must be considered as fogging 
light, rather than as useful light. 

In further work carried out on Schmidt 
cameras for daytime photography, based on 
any special advantages that may be found over 
lens types, it would be advisable to adapt a 
focal-plane type shutter to the camera. It seems 
more likely that the Schmidt camera will find 
an application for night photography in view 
of its fast speed and maximum photographic 
contrast. The present instrument probably 
represents the nearest approach that can be 
made to an effective utilization of the Schmidt 
camera for daytime photography over an ap¬ 
preciable film area. 


15 - 2 Two-Mirror Solid Schmidt Camera 
(Mount Wilson) 22 ’ 23 

The constant need throughout the war for 
lenses of short focal length and high aperture, 


which was further increased by the introduc¬ 
tion of the flash discharge method, led to the 
initiation of several projects making use of 
Schmidt cameras. One of these is known as the 
solid glass Schmidt camera in its 2-mirror 
form, developed at the Mount Wilson Observa¬ 
tory. 

Figure 86 shows the optical system and 
mounting, inserted in a standard 24-in. be- 
tween-the-lens shutter. The clear aperture is 3 
in., and the focal length is also 3 in., so that the 



Figure 86. f/1.4 2-mirror solid Schmidt in K-17 
shutter. 

relative aperture is //1. The loss of light caused 
by the secondary mirror is so large, however, 
that the effective speed is //1.4. 

The optical design of the solid glass Schmidt 
is similar to that of the 2-mirror Schmidt 
camera discussed in Section 1.5.1, except that a 
glass medium has been substituted for air. One 
gains in speed by a factor of n in the /-number, 
so that an //1.6 air Schmidt would perform at 
f/1 in glass. Moreover, the optical aberrations 
of the glass Schmidt at f/1 are identical in 
character and magnitude with those of the air 
Schmidt at //1.6. 

The most serious aberration of the solid glass 
Schmidt camera, unfortunately, is lateral color, 







































SCHMIDT CAMERAS 


101 


which ordinarily is the most serious aberration 
that affects resolution. Thus, good perform¬ 
ance cannot be expected of the solid glass 
Schmidt in even fairly restricted fields at the 
focal length used. Limitation of the spectral 
range by means of filters is not adequate for 
overcoming the large amount of lateral color 
present. 

Figure 87 shows a star field photographed 
with the solid Schmidt camera. The radial elon¬ 
gation of images in the outer part of the field 
is evident to the eye. It is clear that the optical 
system must be achromatized before any useful 
results can be realized with this form of cam¬ 
era. On the other hand, if this achromatization 
can be realized, there are very few other diffi¬ 
culties remaining. The system is compact and 
adaptable to standard equipment. The film can 


easily be deformed to fit the 30-degree field by 
pressure from a flexible pad. 

It is probable that the system is not as useful 
as the Rochester f/1 lens with curved field, and 
that further efforts might appropriately be con¬ 
fined to such lenses. If the solid glass Schmidt 
were achromatized to give the same perform¬ 
ance as the f/1 lens, it is probable that the same 
number of elements would suffice for an im¬ 
proved all lens system of greater field, and that 
aspheric surfaces could be avoided. 

15,3 Schmidt Camera for Use With Electric 
Flash Night Photography 
(Harvard ) 24 

Early in 1945 an adaptation of the ordinary 
Schmidt camera for night photography was 



Figure 87. Star test with //1.4 Schmidt. 




102 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


proposed under the Harvard contract, of par¬ 
ticular use with the electric flash system. Dur¬ 
ing March and April of 1945 a prototype was 
built in the form of an inexpensive mockup for 
test purposes. 

The fundamental development leading to the 
type of Schmidt scheme proposed is that roll 
film can be made to fit a spherical focal surface, 
provided the film is narrow relative to the 
length of the picture along the roll. The ex¬ 
perience at hand with vacuum and pressure 
methods proved that good contact on a spheri¬ 
cal surface can be obtained over very large 
areas of field. The roll film Schmidt camera can 
also make use of such pressure or suction, if 
necessary, but to a large extent gains its field 
of view by a very long picture across the line 


of flight, and a relatively short picture with 
overlap in the line of flight. 

The prototype model made use of unperfo¬ 
rated 35-mm film, pressed to a sphere of 5-in. 
radius with a picture size of 1x4 in. The pre¬ 
dominant curvature along the 4-in. direction is 
followed by the natural rolling of the film 
around the curve. To aid in holding the film 
against the focal surface, the film is kept under 
moderate tension and the outer edges of the 
film are pressed against the defining sphere by 
means of spring riders on either side. Contact 
within 0.002 in. over the entire picture area 
was observed. 

Figure 88 shows the prototype design. The 
heavy black line represents the path followed 
by the film, which is allowed to move continu- 



Figure 88. Prototype night flash Schmidt camera. 

















































































SCHMIDT CAMERAS 


103 


ously, even during the exposure, because of the 
short focal length and short exposure time of 
the flash unit. Tension on the moving film is 
provided by a braking action on the left-hand 
spool. The film, on its way into the Schmidt 
camera, passes under rollers just below the 
level of the spherical focal surface. With ten¬ 
sion and edge pressure, the film moves along 
at a constant rate, always maintaining contact. 

The angular dimensions of the picture in the 
prototype model are 11x45 degrees. Synchroni¬ 
zation of the flash exposure time with length of 
film produces the desired overlap in the 11-de¬ 
gree direction. The 35-mm film then shows a 
series of pictures, each 4 in. long, represent¬ 
ing a 45-degree transverse coverage, and each 
with some overlap relative to the preceding 
picture. The film is intended for contact print¬ 
ing directly, whereafter the roll of prints is cut 
up and spliced together to give a final print 4 
in. wide and as long as the run taken. It is esti¬ 
mated that 200 ft of film will provide a mosaic 
covering 120 miles long by 45 degrees wide. 

A second much smaller model was con¬ 
structed for production purposes. This model is 
shown in Figure 89. The principles are much 



Figure 89. The 16-mm night flash Schmidt. 


the same, except that 16-mm unperforated film 
is to be used. The aperture is //1 with plastic 
optics. The device was intended for a small re¬ 
cording camera for night bombing purposes 
with planes making use of very small flash 
equipment. The end of World War II prevented 


testing of the one unit that was nearly com¬ 
pleted. 

A third production unit was planned and par¬ 
tially constructed. This system was to contain 
a quartz mirror and was to be mounted for con¬ 
stant focus by means of Invar connecting rods 
between mirror and film. The rest of the camera 
was to be designed primarily for compactness. 
Two 10-in. diameter quartz mirrors and one 
correcting plate were completed before the end 
of the Harvard contract. These systems were to 
operate at f/1 with 8-in. focal length. 

i.s . 4 Miscellaneous Proposals and Data 

An f/1 Schmidt Camera with 
8-in. Focal Length 

Experiments made at the Chicago Aerial 
Survey Company indicated that photographic 
roll film could be forced temporarily into ex¬ 
treme spherical form by use of hydraulic pres¬ 
sure. To this end work was begun on an ordi¬ 
nary Schmidt camera under Harvard Contract 
OEMsr-474. A 10-in. diameter Pyrex mirror 
was to be used in connection with this camera. 
The system was later discontinued because of 
interruption of the experimental work on the 
film mechanisms, and the correcting plate was 
thereafter used in conjunction with the 10-in. 
quartz mirror for the Schmidt described under 
Section 1.5.3. 

An f/1 Schmidt Camera with 8-in. Focal 
Length for Use in Strip Photography 241 " 

Work at the University of Rochester on 
searchlight strip photography indicated that a 
strip Schmidt camera would be of considerable 
promise. Briefly, the ordinary Schmidt was to 
be folded by means of a 45-degree mirror be¬ 
tween correcting plate and spherical mirror. 
Parallel light on axis was to pass through the 
correcting plate, be reflected from the 45-degree 
mirror to the primary spherical mirror, and 
then converge to a focus through a narrow slit 
opening in the 45-degree mirror. Roll film 
stretched on a surface of double curvature was 
to move past a curved slit across the focal sur¬ 
face. The speed of movement of the film was 
to be synchronized with the speed of the air¬ 
plane in the usual strip fashion. Finally, the 



104 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


camera was to be boresighted with an illumi¬ 
nating searchlight so arranged that ground il¬ 
lumination covered only slightly more than the 
projected image of the slit in the focal surface 
of the Schmidt camera. 

The end of World War II interrupted this 
work, which for a long time was given low 
priority. A 12-in. Pyrex spherical mirror was 
finished, and a blank for a correcting plate was 
brought to plane parallelism. 

Approximate Ray-Tracing through an 
f/1 Schmidt Camera 240 

Table 10 represents a compilation of image 
errors found in an ordinary f/1 Schmidt camera 
of 8 in. focal length, as provided by Harvard 
from approximate formulas. The figures should 
agree with exact calculations within 10 per 
cent. Figure 90 defines the rays as numbered 
and plots the results. The coordinates A y and A z 
are in image space on the surface of the focal 
sphere, and therefore represent the final inter¬ 
cepts of individual rays in the focal surface. 


Table 10. Ray intercepts (image errors) of a 
Schmidt camera. 




// l.o 

A y A z 

(mm) (mm) 

//1.4 
A y 

(mm) 

Az 

(mm) 

14 degrees 

1 

—0.261 

0.000 

—0.023 

0.000 

off axis 

2 

—0.214 

0.071 

—0.012 

0.018 


3 

—0.115 

0.080 

0.008 

0.016 


4 

—0.036 

0.041 

0.014 - 

-0.001 


5 

0.000 

0.016 

0.000 - 

-0.011 


6 

0.035 

0.041 

—0.014 - 

-0.001 


7 

0.115 

0.081 

—0.008 

0.016 


8 

0.214 

0.070 

0.012 

0.018 


9 

0.261 

0.000 

0.023 

0.000 

20 degrees 

1 

—0.513 

0.000 

—0.046 

0.000 

off axis 

2 

—0.422 

0.138 

—0.024 

0.036 


3 

—0.226 

0.159 

0.016 

0.032 


4 

—0.071 

0.081 

0.027 - 

-0.002 


5 

0.000 

0.032 

0.000 - 

-0.022 


6 

0.071 

0.081 

—0.027 - 

-0.002 


7 

0.226 

0.159 

—0.016 

0.032 


8 

0.422 

0.138 

0.024 

0.036 


9 

0.513 

0.000 

0.046 

0.000 


The aberration is a mixture of slight astig¬ 
matism with large oblique spherical aberration. 
At 20 degrees off axis, the light in the zone be¬ 
tween //1.4 and //1.0, which contains half the 
light, has considerable aberration. Vignetting 


can be used to a moderate degree to limit the 
aberration, but it is clear that contrast far off 
axis will be low, particularly if the central core 
of the image must be removed by the film 
holder. The better correction of the skew rays 
helps out to some extent. 

Coordinates of an 8-in. Aperture Schmidt 
Correcting Plate for an f/1 Camera 

Table 11 provides the coordinates of the as¬ 
pheric curve on the face of an f/1 Schmidt 
camera of 8-in. focal length. Minimum chro¬ 
matic aberration requires that the zpne of zero 
deviation lie approximately 87 per cent of the 
way toward the edge. The focal length is 8.000 
in., the clear aperture 8.000 in., and the radius 
of the primary mirror is 15.59 in. 


Table 11. Coordinates of an 8-in. aperture f/1 
Schmidt plate. 


Zone height 
on correcting 
plate (in.) 

^-coordinate (in.) 
(by series) 

^-coordinates (in.) 
(exact) 

0.00 

0.000 000 

0.000 000 

0.20 

0.000 125 


0.40 

0.000 500 

0.000 502 

0.60 

0.001122 


0.80 

0.001 964 

0.001 964 

1.00 

0.003 040 


1.20 

0.004 303 

0.004 288 

1.40 

0.005 737 


1.60 

0.007 296 

0.007 290 

1.80 

0.008 980 


2.00 

0.010 726 

0.010 710 

2.20 

0.012 488 


2.40 

0.014 234 

0.014 200 

2.60 

0.015 855 


2.80 

0.017 352 

0.017 314 

3.00 

0.018 599 


3.20 

0.019 565 

0.019 488 

3.40 

0.020 080 


3.60 

0.020 111 

0.020 026 

3.80 

0.019 488 


4.00 

0.018 162 

0.018 060 

4.20 

0.015 855 


4.40 

0.012 628 

0.012 521 

4.60 

0.008 122 



16 ANTIVIBRATION MOUNTS FOR 
AERIAL CAMERAS 25 

After a thorough study of the theory of anti¬ 
vibration mounting and of types of damping, 
two designs were worked out and constructed 
for standard aerial cameras by the Eastman 











ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


105 


Kodak Company under Contract OEMsr-392. 
A third design was constructed for use with the 
multiple camera installation in the F-5E air¬ 
craft. Symmetrically placed coil springs in 
compression, combined with dry friction 
dampers, accomplish efficient antivibration con¬ 
trol for all degrees of freedom without compen¬ 
sation for center-of-gravity shifts. 

Thorough testing both on the laboratory 
shake table and on flight tests has proved their 
superiority over standard mounts. It is recom¬ 
mended that they be used with high-resolution 
aerial photographic equipment. 

The material in this report falls naturally in 
the following group arrangement: (1) Theory 
of antivibration mounting. (2) Descriptions of 
the test models and final designs. (3) Multiple 
camera mount for F-5 aircraft. (4) Laboratory 
tests of a number of aircraft mounts. (5) Re¬ 
sults of flight tests over resolving power tar¬ 
gets. (6) Conclusions. 

1,6,1 Theory of Antivibration Mounting 

The term vibration is understood to apply to 
the higher frequency range of cyclic motions, 


both rotational and linear in nature. More spe¬ 
cifically, the range of frequencies from 800 
cycles per minute upward has received special 
attention in this report. The motions arising 
from aircraft engine and propeller dynamic un¬ 
balance and propeller aerodynamic unbalance 
are therefore the subject of consideration, as 
distinct from lower frequencies of motion in the 
category of roll, pitch, and yaw. 

It is recognized that only the rotational move¬ 
ments, and most particularly those about the 
longitudinal and transverse axes, are of im¬ 
portance as bearing upon picture definition. An¬ 
tivibration mounting or filtering action against 
linear movements may be desirable, not only for 
protection of the camera from mechanical 
shock, but also for reasons to be pointed out 
shortly. 

Center-of-Gravity Principle 

A center-of-gravity mount is one for which 
the resultant force of the mounting elements is 
always directed through the center of gravity. 
Linear components of motion cannot be con¬ 
verted into definition-destroying rotational mo¬ 
tion in a center-of-gravity mount because no 


Ay IN MM 

0.5 0.4 0.3 0.2 0.1 0 -0.1 ~0.2 -0.3 -0.4 -0.5 








106 


EQUIPMENT FOR AERIAL PHOTOGRAPH! 


torque can develop. This is true even if no filter¬ 
ing of linear motions is provided. 

The first requisite of a good mount is, there¬ 
fore, that it make use of the center-of-gravity 
support as far as practical. However, the center 
of gravity of a camera changes vertically with 
the amount of film load and horizontally as the 
film is fed from supply to take-up reel. In addi¬ 
tion, the center of gravity with respect to the 
mounting trunnion varies with camera model 
and even between individual cameras of the 
same model. 

Filter Action for Linear Motions 

After bringing the average position of the 
center of gravity to the most favorable location, 
we still have a significant departure from the 
ideal in most practical cases. There remains 
one alternative, namely, the use of adequate 
filtering action for linear motions as well as for 
rotational motions. 

To say that the amplitude of a disturbance 
has been reduced through the filtering action of 
a mount is equivalent to saying that the forces 
of acceleration acting on the isolated member 
have also been reduced to the same extent. 
Therefore, if we do not have a center-of-gravity 
mount, we can still obtain satisfactory opera¬ 
tion if the linear acceleration forces acting at a 
small distance from the center of gravity are 
sufficiently reduced to keep the resulting angu¬ 
lar motion within tolerable limits. 

Natural Frequency 

The filtering effectiveness of a mount is re¬ 
lated to its natural frequency. The lower the 
natural frequency, other things being equiva¬ 
lent, the better the filtering action for a given 
disturbance. The natural frequency must be 
lower than the disturbing frequency in order to 
produce any filtering action. 

Two of the elements which are always pres¬ 
ent in a mechanical filtering system are: (1) 
the mass from which the vibrations are to be 
filtered, and (2) an elastic support, such as 
rubber or spring members, which may be called 
the compliant member. If such a system, con¬ 
taining only these two elements, is set in mo¬ 
tion and not disturbed by external forces of 
friction, etc., it will continue to oscillate at a 


characteristic frequency called the natural fre¬ 
quency. Let C be compliance measured in inches 
of deflection per pound of load (reciprocal of 
spring constant) and let W be weight of mass 
in pounds; then 


187.6 

* " VWC’ 


( 1 ) 


where F n is natural frequency in cycles per 
minute. This formula applies to any linear di¬ 
rection of motion if the compliance C applies 
to that direction. It is of interest to note that, 
as applied to the vertical direction, if the spring 
deflects statically by the amount D inches from 
zero load to a load equal to W, then 


F n = 


187.6 

Vd' 


( 2 ) 


We now consider the case of a rotational de¬ 
gree of freedom with a restoring torque about 
the axis of rotation. If k is the radius of gyra¬ 
tion of the mass about its axis of rotation and r 
is the radius at which the springs act, then 


F n 


r 187.6 
k VWC’ 


(3) 


where F n is the rotational natural frequency in 
cycles per minute, W is again the weight of the 
mass in pounds, and C is the combined linear 
compliance of the springs (inches per pound). 

The second requisite of a practical mount is 
that the natural frequency for all components, 
linear as well as rotational, be low. Only then 
is good filtering obtained for those disturb¬ 
ances above the natural frequency. 

Low natural frequency requires high com¬ 
pliance or “softness” in the mount. As a con¬ 
sequence, a point is reached where the in¬ 
creased instability of the mount permits an 
intolerable amount of camera tilt owing to hori¬ 
zontal shift of center of gravity with film move¬ 
ment. There are other factors which place a low 
practical limit to the natural frequency. Ex¬ 
amples are the stiffness of such cable connec¬ 
tions as may be required, forces of air turbu¬ 
lence against lens cone, and convenience in 
handling. The shift of center of gravity seems 
to be the most important factor, however, and 
a figure of 80 to 200 cycles per minute is a prac¬ 
tical range. 


Damping 

With a proper mounting system of low nat- 










ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


107 


ural frequency, vibration disturbances are well 
filtered out, even if no damping is applied. It is 
for proper behavior of the mount at and near 
the natural frequency that damping must be 
supplied. The slow, continuous roll and pitch of 
the airplane may contain harmonics which 
would cause the camera to oscillate at the nat¬ 
ural frequency with large amplitude if damp¬ 
ing is not provided. 

All disturbances of a transient nature, such 
as bumps and jerks from air pockets, are sure 
to set up oscillations, and the properly damped 
mount will recover from such transients in the 
minimum of time. 

The type of damping and method of applica¬ 
tion are very important. In general, it may be 
said that the better the control of amplitude at 
the natural frequency, through application of 
damping, the poorer the filtering action at 
higher frequencies. This is true because the 
damping mechanism increases the coupling be¬ 
tween camera and vibrating support, thus in¬ 
troducing more vibration into the camera. The 
effect is different for different types of damp¬ 
ing. 

Types of Damping and Method of 
Application 

Under this heading will be treated the vis¬ 
cous and dry-friction types of damping as ap¬ 
plied either directly or through the medium of 
an auxiliary compliance. The performance of a 
mount can be shown to good advantage by plot¬ 
ting what is called magnification factor (also 
called transmissibility) as a function of the fre¬ 
quency of a simple harmonic disturbing vibra¬ 
tion. The magnification factor is the ratio of 
camera amplitude to the amplitude of disturb¬ 
ance. Magnification factor curves may be ob¬ 
tained experimentally on a test stand but much 
can be learned preliminary to the design of a 
mount by calculating magnification factor 
curves from damped vibration formulas. This 
method is used to demonstrate the effect of 
damper type and of the use of an auxiliary com¬ 
pliance. 

Only one component of movement will be con¬ 
sidered, and the constants will be selected to 
give a free (damper removed) natural fre¬ 
quency of 100 cycles per minute and a maxi¬ 


mum magnification factor of 2. The curves 
apply equally well for either a translational or 
a rotational vibration, although the schematic 
sketches indicate only translational move¬ 
ment. 25 ' 1 

Figure 91 is the schematic arrangement for 
direct viscous damping. Viscous damping is 
characterized by the fact that the resisting 
force is proportional to the velocity of relative 
movement. It is obtained by the use of an oil 
film or by such arrangements as dashpots and 
bellows. Sponge rubber may be considered as 
supplying viscous damping, although usual de- 


MAIN -— 

COMPLIANCE 


' "^ -VIBRATING 
MEMBER 


'VISCOUS 
DAMPER 

AUXILIARY 
COMPLIANCE 


CAMERA 


Figure 91. Direct viscous damping. 


signs seldom supply enough damping. Figure 
92 shows the magnification factor curve for di¬ 
rect viscous damping in a sufficient amount to 
bring the maximum factor to 2. 

Figure 93 shows the arrangement when vis¬ 
cous damping is employed with an auxiliary 
compliance, and the magnification factor is also 
shown in Figure 92. The curve of Figure 92 
was calculated for the case of an auxiliary com¬ 
pliance equal to one-half of the main compliance 
and the proper damping to bring the maximum 
factor to 2. 

Even though the magnification factor does 
not go above 2 for the case of the auxiliary 
compliance, it appears from Figure 92 that 
resonance control is less satisfactory than that 
for direct damping, as the peak is broader and 
extends to higher frequencies. There are, how¬ 
ever, two very important advantages which 
more than offset this disadvantage. The first 
is that the magnification factor above about 500 
cycles per minute is lower and becomes in¬ 
versely proportional to the square of the fre¬ 
quency. The magnification factor is inversely 











108 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


proportional to the first power of frequency for 
the case of direct viscous damping. The second 
advantage is that the amount of damping may 
change over a wide range (as may happen 
with temperature changes, dust-clogged orifice 
holes, or improper adjustment, etc.) without 
adversely affecting the amount of filtering 
obtained at the higher frequencies. 

In considering damping means for the first 


ing action. This may be best explained by re¬ 
membering that the rate of energy absorption 
by a damper is equal to the product of force and 
velocity. Now, since the force for a viscous 
damper is proportional to the velocity, it fol¬ 
lows that the rate of energy absorption is pro¬ 
portional to the square of the velocity. For the 
dry-friction type of damping, however, the 
force is substantially independent of velocity. 


AUXILIARY COMPLIANCE = 1/2 MAIN COMPLIANCE 
DAMPING SUCH AS TO MAKE MAXIMUM FACTOR =■ 2 



l 


COMPLIANCE 


U 


^-VIBRATING 
MEMBER 


-•-VISCOUS 
DAMPER 


V 

— -CAMERA 


Figure 93. Viscous damping with auxiliary 
compliance. 


COMPLIANCE 



FRICTION 

DAMPER 


CAMERA 


Figure 94. Direct dry-friction damping. 


spring mount, it was decided to try dry-friction 
or coulomb damping because of its greater con¬ 
venience. This is apparent when it is considered 
that one small friction unit may supply damp¬ 
ing in two or three degrees of freedom, thus re¬ 
placing several larger and more delicate units. 

It was discovered that dry-friction damping 
also possesses an important advantage over vis¬ 
cous damping from the viewpoint of the filter- 


We may say then that the rate of energy ab¬ 
sorption is about directly proportional to the 
velocity for dry-friction damping. 

If we consider two different oscillating move¬ 
ments, both of the same amplitude but of dif¬ 
ferent frequency, and if we have viscous and 
dry-friction dampers which absorb the same 
energy at the lower frequency, we shall dis¬ 
cover that the viscous damper absorbs much 























ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


109 


more energy at the higher frequency because, 
while the amplitudes are the same, the effec¬ 
tive velocity is greater at the higher frequency. 
This is equivalent to saying that the dry-fric¬ 
tion damper offers less disturbing force at the 
higher operating frequency, while providing 
adequate control at the natural frequency. 

The exact nature of the dry-friction damper 
must be known in order to calculate correctly 
the magnification factor curve near resonance. 
The behavior at the higher frequencies is less 
dependent on the nature of the friction and can 
be calculated with adequate accuracy. Figure 
92 shows the calculated curve for direct dry- 
friction damping for higher frequencies. The 
magnification factor for higher frequencies be¬ 
comes inversely proportional to the square of 
the frequency and is lower than that for viscous 
damping, even with auxiliary compliance. Fig¬ 
ure 94 gives the schematic arrangement for 
direct dry-friction damping. The effect of using 
an auxiliary compliance with dry-friction 
damping is almost negligible as far as filtering 
is concerned. 

The relative performance of actual mounts 
may easily be calculated for one component of 
vibration if the natural frequency and type and 
amount of damping are known. If the displace¬ 
ment of the image relative to the film is known 
as a function of time, then the relative distance 
through which the image moves during expo¬ 
sure is found by observing the movement dur¬ 
ing the change of time from t 0 to t 0 + exposure 
time. The worst condition occurs when expo¬ 
sure starts just before and ends just after the 
time of maximum relative velocity. Therefore, 
if the disturbing amplitude and frequency, the 
magnification factor, and the exposure time are 
known, the blurring or movement of image 
relative to film during exposure can be de¬ 
termined. If we take the somewhat severe case 
of the mount support members, which are 20% 
in. apart, having a vertical vibration amplitude 
of 0.015 in. 180 degrees out of phase, a rota¬ 
tional disturbance is set up in the camera which 
would produce an image movement relative to 
film, with an amplitude of 0.0349 in. for a 24- 
in. lens if the camera is mounted rigidly to the 
support. Figure 95 shows the maximum amount 
of blurring movement for % 0 -sec exposure for 


this case, with the three types of damping 
shown on Figure 92 for frequencies between 
800 and 2,200 cycles per minute. 25b 

Caution must be exercised in interpreting 
Figure 95 in other than relative values; in the 
actual case, the combined effect of all compo¬ 
nents of all vibrations at different frequencies 
must be considered with the complicated inter¬ 
actions of the different components when the 



CYCLES PER MINUTE 

Figure 95. Effect of damping type upon picture 
blurring. 


center of gravity does not coincide with the 
center of suspension. 

The third requisite of a good mount is, 
therefore, suitable damping elements of 
which the dry-friction type has definite advan¬ 
tages. 


162 Description of Test Models 

The first experimental mount consisted of a 
double-ring gimbal arrangement with ball¬ 
bearing gimbal axes. An air-bellows damper 
was used. The bellows was of metal construc¬ 
tion with a long leakage path through felt pads. 
The compliance of the metal bellows supplied 
the main compliance while that of the air vol- 




no 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


ume acted as an auxiliary compliance for the 
damper. 25c Tests proved this mount to be very 
effective for rotational disturbances. 25 * 1 It was 
soon discovered, however, that either an accu¬ 
rate automatic counterbalance was needed to 
keep the center of gravity at the intersection of 
the gimbal axes or effective filtering for linear 
components of motion was needed. 

The use of compression spring elements with 
friction dampers to give complete linear and 
rotational filtering action proved to be the sim¬ 
plest and most effective solution. 

The first design of a spring mount made use 
of a standard A-8 mount. The only change was 
to replace the four sponge-rubber buffers with 
enclosed spring and friction damper units. 25e 
This produced an experimental mount quickly 
and proved the effectiveness of the design prin¬ 
ciples in subsequent laboratory and flight tests. 
This model was also provided with a combina¬ 
tion intervalometer and sweep mechanism for 
ground-speed compensation. 

A second design was then worked out and 
four models constructed. Figure 96 is a photo¬ 
graphic view of this design, and Figure 97 
shows details of the spring and damper units, 
the trunnion clamp, and the bearing for sweep 
motion. These parts are shown to scale and in 


their correct relative heights. The outer ring of 
the A-8 mount is again used but the sponge- 
rubber pads are replaced by rigid connectors. 
The springs for this design are located on an 
internal ring structure at the corners of an 
imaginary rectangle 10x14 in., with the long 
dimension along the longitudinal direction. This 



Figure 96. Spring mount, second design. 


location of the springs gives greater stability 
about the transverse axis at which greater un¬ 
balance torques occur owing to film transport. 
The spring location in the first design was in- 



FILTER UNIT 


SWEEP AXIS 


Figure 97. Details of spring mount, second design. 





































































ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


111 


correct for this consideration. Also, the springs 
of the first design were in a lower horizontal 
plane than the average center of gravity for 
the K-17 and K-22 cameras with 24-in. lens. 
This condition is corrected in the second de¬ 
sign. The spring and damper design of Figure 
97 accomplishes symmetrical damping by 
means of the internal ball-and-plunger arrange¬ 
ment with the small loading spring. Both stain¬ 
less steel balls are soldered on their lower sides 
to the supporting steel member, while on the 
upper sides they are free to slide in the syn- 
thane (bakelite fiber) sockets. By virtue of this 
sliding friction, damping action is provided for 
all horizontal linear motions. The steel plunger, 
sliding in the synthane sleeve, in turn provides 
friction damping for vertical motions. The 
plunger fit is not sufficiently snug for any air 
dashpot action. A symmetrical damper unit of 
compact and ragged design has thus been pro¬ 
vided with operation in three degrees of free¬ 
dom. 

The camera weight and radius of gyration 
may be varied by about plus or minus 15 per 
cent without appreciably affecting the perform¬ 
ance. The springs can, of course, be changed for 
other camera weights. The camera center of 
gravity should remain within a radius of ap¬ 
proximately 1 in. of the geometrical center of 
the spring locations. 

The damping friction is best determined 
under actual test conditions. The damper ad¬ 
justment is, however, not critical. A good cri¬ 
terion in judging the damping action is to push 
the loaded mount all the way against the rub¬ 
ber stops (approximately % in.) in one direc¬ 
tion, and if upon release the mount comes to 
rest in two or three cycles, the amount of damp¬ 
ing is correct. 

The method of ground-speed compensation, 
described in Section 1.6.8, calls for a variable- 
speed electric motor on the mount to swing 
the camera back and forth. The first mount 
arrangement places this motor on the same side 
of the spring suspension as the camera. It is 
feasible to place the motor drive outside of the 
spring supports as has been done in the later 
design. This safeguards the camera from motor 
vibrations without any reliance upon motor 
suspension. 


16 3 Multiple Camera Mount for F-5E 
Aircraft 

Construction of spring and damper elements 
for the multiple camera mount was undertaken 
at a somewhat later date than the development 
of the single camera mounts described above. 
A description of the multiple mount is of inter¬ 
est at this point because of the unique damping 
arrangement employed and because it illus¬ 
trates the possibility of providing a center-of- 
gravity mount while utilizing previously de¬ 
signed supporting structures of apparently un- 
symmetrical location. 

The F-5E (a photographic conversion model 
of the P-88 fighter aircraft) makes use of a 
welded frame structure of 1-in. seamless steel 
tubing; within this structure is another welded 
frame of %-in. seamless steel tubing adapted 
to support various combinations of from one to 
three cameras in the nose of the ship. The vi¬ 
bration filtering elements are placed between 
the two frames to provide a support for the 
inner frame which, with the cameras, acts as a 
rigid unit. 

The total maximum weight of the inner 
structure with cameras is approximately 220 lb. 

The arrangement found to utilize best the 
supporting members of the frame structure 
already constructed makes use of two large 
compression springs, one on each side of the 
frame and 5% in. back of the combined center 
of gravity. Each large spring supports about 
81 lb of the total load. Two smaller springs in 
tension are located 14% in. in front of the 
center of gravity, each supporting about 29 lb. 
The front springs are coiled loosely around a 
stiff steel wire calculated to give the necessary 
lateral compliance not provided by the tension 
springs. The rear springs are also below the 
center-of-gravity level, but the front springs 
are correspondingly high. The static deflection 
of all springs is the same (a little less than 1 
in.) ; this is a necessary condition for a center- 
of-gravity mount. 

Figure 98 shows the four mounting elements 
in correct relative position except for being 
crowded much closer together than they would 
be for normal use. The rubber-faced stops next 
to the large springs fit around trunnions pro- 



112 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


vided on the inner structure. The caps at the 
upper ends of the tension springs also house 
stops. The picture shows quite clearly the four 
synthane friction pads inside each of the large 
springs. The lower ends of the two lower pads 
and the upper ends of the two upper pads are 
held to their respective spring end caps by a 
loose-fitting pin in a hole through the pad. Be¬ 
tween each pair of pads is an expansion spring 
keeping the pads pressed against the inside sur¬ 
faces of the large spring coils. In the normal 
compressed condition, the upper pair of pads 
telescope between the lower pair. The friction 
between the pads and the spring coils provides 
the requisite damping in all three degrees of 
freedom. For this type of damping, which is 
extremely simple and requires no extra space, 
it is necessary to design the spring with a suf¬ 
ficient number of coils to afford a good fric¬ 
tion contact for the pads. 


16,4 Laboratory Test of a Number of 
Mounts 

The test stand provides a standard mount 
support which can be vibrated rotationally 
about a horizontal axis or translationally in a 
horizontal direction over a wide range of fre¬ 
quency and with adjustable amplitude. The am¬ 
plitude remains constant throughout each test. 
The range from about 10 cycles per minute to 
about 800 cycles per minute can be covered 
with ease of adjustment to any particular fre¬ 
quency. The frequency is measured by the use 
of a strobotac and a segment disk on the eccen¬ 
tric shaft. Segment rings of 60, 30, and 10 divi¬ 
sions were found useful. 

The rotational movement of the camera was 
measured by noting the movement of a focused 
light beam reflected from a small mirror on the 
camera. The readings were carried out in a 
semi-darkened room. One observer could adjust 
speed, read amplitude, and plot a curve directly 
with convenience. The resulting curve of am¬ 
plitude versus frequency is in effect a magnifi¬ 
cation factor curve if the proper ordinate scale 
factor is applied. 

The testing technique found to be most suit¬ 
able requires two steps: First, a curve of re¬ 


sponse is determined for a pure rotational in¬ 
put (about a horizontal axis through the center 
of gravity of the camera for full film load) at 
an amplitude of 4.3 min of arc; and, second, a 
curve of response for pure horizontal transla¬ 
tion (along the axis perpendicular to the axis of 
rotation) with an amplitude of 0.015 in. These 
test curves are plotted along with a third com¬ 
posite curve obtained by addition of the other 
two curves. These composite curves provide a 
ready comparison between mounts and of 
changes in a given mount. 



Figure 98. Parts of antivibration mount for 
F-5E aircraft. 

It is possible to report here only a small part 
of the tests made. 25f The most interesting re¬ 
sults are those for the standard A-8 mount, the 
A-ll mount, and the Eastman-NDRC Robinson 
spring mount. Figure 99 gives the test curves 
for these mounts. These are composite curves 
determined by the procedure outlined above, 
except for the case of the A-8 mount for which 
the rotation and translation components of the 
shake table were simultaneously applied. The 
composite method would show a less favorable 
curve for this mount. 




ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


113 


The curves for the A-8 and A-ll mounts 
show two characteristic peaks. The second peak 
in each case is caused by the translational reso¬ 
nance converting into rotation. Greatly im¬ 
proved operation would be obtained by: (1) 
reducing the translational natural frequency, 
(2) relocating the center of gravity (the center 
of gravity comes much too high for the K-17 
or K-22 cameras with 24-in. lens in both 
mounts), and (3) providing better damping 
action. The first peak in each case is that caused 
by the rotational resonance. For the case of the 
A-8 mount this is seen to occur at a much 
higher frequency than that dictated from con¬ 
siderations of best filtering action compatible 
with stability. 

The A-ll mount presents a case of unusually 
poor damping control of the rotational reso¬ 
nance peak. The A-8 mount would also profit 
by better damping control, the action at present 
being more in the nature of a snubbing action 
rather than true damping. 

The spring mount curve shows satisfactory 
operation in conformity with the established 
design principles. 

1,6 3 Results of Flight Tests on 

Resolving-Power Targets 

Table 12 gives the summary of results ob¬ 
tained in the comparative test of the first mount 
design with a standard A-8 mount. 


vided so that two cameras and two mounts 
could be used simultaneously. 

Two K-17 cameras with 24-in., //6 lenses 
were used in the test. One had a Bausch and 
Lomb lens which had been previously tested 
and focused at Kodak Park in Rochester, New 
York. The other K-17 had an Eastman lens and 
was not prechecked for focus. Super-XX film 
with No. 12 minus-blue filters was used. 

Electric connections were made so that both 
shutters could be tripped together, either manu¬ 
ally or by the sweep intervalometer of the East¬ 
man mount. A standard viewer with 10-in. lens 
and ground glass was arranged to show when 
exposures should be made. Three pictures were 
made in each camera at each pass over the tar¬ 
get, using the standard overlap of 60 per cent. 
The first and last shots placed the target ap¬ 
proximately 8.5 degrees off the camera axis, 
while the middle shot was at approximately 0 
degrees. 

The altitude was 10,000 ft and the ground 
speed approximately 160 mph, resulting in an 
interval of approximately 6.5 sec. 

Two passes were made for each of the sixteen 
possible combinations of mount location in the 
plane, camera, exposure setting, and sweep on 
or off. A few shots missed the target, but ninety 
usable pictures were obtained in each camera. 

Following the aerial pictures, about eighteen 
ground pictures with each camera were made 
of a resolving-power chart at 386 ft, with 
spacers behind the lenses. The cameras 


Table 12. Results of flight tests on Eastman Kodak and A-8 mounts made at Wright Field, July 15 and 
16. 1943. 


Shutter 

speed 

(sec) 

Aperture 

Degrees 
off axis 

No. of 
observa¬ 
tions 

Avg. resolving power (lines/mm) 

Per cent 

A-8 EK Improve- 

mount mount ment 

Sweep 
for EK 
mount 



For lines parallel to flight 




1/150 

f/8 

0 and 8.5 

46 

7.07 

7.26 

2.7 


1/50 

f/11-16 

0 and 8.5 

44 

5.99 

6.43 

7.3 




For lines perpendicular to flight 



1/150 

f/8 

0 and 8.5 

23 

6.56 

6.84 

4.3 

off 

1/50 

f/11-16 

0 and 8.5 

24 

4.00 

4.13 

3.3 

off 

1/150 

f/8 

0 and 8.5 

23 

6.52 

7.50 

15.0 

on 

1/50 

f/11-16 

0 and 8.5 

20 

3.97 

8.59 

116.4 

on 


The tests were carried out in an F-2 Beech- were placed horizontally on a sturdy table and 
craft two-motor plane at Wright Field on July the same exposures and angles were used as in 
15 and 16, 1943. Two mount supports were pro- the plane. The cameras gave nearly equal re- 







114 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


solving power, averaging 10.8 lines per mm at 
//8, and 11.7 lines per mm halfway between 
f /11 and //16. 

Table 13 shows the average resolving power 
for the 8.5-degree shots combined with the 
0-degree shots. This gives predominance to the 
8.5-degree shots since there were approxi¬ 
mately twice as many, but it is felt that this is 
a fair estimate for overall picture quality. 

The antivibration feature of the mount 


ished. This could be detected unmistakably by 
touching the camera or suspended parts lightly 
with the fingers. 

The results were to some extent masked by 
the bad conditions of roll and pitch in the F-2 
airplane. 

Bedford Tests of Second Mount Design 

On August 8, 1945, a test flight took place 
from the Bedford Army Air Field. Many of the 




showed a consistent but small improvement. 
The sweep mechanism showed a definite and 
valuable improvement, particularly at % 0 sec 
exposure. 

The Eastman-NDRC mount was observed to 
remove vibration from the camera to a marked 
degree, while for the A-8 mount the vibration 
appeared to be transmitted almost undimifi- - 


conditions of the test were much more favor¬ 
able than for the first test. A B-17 bomber was 
used; this proved to be quite stable with the air 
conditions prevalent, so that the factors of pitch 
and roll were much reduced. 

A camera equipped with a Harvard-NDRC 
40-in. //5 telephoto lens was used for this test. 
This was equipped with a thermostat tempera- 














ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


115 


ture control and automatic focus, and had been 
checked on previous flights. One of the four 
mounts of the final design had previously been 
equipped with springs and sweep eccentric for 
this camera. Pan-X Aero film was used through¬ 
out the test. A number of individual readings 
gave a resolving power of 30.8 lines per mm. 
Thus the camera-film resolution in angular units 
was about 4.5 times that for the first test. 

The test targets used were those constructed 
by Harvard on the abandoned air field at 
Orange, Massachusetts. Sensitometric checks 

on the film indicated a contrast ratio of 5/6 for 

the arrays of parallel-line test targets used. 

Table 13. Results of flight tests of Eastman 
Kodak Model II mount and A-8 mount, made at 
Bedford, Massachusetts, August 8, 1945. 

Avg. resolving 
power 
(lines/mm) 

Shutter No. of Per- Sweep 

speed Aper- obser- pen- for EK 

(sec) ture vations Mount Parallel dicular mount 


1/150 

//10 

48 

A-8 

18.2 

5.1 


1/150 

Z/io 

24 

EK 

21.1 

5.6 

off 

1/150 

//10 

14 

EK 

24.4 

27.9 

on 

1/800 

//5.6 

11 

A-8 

21.6 

19.4 


1/800 

//5.6 

9 

EK 

23.2 

23.7 

on 


Most of the pictures were made at % 50 sec, 
//10, with No. 12 yellow filter. The longer ex¬ 
posure time gives maximum emphasis to the 
motion factors. Some pictures were made at 
Ys oo sec and //5.6 with the No. 12 filter. 

The altitude was 9,800 ft and the speed about 
200 mph. The sweep interval was set at 3.0 sec. 
When the sweep was not used, the shutter was 
tripped manually by an electric connection at 
an estimated interval of 3 sec. An average of 
about eleven target images was thus obtained 
on each pass over the target with random posi¬ 
tioning in the picture area. Ten useful passes 
were made. 

Table 13 shows the average resolving power 
for lines parallel and perpendicular to flight 
and for each test condition as shown. 

The use of the antivibration mount without 
the sweep mechanism improves the resolving 
power for lines parallel to flight by 16 per cent 
for %5 0 “ se c exposure. The improvement is in¬ 
creased to 34 per cent for lines parallel to flight 


when the sweep is used. The low resolution for 
lines perpendicular to flight without the sweep 
has a definite influence upon the determination 
for lines parallel to flight; the lines are smeared 
out to a greater length thus affecting exposure 
and in some cases producing overlapping of 
charts. 

The use of the sweep mechanism with M 50 - 
sec exposure time improves the resolution for 
lines perpendicular to flight by 400 per cent 
over that obtained with the antivibration 
mount without sweep, and by 450 per cent over 
that obtained with the A-8 mount. 

The magnitude of the overall improvement 
can best be judged by comparing 5X enlarge¬ 
ments of the targets. Figure 100 shows one 
group of resolving-power targets and is repre¬ 
sentative of the average result obtained at y 150 
sec with the A-8 mount. Figure 101 shows the 
same group of targets taken at y 150 sec with 
the antivibration mount with sweep working, 
and it is representative of the average for these 
conditions. 

The crabbing angle was estimated at 5 de¬ 
grees and adjusted accordingly at the time of 
the test. Later inspection of the negatives 
showed that this was too much adjustment. The 
effect is small but is greater for lines parallel 
than for lines perpendicular to flight. This, to¬ 
gether with the fact that the roll effect is 
greater than the pitch effect, explains why 
greater resolution is obtained for the perpen¬ 
dicular lines than for the parallel lines. 

At a shutter speed of y 800 sec, the antivibra¬ 
tion mount with sweep shows an improvement 
of 22 per cent for the lines perpendicular to 
flight. The lines parallel to flight show an im¬ 
provement of over 7 per cent. It is quite evi¬ 
dent that the new mount with its sweep mech¬ 
anism brings about a very worth-while im¬ 
provement. The use of any high-resolution sys¬ 
tem, and especially those involving longer focal 
length lenses, should not be considered without 
the use of such a mount and ground-speed com¬ 
pensation. 

Additional Flight Tests 

During the period from July to November, 
1945, a number of both day and night flights 
were carried out from Bedford in connection 







116 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


with the work at Harvard under Contract 
OEMsr-474. 10b 

In a large percentage of the day flights the 
Eastman-NDRC mount was used, and for the 
most part with the Harvard-NDRC 40-in. tele¬ 
photo //5 lens. These tests were not planned 
primarily to compare various mounts. How¬ 
ever, very consistently high resolution was ob¬ 
tained on a number of flights (the highest 
being 41 lines per mm 10c on Pan-X film). 

The night tests were made to yield quantita¬ 
tive information on the amount and type of 


showed the Eastman-NDRC mount to be in¬ 
ferior to either the A-ll or A-8 mounts. Further 
analysis, however, shows that for the particu¬ 
lar flights on which the Eastman-NDRC mount 
was tested the motions of the aircraft were con¬ 
siderably larger than for the flights on which 
the A-ll and A-8 mounts were tested. The 
average magnitude of aircraft motion was, in 
fact, greater by a larger ratio than the mount 
performance ratios. The data, while not con¬ 
clusive, do indicate a margin in favor of the 
Eastman-NDRC antivibration mount. 



Figure 100. 5X enlargement of target, A-8 mount, 40-in. lens, tiso sec at //10, Pan-X Aero film, No. 12 
filter. 



Figure 101. 5X enlargement of target, EK mount with sweep, 40-in. lens, Uso sec at //10, Pan-X Aero 
film, No. 12 filter. 


actual motion from the analysis of dot trails 
produced by flashing strobolux and neon lights. 
A number of other analytical test targets and 
devices 100 were employed for both day and 
night tests. 

In the analysis of night flight tests, all mo¬ 
tions with periods shorter than 1 sec were ar¬ 
bitrarily ascribed to the camera-mount system, 
while only those with periods longer than 1 sec 
were ascribed to the aircraft. The summary 
tabulation 106 of “average median angular veloc¬ 
ities in mils/sec of the camera-mount system” 


The frequencies of mount vibration 101 as de¬ 
termined from the light trails by visual inspec¬ 
tion are in line with what would be expected 
from the laboratory tests of the mounts. The 
Eastman-NDRC mount with few exceptions 
showed no frequencies higher than its natural 
frequency of 1.7 to 2.0 c. The A-8 and A-ll 
mounts showed higher frequencies ranging 
quite generally up to 12 c. 

Earlier flight tests at Wright Field in the 
F5-E aircraft were also analyzed. 10 ^ Since the 
frequencies reported for the experimental 
























ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


117 


spring mount are well above the natural fre¬ 
quency for the suspension, there is good reason 
to conclude that the suspension was not free to 
function as intended. 

The method of flash trail analysis is a pow¬ 
erful tool and can be made to yield much needed 
information. It is unfortunate that the reduc¬ 
tion requires much tedious and time-consuming 
labor. The value for both design and test pur¬ 
poses would be very much enhanced if the 
analysis in the form of an amplitude-frequency 
spectrum could be quickly and accurately de¬ 
rived. 


16 ’ 6 Center-of-Gravity Mount 

(Harvard) 1011 

Although almost every standard and experi¬ 
mental mount has approximated the condition 
that airplane forces imparted to the camera 
should pass only through the center of gravity, 
vibration studies indicate that no mount has 
succeeded altogether in this respect. In princi¬ 
ple, if a camera is suspended at its exact center 
of gravity, no rotational forces can be imparted 
to change the direction of the optical axis. 
Translatory forces can only displace the optical 
axis by a negligible amount, as seen from the 
ground. 

The application of the center-of-gravity prin¬ 
ciple assumes that the optical system within 
the support is a rigid body. In practice, such is 
not exactly the case. Residual translatory vi¬ 
brations imparted through the precise center of 
gravity can set up vibrations within component 
parts of the camera system, and these in turn 
may affect the direction of the optical axis. 
For example, translatory vibrations might set 
up lateral vibration of a loose lens element, 
which in turn would shift the optical axis di¬ 
rectly. 

An attempt was made under the Harvard 
Contract OEMsr-474 to apply the center-of- 
gravity principle in detail and at the same 
time to provide for twin-mounted 40-in. tele¬ 
photos that would give double coverage picture 
mosaics. Figure 102 shows a view of the ap¬ 
paratus developed. 

The angle between the two telephotos has 


been chosen to give an overlap of */2 in. across 
the line of flight. The final mosaic therefore has 
a width of 17.5 in. The two telephotos are 
oriented, about a vertical axis, 90 degrees from 
the usual orientation of single cameras. This 
arrangement provides for opposed shutter re- 



Figure 102. The center-of-gravity mount with 
two 40-in. //5 telephotos. 


coil within the two camera bodies, which in 
turn should eliminate the small angular recoil 
of the optical axis of a camera when a focal 
plane shutter is tripped, although it will not 
necessarily eliminate secondary vibrations im¬ 
parted by release of the shutters. The cross 
mounting also permits equal and opposite film 
winding to preserve the exact center of gravity 
of the mount as a whole, as the film is 
used. 

The entire 260-lb double assembly is sus¬ 
pended by a small steel ball (% in. diameter) 
in a hardened steel cup at the exact center of 
gravity. The vertical adjustment of the center 
of gravity is obtained by a screw and lock nut. 
There is no arrangement provided for adjust¬ 
ment in a horizontal plane. Experience proved 
that such adjustment is highly desirable, and 
that small discrepancies in balance in the ab- 




118 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


sence of restoring forces upset the alignment of 
the twin mount, which is analogous to the 
principle of weighing on an ordinary balance. 

The center-of-gravity mount rested on a 
wooden truss, supported on sponge-rubber pads 
placed on the floor of the airplane. In this way 
translational vibration was reduced. 

Flight tests on the program at Orange, 
Massachusetts, conducted at night over flashing 
neon lights proved that this center-of-gravity 
mount gave better performance than any other 
mount tested. With the 40-in. NDRC telephotos 
the center-of-gravity mount gave an angular 
velocity of 0.99 mils per sec in roll and 0.67 
mils per sec in pitch from the average of four 
films, which is approximately one-half of the 
velocities given by the next best mount. These 
results were obtained in spite of the following 
points: 

1. The lenses were procured from Army 
production and were not checked beforehand 
for tightness of lens elements in their cells. 
Since there are seven elements in each system, 
a slight looseness of one or more is probable. 

2. The exact center of gravity was not 
achieved, since no adjustments in a horizontal 
plane had been provided, and since the power 
cables upset the balance in the plane. 

3. An error in machining caused one camera 
to be off laterally by Vs in. in position. Balanc¬ 
ing was accomplished by weights taped on in 
the best available places. 

4. The lower ends of the cameras were 
clamped together by means of a protective thin 
felt separator, rather than by rigid contact, as 
would be desirable. 

5. The lack of restoring forces makes it diffi¬ 
cult to control long period motion in yaw. The 
off-center position of the cameras accentuates 
yaw difficulties. All steadying was done by 
hand, with residual linear motion. 

6. The clamping of the adjustable screw for 
vertical center-of-gravity correction was mis¬ 
construed in the machining for easy fit. Conse¬ 
quently, the cameras were not rigidly connected 
to the center of gravity. 

7. Each telephoto contains the automatic fo¬ 
cusing unit, which controls the position of the 
rear element by spring and bellows. It is prob¬ 
able that the rear element will react to trans¬ 


lator vibrations imparted through the center 
of gravity, although all flight tests to date show 
no ill effects up to perhaps 30 lines per mm. 

8. The observer was often forced to hold the 
mount steady for some moments by hand in 
order to overcome tilting caused by swinging 
cables and plane movement. On at least one 
flight the center of gravity was markedly below 
the point of support in a pendulous suspension 
through a misinterpretation by the observer at 
the time. 

In spite of these eight points, the mount still 
gave the performance indicated above. All in 
all, it would seem that if the center-of-gravity 
principle were followed in all its exacting detail 
down to the last piece of metal, the vibration in 
angle would be almost entirely eliminated. Nat¬ 
ural frequencies of individual parts of the ap¬ 
paratus should be watched with a view to mini¬ 
mizing angular effects of translatory vibrations 
through the center of gravity. 

Daytime flight tests at Orange gave resolu¬ 
tion results very much lower than those ob¬ 
tained with standard equipment. The results 
were so consistently low and so strongly at 
odds with the low vibration rates of the night 
flights that the conclusion must be drawn that 
the lenses were not in good optical adjustment 
in the air. The most probable explanation of the 
discrepancy is that the clamps provided for the 
individual lenses at the lower end, which are 
tightened by nuts at the time of installation by 
machinists in the plane, were drawn up en¬ 
tirely too tight. The 8-in. diameter elements 
are sensitive to strain. The wall thickness of 
the telephoto is about y 10 in. in the vicinity 
of the clamps. The resolution results were con¬ 
sistently lowest in the direction of flight, out 
of keeping with the average figures obtained 
from uncompensated mounts. The clamps would 
tend to produce maximum strain in the line 
of flight. 

Later laboratory tests made with one of the 
lens systems proved that a resolution on Super- 
XX of 45 lines per mm could be obtained. 

Discussion 

Considerable effort should be put on the de¬ 
sign and construction of a new center-of- 
gravity mount for general use. This mount 




ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


119 


should incorporate soft restoring forces, quite 
possibly without damping, and by one means 
or another compensate for ground movement. 
It is evident that if angular vibration rates as 
low as 0.5 mils per second are obtained most 
of the time, and if ground movement is com¬ 
pensated, average resolution will remain at a 
level nearly equal to laboratory resolution in 
the presence of the haze encountered in the air, 
with the lens-film combination. Some attention 
should be given to activating the soft restoring 
forces by means of gyro control and servo 
mechanisms for maintenance of a good nadir. 
Careful attention should be given to the elimi¬ 
nation of yaw. 

If all the above points are carried out, it will 
prove possible to use longer exposure times 
and more efficient shutters, and what is even 
more important, to use slow-moving shutters 
that are unlikely to impart vibrations to the 
system. 


1 ' 6 ’ 7 Stabilized Aerial Camera Mounts 

A thorough study was made at Eastman 
(Contract OEMsr-392) of the possible methods 
of stabilization control in cooperation with 
antioscillation control to obtain complete mo¬ 
tion isolation from the camera. It was decided 
that the most promising method is that of de¬ 
veloping the stabilization torque relative to 
gravity, that is, by shifting a weight or the 
camera itself relative to the suspension axis. 
This removes the burden of following large or 
rapid motions with the servo system and main¬ 
tains at all times a center-of-gravity mount 
which is not susceptible to linear accelerations 
and permits a very low natural frequency for 
the antivibration suspension. 

A further feature of the stabilization method 
as worked out is the provision of an “integrat¬ 
ing” mechanism whose function is to compen¬ 
sate for the gradually accumulating film unbal¬ 
ance or other steadily applied torques without 
additional burden on the servo mechanism or 
the introduction of alignment errors. 

A test model of the electromechanical follow¬ 
up mechanism was constructed and found to 
operate in complete conformity with the re¬ 


quirements. Further work would undoubtedly 
bring about simplifications and produce a model 
for flight tests. 

Work was started on a stabilized mount util¬ 
izing an inertia-controlled mirror and having a 
very low natural frequency. The mount has a 
number of attractive features. No tests have 
yet been possible. 

A stabilized camera mount using a different 
type of servo mechanism was developed by the 
Lawrence Aeronautical Corporation (Contract 
OEMsr-1366). See Section 1.8.6. 

Eastman Servo-Controlled 
Stabilized Mount 26 

Introduction. It has been pointed out 25g that 
it is not practical to place the natural frequency 
of spring-filtered aerial camera mounts much 
below 100 cycles per minute. This limit is set 
because of the shift of the center of gravity 
which results from the motion of the film in a 
camera, and also because of differences in loca¬ 
tion of the center of gravity in individual 
cameras. These effects make it impractical to 
maintain rigorously a center-of-gravity suspen¬ 
sion under service conditions. It is thus neces¬ 
sary to keep the suspension springs sufficiently 
stiff so that alignment between the optical axis 
of the camera and the vertical may be main¬ 
tained, and this results in a lower limit for the 
natural frequency. However, even with these 
limitations there is no doubt that the spring- 
type camera mount provides considerable im¬ 
provement of aerial photographs, and there is 
reason to believe that further gains in resolv¬ 
ing power could be made if mounts were avail¬ 
able which would eliminate all disturbances in¬ 
cluding those of zero frequency. Such a mount 
would also maintain a certain orientation of the 
camera regardless of any shift of the center of 
gravity or motion of the airplane, a property 
which should be of considerable value in map¬ 
ping operations. 

Choice of Control Mechanism. Considering 
various possible control mechanisms, it is rec¬ 
ognized that none is effective over the entire 
frequency range of possible disturbances, but 
that any of the possible devices operates best in 
a finite frequency band. The spring-mass type 
of filter becomes very unstable if it is used at 




120 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


low frequencies; on the other hand, it is difficult 
to construct electromechanical follow-up or 
servo mechanisms which are effective at high 
speeds. It seemed therefore convenient for 
economical design to make use of both devices 
and to provide for sufficient overlap to cover the 
entire frequency spectrum satisfactorily. If F 
is the crossover frequency at which a spring- 
mass filter and the follow-up type of control 
should both be effective, it is judged that the 
natural frequency of the former should be not 
higher than F/ 2, and that of the latter not 
lower than 2 F. In view of previous experience 
with the two types of control, it was thought 
a good compromise to make F equal to 30 cycles 
per minute. 

Design of Folloiv-up Mechanism. The pur¬ 
pose of the follow-up mechanism is to maintain 
alignment between the optical axis of the 
camera and a gimbalized gyroscope at a high 
degree of accuracy. The torques which tend to 
disturb the alignment, once it has been at¬ 
tained, are due to low-frequency linear acceler¬ 
ation forces and rotational torques of the air¬ 
plane frame transmitted through the springs 
supporting the camera, and also to disturbances 
created within the camera by the motion of the 
film between exposures, acceleration forces of 
the shutter mechanism, air currents, etc. If the 
alignment between camera and gyroscope has 
been disturbed for some of these reasons, it may 
be restored (1) by driving the camera directly 
into the correct position through a motor at¬ 
tached to the airplane frame, (2) by generating 
restoring torque through compliant connections 
to the airplane, or (3) by applying restoring 
torque relative to space or gravity without ref¬ 
erence to the airplane structure. The last alter¬ 
native seems particularly attractive because it 
avoids mechanical connections between camera 
and airplane. These would add stiffness to or 
completely eliminate the benefits of the anti¬ 
vibration mount and thus transmit high-fre¬ 
quency disturbances. A further advantage is 
that mechanical motions of the correcting 
mechanism can take place relative to the stabil¬ 
ized platform itself rather than relative to the 
airplane frame. It becomes unnecessary then to 
compensate for possible large and rapid motions 
of the airplane by corresponding motions of me¬ 


chanical members through which correcting 
torques are transmitted. 

Of the several possible ways of applying 
torque relative to space, at least three merit 
consideration. The first makes use of a combi¬ 
nation of cross springs and coil springs. The 
cross-spring structure has some positive stiff¬ 
ness and tends to oppose rotation of the frame, 
whereas the coil spring produces a torque that 
tends to pull it farther away from its rest posi¬ 
tion and thus supplies what may be called 
negative stiffness. The springs may be designed 
to produce stiffness of equal value but of oppo¬ 
site signs, and this results in a structure of in¬ 
finite compliance. If, now, one end of the coil 
spring is shifted, it is seen that a torque pro¬ 
portional to the displacement is applied to the 
platform without reducing the compliance of 
the connection. 

Another possibility is to make use of the re¬ 
action torque which is set up if a rotating fly¬ 
wheel is accelerated or decelerated. However, 
the torque obtained in this way is proportional 
to acceleration and it is not possible to maintain 
it for any length of time without reaching ex¬ 
cessive flywheel speeds. The device is not suit¬ 
able to compensate for static unbalance of the 
camera. 

The last method, which was considered in 
connection with this work, produces restoring 
torque by moving a weight attached to the 
camera, or the camera itself, relative to the 
axis of rotation of the stabilized platform. Com¬ 
pared with the two other possibilities men¬ 
tioned previously, this has the overwhelming- 
advantage that any shift of the center of grav¬ 
ity occurring within the system is compensated 
for by rebalancing the camera. Thus, a center- 
of-gravity type suspension is continuously 
maintained, and an antivibration filter of very 
high rotational compliance (low natural fre¬ 
quency) may be used to effect filtering in the 
frequency range not covered by the follow-up 
mechanism. 

For these reasons, the method of moving a 
weight was adopted to produce both temporary 
and continuous torques required to maintain 
alignment between camera and gyroscope axes. 

General Layout of Stabilizer. At present only 
a laboratory model has been completed. This has 






ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


121 


one axis of stabilization and uses brass weights 
in place of the camera. The contemplated ar¬ 
rangement of elements in a completed model is 
as follows. 

To an inner gimbal ring which supports the 
camera is attached a gimbalized gyroscope with 
the spin axis vertical. Only a few degrees of 
angular motion need be provided in the gyro¬ 
scope gimbal, which should preferably have 
cross-spring axes to avoid friction. 

Two electric signals are provided by pickup 
elements at the gyroscope. These signals are 
proportional to the angular misalignment be¬ 
tween the camera and the gyroscope with re¬ 
spect to the two horizontal axes of stabiliza¬ 
tion. With suitable amplification the signals 
operate two servo follow-up mechanisms, also 
mounted on the inner camera gimbal ring. 

Either the camera may be given two degrees 
of freedom of horizontal linear motion with re¬ 
spect to the inner gimbal ring or the camera 
may be clamped rigidly to this ring and moving 
weights provided for the purpose of producing 
balancing torques. In either case the two servo 
mechanisms operate to shift the center of grav¬ 
ity to counteract any disturbances which would 
tilt the camera in space. Due consideration is 
given to the proper damping of oscillatory ten¬ 
dencies and to the balancing of permanently 
applied torques without burden upon the servo 
mechanism or the introduction of alignment 
errors. 

The inner camera gimbal ring is to be sup¬ 
ported by two small diametrically opposite com¬ 
pression springs. These perform the dual pur¬ 
pose of providing suitable antivibration opera¬ 
tion for linear vibrations in all directions and 
of acting as one pivot axis of the gimbal. These 
springs offer extremely low rotational stiffness 
to camera motion, the natural frequency in ro¬ 
tation being in the order of 15 cycles per min¬ 
ute and well below the upper limit of function¬ 
ing of the servo mechanism. These two springs 
are supported by an intermediate gimbal ring 
which, in turn, is supported by two more 
springs providing the other gimbal axis at 90 
degrees to the first and also assisting in linear 
vibration control. The outer springs are sup¬ 
ported by the airplane structure. 

The gyroscope is preferably of the self-erect¬ 


ing type, but a neutral gyroscope may be used 
if a close approach to the true vertical need not 
be maintained and some recaging device is 
provided. 

Because of the motion of the film between 
exposures, the center of gravity of the camera 
is shifting continuously. Some feature must be 
provided whereby continuous compensation 
torques can be maintained without requiring 
continuous input signals into the servo and 
hence continuous error of alignment. This is 
accomplished by an integrating mechanism 
which sums up the corrections applied by the 
mechanism and generates a torque equal to 
their time average, thus relieving the servo sys¬ 
tem from steady loads. The time constant of the 
integrator is chosen such that the error of 
alignment of the camera does not exceed a few 
minutes of arc at the highest rate of film motion 
required. 

Design of Servo Mechanism. The first de- 
sign 2Ga made use of mechanical members to pro¬ 
vide the functions of damping and integration 
as well as the direct displacement signal. The 
use of rotary viscous members employing Dow- 
Corning fluid in conjunction with various ar¬ 
rangements of springs, etc., made the method 
look very attractive. Only a comparatively small 
power output was required from the amplifier 
while the major portion of the work was to be 
done by a contact-actuated reversible motor op¬ 
erating directly from the 28-v d-c line. 

Various arrangements of the components 
were tried and successful operation was ob¬ 
tained up to 30 cycles per minute. However, 
some trouble was experienced with friction and 
the pressure needed for positive contact opera¬ 
tion. Servo motors with sufficient power for 
direct operation of the weight-shifting mechan¬ 
ism and with built-in generators for rate sig¬ 
nals became available and proved to be more 
satisfactory although bigger amplifiers were re¬ 
quired, lower electric efficiency resulted, and a 
balancing slide-wire bridge was required. 

Figure 103 shows a schematic circuit dia¬ 
gram. A variometer V is used to detect mis¬ 
alignment, one coil being fastened to the gyro¬ 
scope G and the other to the camera C. The sig¬ 
nal generated in this way is added to the out¬ 
put of bridge B-l t amplified in channel AM, 



122 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


filtered in a band-pass filter F- 1, and after 
further amplification, is fed into the control 
phase of a two-phase servo motor Af-1. This 
motor drives a contact arm along slide wire S -1 
of the bridge until it finds a position such that 
the output of the bridge cancels the signal de¬ 
livered by the variometer. Since the shaft of the 
motor is also connected to the weight W-l, the 


loop in itself. Its natural frequency must be 
high compared to that of the overall servo in 
which the action of gravity on the weight W-l 
supplies the restoring torque. A frequency of 
about 200 cycles per minute was found satis¬ 
factory. The damping of the minor servo loop 
just described is supplied electrically. A small 
induction generator G-l, located inside the 



N-2 P-2 

Figure 103. Schematic diagram of electrically damped servo. 


weight is moved by a distance which is propor¬ 
tional to the angular displacement of the 
camera as measured by variometer V, and a 
proportional restoring torque is applied to the 
camera. 

The circuit by which the position of the 
weight is continuously matched to the misalign¬ 
ment between camera and gyroscope is a servo 


shell of motor M- 1, delivers a 400-c voltage pro¬ 
portional to its instantaneous velocity. This sig¬ 
nal is amplified and added to the main displace¬ 
ment signal. The speed of the motor Af-1 is pro¬ 
portional to the rotational velocity of the 
camera, and it was thought at first that the out¬ 
put of G-l could also be used to generate damp¬ 
ing torque for the major servo loop including 








































































































































ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


123 


the camera. This is, however, incorrect for one 
loop when it is corrected for the other loop and, 
although the frequency at which damping is re¬ 
quired is quite different for the two loops, none 
of the schemes to use the same signal proved 
successful. There are ways of overcoming this 
difficulty which are very promising. Owing to 
lack of time, a rather mechanically clumsy but 
otherwise straightforward method was chosen. 
This is shown in the lower half of Figure 103. 
The rate signal generated by G-1 is added to 
the output of bridge B- 2, which is electrically 
similar to B- 1. The resulting voltage is fed into 
an amplifier channel N- 2, P-2, similar to that 
already described, and drives the servo motor 
M-2 until the voltage produced by P-2 matches 
the output of G-l. The weight W -2 is thus dis¬ 
placed, depending on the velocity of the camera, 
and generates a damping torque through 
gravity. 

Damping of the second minor servo loop 
formed by generator G-l, bridge P-2, amplifiers 
N-2, P-2, and motor M-2 is done in the same 
way as described in connection with the first 
minor servo loop. The rate generator G-2 fur¬ 
nishes the necessary signal. 

This system is wasteful of space and power 
because it duplicates all elements of the first 
servo loop. However, considering the short time 
available to finish up the project, and the ex¬ 
perience that had been gained in constructing 
the displacement signal servo loop, it was 
thought best to proceed in this way. 

The device employed to compensate for dif¬ 
ferences in mass distribution between indi¬ 
vidual cameras and changes, such as caused by 
the motion of the film in the camera, consists 
of springs Y-1, Y'-l, and the dashpots D- 1, 
D'- 1. The dashpots are filled with Dow-Corning 
silicone fluids which exhibit only a small change 
of viscosity with temperature. It is seen that 
for high-frequency motions of the contact arm 
of bridge P-1 the slide wire P-1 remains essen¬ 
tially stationary because of the stiff dashpots. 
If, however, some mass unbalance demands a 
new average position of the weight W- 1, in 
order to level the camera, and thus produces 
a steady unbalanced signal in the servo, the 
springs Y-l, Y'-l will eventually move the slide 
wire P-1 under the contact arm until the bridge 


is again balanced with the weight occupying its 
new position. Instead of moving P-1 a similar 
result could, of course, be obtained on the other 
side of the bridge by moving the ground point 
on P'-l. 

A stable follow-up system that maintained 
the position of the dummy camera within ap¬ 
proximately 1 min of arc and operated with a 
natural frequency of 60 cycles per minute has 
been built. A number of obvious improvements 
have occurred during the construction of the 
test model. The performance already attained 
is highly gratifying and indicates with a high 
degree of certainty that a completely satisfac¬ 
tory stabilized mount can be built. The elimina¬ 
tion of the effects of all normal aircraft motions 
of any nature is assured. 

Mirror Stabilized MouNT 26b 

The sketch of Figure 104 illustrates a pos¬ 
sible solution to the aerial camera stabilizing 
problem through the use of a mirror. Work was 
started on such a mount at the Eastman Kodak 
Company, but time did not permit completion. 

The mirror, with its associated parts, is sus- 


Figure 104. Schematic diagram of mirror sta¬ 
bilizer. 

pended by a pivot A at the center of gravity. 
The linkage connection at C supports no weight. 
The mass is, in turn, pivoted at B just above 
the combined center of gravity of mass and 
mirror. The system is therefore pendulous and 
self-leveling. It is expected that the natural 
period of the pendulum or center of gravity 
arrangement can be made long compared to the 
3- or 4-sec period of roll and pitch most com¬ 
monly encountered. The point C is prevented 
from having up-and-down motion by the link 
running from C to D. For rotations about an 




















124 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


axis perpendicular to the paper, the mass essen¬ 
tially remains fixed in space, and the ratio of 
the distance AB to AC (essentially 1 to 2) is 
chosen such that the mirror is rotated through 
one-half the angle of the disturbance. The effect 
of the disturbance is therefore eliminated. For 
rotations about the optical axis of the lens, 
pivots B and C (having at least 2 degrees of 
freedom) allow both the mass and the mirror 
to be undisturbed. 

While the natural frequency of such an in¬ 
ertia system cannot be made as low as that of 
a gyro system, it is believed that it can be made 
low enough for effective operation in view of 
the fact that the system is enclosed, free from 
air currents, and is not subject to shift of center 
of gravity. 

The actual construction requires careful at¬ 
tention to rigidity and the elimination of play 
and friction at the pivots. It is expected that 
extensive use can be made of the spring hinge, 
which can be designed for ample rigidity and 
has no lost motion. The higher frequency mo¬ 
tions can be eliminated by conventional anti¬ 
vibration means, thus avoiding the need of ex¬ 
treme rigidity. 

It will be noted that the mirror causes a re¬ 
versal of the image on the film. This means that 
contact prints (unless printed through the film 
base) are reversed left to right. Projection 
prints, enlargements, or contact transparencies 
(viewed through the back), however, provide 
nonreversed views and are, in general, to be 
preferred. 


168 Ground-Speed Compensation Mount 

Theory of Ground-Speed Compensation 

The blurring effect produced by the forward 
speed of the airplane relative to the ground may 
be effectively eliminated if the camera is ro¬ 
tated in such a way as to cause the optical axis 
to pass through a fixed point on the ground 
during exposure. (The change of angular posi¬ 
tion and distance of ground objects relative to 
the camera are of secondary importance for the 
usual exposure time, speed, and altitude.) This 
method of compensation may be called the 
“sweep” method. There appear to be certain 


advantages of this method over other methods 
which may require optical complications and 
accurate moving mechanisms with attendant 
risk of deterioration of the optical definition. 

The sweep mechanism may be conveniently 
combined with the functions of the inter- 
valometer as the following equation 2511 shows: 

3.6 

V — —jT> 

where v is required compensating velocity 
(inches per second) of lens relative to the film 
for 9-in. film width and 60 per cent overlap, 
and T is picture interval in seconds. 

Since the required velocity of lens relative to 
film is dependent only upon the interval T, it 
follows that the sweep mechanism may be 
automatically set without further adjustment 
when the intervalometer is properly set. 

The sweep mechanism may impart any suit¬ 
able movement to the camera as long as the 
proper velocity is imparted during exposure 
and the camera returned to the initial position 
so that the cycle may be repeated at an interval 
(or subinterval) of T seconds. The duration of 
exposure is extremely short compared to the 
interval T, so that a sinusoidal movement is en¬ 
tirely adequate if exposure takes place at the 
mid-position of the backward sweep. For a lens 
of 24-in. focal length, the angular amplitude 
of the sweep motion is fixed at 1° 22' or a 
total sweep angle of 2°44'. 

Details of Sweep Mechanism. The theory of 
ground-speed compensation serves as an indica¬ 
tion of what the general design should be. Ex¬ 
act details are less important and depend upon 
many factors, such as expediency and prefer¬ 
ence. The box housing the sweep mechanism of 
the Eastman-NDRC mount is shown in the 
photograph of Figure 96. 

The mechanism is mounted on the trolley 
arrangement of the crabbing adjustment roll¬ 
ing on the outer ring of the A-8 mount and 
moves the inner carriage on its transverse pivot 
axis. Very nearly sinusoidal movement is im¬ 
parted through an eccentric and connecting rod 
arrangement. The connection between the 
lower end of the connecting rod and the inner 
carriage is accomplished through a lever ar¬ 
rangement providing a smooth leveling adjust¬ 
ment about the transverse axis. 




ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


125 


The upper end of the connecting rod is driven 
by a crank shaft which makes one revolution 
during the time interval between pictures. The 
eccentric is easily changed on the second design 
to permit different per cent overlaps or differ¬ 
ent camera and lens combinations. 

The gear on the eccentric shaft carries a 
fiber button. This actuates a set of contacts to 
perform the functions of tripping the camera 
shutter at the proper time and of stopping the 
sweep motor only when the camera is in the 
level position after the manual off-switch is 
thrown. A short length of cable is connected 
permanently to the sweep mechanism and plugs 
into the camera in the usual intervalometer 
connection without alteration of wiring. 

The speed of the sweep motor is controlled 
and adjusted by means of a mechanical gover¬ 
nor in which the centrifugal force pulls a ro¬ 
tating disk against stationary adjustable fiber 
buttons. A range of from 3 to 20 sec of time 
interval is covered in the final design. Still 
shorter intervals can be easily accommodated 
by a change of gears which would also shorten 
the maximum interval correspondingly. 



Figure 105. The rotating prism compensator. 

The use of the sweep mechanism on numer¬ 
ous flight tests has thoroughly proved its effec¬ 
tiveness and its complete practicability. In some 
cases, as large as 400 per cent improvement 251 
at % so sec exposure, 10,000 ft altitude, and 200 
mph speed has been obtained. In fact it is quite 


probable that such a ground-speed compensat¬ 
ing device represents the only single improve¬ 
ment that yields such large results in propor¬ 
tion to the amount of expense and effort re¬ 
quired. 

The strongest possible recommendation 
should be made to incorporate ground-speed 
compensation into standard photographic 
equipment. 

Summary. Compensation by the method of 
swinging the camera about the transverse axis 
was incorporated into the antivibration 
mounts. The correct sweeping rate is auto¬ 
matically obtained when the picture-taking in¬ 
terval is manually set. Highly satisfactory re¬ 
sults were obtained on flight tests showing 400 
per cent improvement in resolving power at 
% 5 o sec exposure and 10,000 ft altitude. 

Rotating Prism Unit (Harvard) 101 

Several independent investigators have sug¬ 
gested the possibility of using rotating low- 
angle prisms below the aerial camera for the 
purpose of eliminating ground movement of the 
image. In order to obtain to-and-fro harmonic 
oscillation of the image in the line of flight, it 
is necessary to use two identical prisms, which 
at the moment of exposure are moving in oppo¬ 
site directions and have zero deviation and 
therefore zero color. The important thing is not 
the deviation but the rate of change of the devi¬ 
ation. 

The idea was put into practice under the 
Harvard contract. The end of World War II 
prevented completion of the device in the form 
of a workable and producible unit, but enough 
was accomplished to prove that the method 
would work satisfactorily if fully developed. 
The prisms are always moving at constant rates 
and therefore will not impart accelerations to 
the camera. 

Figure 105 shows a view of the unfinished 
rotating prism unit built at Harvard. The re¬ 
port states that some of the engineering de¬ 
tails require review and that haste engendered 
by the end of World War II forced use of im¬ 
proper materials. 

The elementary theory of this device is pre¬ 
sented in the Harvard report. The prism angles 
turn out to be very small, in spite of the rela- 



126 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


tively long period of rotation. If 60 per cent 
overlap is desired, the variation in thickness of 
a single prism is given by 

0.9 d 

'IV — i \ /• 

(n — 1) / 

where d is the diameter of the prism and / the 
focal length of the lens. For a crown-glass 
prism of BSC-2 glass used with a 40-in. lens at 
//5, the prism angle is given by a variation in 
thickness of 0.113 in. or an angle of 0.00716 
radians. 

In the mounting, each prism is set at mini¬ 
mum deviation in its cell. Thus, vibration of 
the prism will cause almost no displacement of 
the image, particularly since the deviation is 
already small. It is believed that the prism pair 
together are extremely insensitive to construc¬ 
tional errors of any ordinary kind, although the 
optical quality must be excellent. 

The preliminary tests of the Harvard unit 
showed erratic rotation, caused by a flexible 
drive between the power unit and the prism 
device. It was thought inadvisable to mount a 
drive on the camera itself. Figure 105, which 
shows a driving motor on the prism unit, was 
made during preliminary tests which were in¬ 
terrupted by the end of the project. Plans were 
under way to make use of the variable speed 
drive manufactured by the Chicago Aerial Sur¬ 
vey Corporation. 

The timing of the prism unit exposure takes 
into account the delay of a focal plane shutter 
in moving across the film. The timing is so set 
that the prisms are in their ideal zero deviation 
position on the only linear portion of the har¬ 
monic oscillation at the moment when the slit 
of the focal plane shutter reaches the middle 
of the picture at a 3-sec interval, with high 
tension K-22 shutter. 

Although the rotating prism device is capable 
of eliminating ground movement without dele¬ 
terious effects on vibration, focus, or color cor¬ 
rection, it is believed that such a solution in the 
long run is not advantageous. The presence of 
four extra glass-air surfaces, even though 
coated, creates additional scattering and loss of 
light. The British found that lens coatings were 
very ineffective on lenses that had been in 
service long enough to have all lens surfaces 
somewhat soiled, and that the gain from coat¬ 


ing was not comparable to losses from dust and 
grease. Since the prisms are in an exposed posi¬ 
tion, relative to internal lens elements, it is 
probable that the loss of efficiency will be felt 
very quickly. Probably the device should be re¬ 
garded as primarily of experimental character 
to ascertain in flight testing the effects of com¬ 
pensation on resolution, and as a guide to toler¬ 
ances on moving film or mechanical sweep 
methods. All in all, it would appear that a per¬ 
fected sweep mechanism is the most nearly 
ideal solution to the problem of image move¬ 
ment compensation. 

1,69 Gun Camera Anti vibration Mounts 2 ' 

Antivibration mounting techniques have been 
worked out for mounting the gun camera as a 
single unit and in combination with a gunsight 
to record the reticle image as well as the target. 
The laboratory tests show highly satisfactory 
performance for the single unit, and very en¬ 
couraging results from the camera-sight com¬ 
bination for which all the antivibration effects 
are produced by a small detachable and self- 
contained mirror unit next to the camera lens. 
Even better results are expected from this unit 
with further development. High-speed motion 
picture studies were made on two Martin 
upper-gun turrets to establish the requirements 
for camera mounts and to guide in shake-table 
tests. 

The development work on this project is 
divided into two parts. The first part deals with 
gun-camera mounts for fixed gun installations. 
The second part deals with flexible gun instal¬ 
lations for which lead computing sights are 
used. The standard gunsight aiming point 
camera [GSAP] was used for both installations. 

Theoretical Considerations 

In considering picture blurring, only two 
components of motion are of prime importance. 
These are the rotational components about the 
vertical axis and the transverse axis of the 
camera as it is normally positioned with hori¬ 
zontal optical axis. The three translational com¬ 
ponents of movement, as such, have no practical 
effect upon the picture blurring. If the mount 





ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


127 


is arranged so that the forces of acceleration 
set up by the translational components act 
effectively at the center of gravity of the 
camera, they can then introduce no rotational 
movements. 

There are, however, three chief advantages 
to be derived from also providing filtering ac¬ 
tion against translational vibrations of appre¬ 
ciable magnitude: (1) The stresses set up in the 
mount and camera mechanism are greatly re¬ 
duced and these parts are thus protected. (2) 
Rotational movements caused by lack of rigid¬ 
ity of the mounting bracket or panel, particu¬ 
larly at its natural frequency, are reduced. (3) 
The strict requirement that translational ac¬ 
celeration forces be applied effectively at the 
center of gravity of the camera is relaxed to 
the extent that translational vibrations are 
filtered out. The translational vibration along 
the axis of the gun is likely to be of very con¬ 
siderable magnitude. 

Vibration filtering action is accomplished by 
the camera mass in cooperation with a mount¬ 
ing of adequate compliance. The lower the 
natural frequency of the mount in comparison 
with the disturbing frequency, the more effec¬ 
tive is the filtering action. The natural fre¬ 
quency must not be made too low because of 
practical considerations of stability. Individual 
circumstances differ, but 100 cycles per minute 
has been found to be a practical lower limit. 

Disturbing frequencies lower than the nat¬ 
ural frequency are not filtered, while those 
neaj* the natural frequency may actually be 
magnified many times. It is the function of the 
damper to control the amplitude for frequen¬ 
cies near the natural frequency. This function 
is essential for proper behavior to transient dis¬ 
turbances, even if no steady disturbance near 
the natural frequency is present. 

Dampers employing viscous fluids are objec¬ 
tionable for reasons of structural complication, 
change with temperature, and possible leaks. v 
Air dashpot dampers are also structurally com¬ 
plicated, are subject to clogging or air leaks, 
and often contain delicate parts. The employ¬ 
ment of simple friction for damping offers 
advantages of extreme simplicity, adaptability 
of design, adjustment, etc. In some cases ordi¬ 
nary sleeve bearings may be used in place of 


antifriction bearings, thus simplifying and 
making the design more rugged, while provid¬ 
ing proper damping. For frequencies above the 
natural frequency, the action of the damper is 
detrimental to filtering action since the damper 
affords a coupling between the support and the 
camera through which some disturbing force 
can be transmitted. The force provided by a 
simple friction damper is practically inde¬ 
pendent of velocity, while that provided by a 
viscous or equivalent damper increases linearly 
with velocity. It follows, therefore, that the 
viscous damper is more detrimental to filtering 
action since the velocity of motion is greater at 
the frequency of filtering than at the natural 
frequency for a given amplitude of disturbance. 

The objection of inadequate boresighting 
may be raised for the friction damper. This is 
the case since the camera may come to static 
rest at any angular position within a finite zone 
determined by the amount of friction and the 
stiffness of mounts. The magnitude of this zone 
is, in fact, a sufficient measure of the damping 
effect and must not be too small if the damping 
is to be adequate. For the gimbal mount as sub¬ 
mitted, the static angular friction zone is ap¬ 
proximately 10 mils wide (±5 mils from the 
mean position). In making shake-table tests it 
was discovered that the dynamic boresighting 
always remained within 1 mil. It is hoped that 
engine vibration in the airplane will alone suf¬ 
fice to maintain this accuracy. In any event, 
the gun firing will quickly establish the proper 
camera alignment. 

There is another source of boresighting in¬ 
accuracy which is quite independent of the type 
of damping or mount structure. This is the 
unbalance produced by the movement of film 
from supply to takeup reel. For the gimbal 
mount as submitted, the misalignment is ap¬ 
proximately ±8 mils of angle about the trans¬ 
verse axis for 30 ft of film. The natural fre¬ 
quency about that axis is 115 cycles per minute. 
By increasing the natural frequency, the mis¬ 
alignment can be reduced at the expense of the 
filtering action. A compensating weight could 
also be employed if the refinement warrants the 
complication. 

Boresighting is accomplished in the gimbal 
mount by the simple expedient of releasing the 



128 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


compliance springs and clamping them again 
when the camera is properly aligned. This as¬ 
sures that the alignment comes at the center of 
the static friction zone. No vibration need be 
present when boresighting is done. 

Gun Camera Spring Mount 

The spring mount idea proved to be a good 
solution to the aerial camera mount problem.- 5j 
For this reason a compact design was worked 
out for the first trial mount of the gun camera. 
A center-of-gravity mount was achieved with 
filtering action for all six components of vibra¬ 
tion. A number of factors combined to make 
this a less attractive solution to the gun camera 
problem. The translational natural frequency of 
a spring and mass combination is a function of 
the static deflection of the spring when the 
mass is placed on it. This is true regardless of 
the actual size or weight of mass involved. The 
proportion of deflection to size is therefore 
larger for the small mass. The gun camera 
must function for a wide range of angular 
attitudes of the airplane, while the aerial 
camera functions only in level flight. These 
facts required the size of the filter units to be 
larger than desirable; also the natural fre¬ 
quency could not be as low as desired. 

Gun Camera Gimbal Mount 

The center of gravity of the gun camera 
moves comparatively little with the shift of the 
film in the magazine (approximately ±0.019 
in. fore and aft for a 30-ft roll). This makes 
the gimbal mount quite feasible even if all 
translational components are not removed. The 
weight of the gun camera is such that plain 
gimbal bearings of adequate strength do not 
present too much friction for damping pur¬ 
poses. This avoids the necessity of using ball 
bearings and makes for a simple, compact, and 
rugged mount. 

Figure 106 shows the camera in one possible 
mounting position in the gimbal mount. The 
trunnions forming the gimbal axes are adjusted 
according to the center-of-gravity location. The 
small springs provide the restoring torque and 
establish the natural frequency about each axis. 
Longitudinal filter action is obtained by sus¬ 
pending the intermediate carriage with four 


parallel arms. The parallelogram arrangement 
avoids any possibility of longitudinal motions 
introducing rotational motions to the gimbal. 
The pivot arms are % in. long, which gives a 
pendulum effect with a natural frequency of 
216 cycles per minute. Rubber stops are pro¬ 
vided as safety limiters for rotation about each 
gimbal axis. None are required for the longi¬ 
tudinal filter system. 

Gimbal Mount Performance 

Figures 107, 108, and 109 show enlargements 
of pictures taken at 16 frames per second with 
the gun camera in the gimbal mount on the 
laboratory shake table. The shake table was not 
in operation for Figure 107 but was providing 
the following disturbances at 800 cycles per 
minute for Figures 108 and 109: (1) 8.1 mils 
total angular rotation about transverse axis, 
(2) 0.023 in. total translational movement 
along vertical axis, and (3) 0.026 in. total 
translational movement along longitudinal axis. 
The mount was blocked in such a way that no 
filtering action took place for Figure 109. 

It is felt that this mount demonstrates ade¬ 
quately the soundness of the principles upon 
which it was constructed and provides a basic 
solution of the gun camera antivibration mount 
problem. 

Camera Mount for Flexible 
Gun Installations 

The problem of mounting a gun camera in 
conjunction with a sight to obtain a combined 
reticle and target image which is not blurred 
is evidently quite involved. The normal func¬ 
tioning of the sight must not be affected, the 
vision of the gunner must not be obstructed, 
and it is necessary to adhere to the rigid space 
limitations. 

The K-15 (Mark 18) sight was used in this 
phase of the camera mount project. Prelimi¬ 
nary to the actual mount construction, exhaus¬ 
tive tests of motions in the Martin upper tur¬ 
ret 27 ® under firing conditions were made with 
the aid of high-speed photographic equipment. 
These were carried out at Patuxent River, 
Maryland, in November 1944, and at Wright 
Field in February 1945. 

The tests showed that filtering action about 




ANTIVIBRATION MOUNTS FOR AERIAL CAMERAS 


129 


both the transverse and vertical axes must be 
provided and that it would be desirable to ob¬ 
tain a 10/1 reduction in amplitude at the firing 
frequency and higher frequencies. 


V&RTSCM. A"SUS 



Figure 106. Gun camera gimbal antivibration 
mount. 


Antivibration mounting of the sight plus 
camera as a unit does not seem feasible be¬ 
cause of the necessity of bringing in the range 
cable with its stiff tension spring, the rather 



Figure 107. Gun camera pictures made in gim¬ 
bal mount without disturbance from shake table, 
16 frames per second. 


stiff electric cable connection, the necessity of 
strict boresighting, and the space limitation. A 
better approach seems to be to locate the cam¬ 
era in such a position that a clear picture of 
the target may be obtained regardless of the 


motions of the sight. The reticle image then has 
some motion, but about 70 per cent of this is 
removed by the action of the gyro mirror. It 
is difficult to find a suitable location because of 
limited space and limited possible locations. The 
most promising approach appears to lie in the 
direction of antivibration mounting of one of 
the mirrors required for relaying the images 
to the camera, preferably the mirror next to the 
camera. The camera is then rigidly connected 
to the sight and the compact mirror mount can 
be more advantageously located and protected 
by an enclosure with none of the worries of 



Figure 108. Gun camera pictures made in gim¬ 
bal mount with 800 cycles per minute disturbance 
from shake table, 16 frames per second. 


shifting center of gravity, electric connections, 
or ruggedness of design. 

The most promising location of the standard 
gun camera, from the standpoint of available 
space in the turret and functional operation, 
and with minimum vision obstruction, seems to 
be at the right side of the sight (as seen from 
the gunner’s position) with the optical axis 
pointing upward and parallel to that of the 
reticle collimating lenses. This is shown by the 
photographic view of Figure 110. It is seen 
that the light is relayed into a small enclosure 
above the camera lens by means of a 45-degree 
fixed mirror located above the combining glass 
on the lead computing side of the sight. An¬ 
other 45-degree controllable mirror is located 
inside the enclosure to direct the light down 
into the camera. The normal combining glass 



130 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


is replaced by one with a 45 per cent reflection 
coating. The camera may be released and ro¬ 
tated to the horizontal position for purposes of 
loading the film. 

As indicated previously, the mirror just 
above the camera lens must be controlled in 
such a manner as to remove the effects of angu¬ 
lar vibration. This is the only vibration control 
in the system since the camera is rigidly con¬ 
nected to the sight and no alteration is made in 
the connection of the sight to its supporting 
yoke. Obviously, the desired result cannot be 
obtained by antivibration mounting of this mir¬ 
ror directly. The mirror can, however, be made 
to execute the required motions by suitable 



Figure 109. Gun camera pictures made on 
shake table with filtering action of mount blocked 
out. Same 800 cycles per minute disturbance as 
for Figure 108. 

linkage to a mass which is antivibration 
mounted and essentially functions as a fixed 
reference. 

Figure 111 shows the camera lens with the 
antivibration controlled mirror. 27b The natural 
frequency about the transverse and vertical 
axes is approximately 150 cycles per minute, 
and dry-friction damping is employed. The unit 
is unaffected by air currents or accidental con¬ 
tacts, is replaceable as an interchangeable unit, 
and is quite rugged. The mirror arrangement 
is such as to reverse the picture left and right. 
This effect is eliminated by turning the film 
over in projecting or assessing. 

The mirror mount was tested by placing the 


entire sight and camera unit on a shake table 
and taking pictures while it was in operation. 
The table was adjusted to provide a rotational 
amplitude of 10 mils about both the vertical and 
transverse axes simultaneously (circular-type 
motion) and at a frequency of 800 cycles per 
minute. The turret tests show that this 20-mil 
total angle of vibration is typical of actual con¬ 
ditions. With perfect filtering action, as regards 
the target image, the reticle image still retains 
30 per cent of the original motion owing to the 
action of the gyro-controlled mirror in the 
sight. The picture quality shows a substantial 
improvement in comparing pictures made with 
the antivibration mirror with those made when 
a fixed mirror is substituted. It is felt, how¬ 
ever, that the mirror mount operation is not as 
good as might be expected and that it can be 
improved further. 

17 SHUTTERS FOR AERIAL CAMERAS 

During the course of World War II a number 
of projects were initiated for the purpose of 
improving the speed and efficiency of shutters 
for aerial cameras. Most of this work was con¬ 
fined to between-the-lens or louvre shutters. 
No NDRC work was carried out on focal plane 
shutters, with the exception of the multiple slit 
focal plane shutter described below. Valuable 
work was accomplished elsewhere on focal 
plane shutters, particularly near the end of 
World War II. 

171 The Improved Metrogon Shutter 28 

The standard Metrogon shutter for the 6-in. 
Metrogon lens provides a least exposure time of 
Ys oo sec. The Mount Wilson Observatory was 
requested under Contract OEMsr-101 to de¬ 
crease the exposure time, if possible, to %oo 
sec without marked change in shutter efficiency. 

Investigation showed that 97 per cent of the 
inertia of the moving parts of the shutter arose 
in the activating cam, whose mass and bear¬ 
ings were not favorable for fastest operation. 
A slight contribution came also from the shut¬ 
ter blade assembly. 

By machining the entire activating cam of 




SHUTTERS FOR AERIAL CAMERAS 


131 


much reduced weight from a single piece of 
metal as permitted by the forces acting, it 
proved possible to reduce the total moment of 
inertia by more than threefold. After several 
experiments on types of material for the cams, 
it was found that the best material was steel, 
carburized deeply at the ball races, but only 
lightly over the rest of the surface. 

Figures 112 and 113 show comparative views 



Figure 110. Gun camera attached to K-15 sight. 


of the new and old cams, both disassembled and 
in position. The new cam weighs less than half 
as much as the standard cam, and its moment of 
inertia is reduced by a factor of 3.3. This re¬ 
sults in a decrease in the exposure time by a 
factor of about 1.7, or nearly \/3.3, as might 
be expected if the cam is the limiting factor. 

Other slight changes in design were carried 
through to aid in the effective operation of the 
new-type cam and to add to the life of the 
assembly. It is stated that the total time taken 
to replace the standard parts by the ready-made 
modified parts is about 2 hours. 


Performance. The shutter speed and effi¬ 
ciency curve were determined by means of a 
photocell and oscilloscope with photographed 
traces. These observations showed that at the 
peak speed, performance is satisfactorily uni¬ 
form from one shutter to another. All modified 
shutters tested proved to vary in a total range 
of 10 per cent, with an average exposure time 
of y 460 sec. The efficiency of the modified shut- 



Figure 111. Antivibration controlled mirror of 
gun camera mount. 


ter proved to be 77 per cent, leading to an 
effective exposure time of % 70 sec. 

Table 14 reproduces observed data on one of 
the modified shutters. 

Table 15 shows values of the total exposure 
times for three modified shutters at each of the 
four available settings. 

Durability. No systematic breakdown tests 
were performed at Mount Wilson. Two of the 
shutters, however, were run two or three thou¬ 
sand exposures each, with cams from the first 
group made up. Cams from a second group 
were installed in five cameras, and each of these 







132 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


was operated about 300 exposures without any 
indication of wear or breakage. 

Discussion. It was believed at Mount Wilson 
that the modification of the 6-in. Metrogon 
shutter was one of the most important accom¬ 
plishments under Contract OEMsr-101. Sev- 


Table 14. Observations on speed and efficiency of 
improved Metrogon shutter. 


Shutter 

setting 

(sec) 

Dot 

fre¬ 

quency 

Num¬ 

ber 

of dots 

Total 

ex¬ 

posure 

(sec) 

Effec¬ 

tive 

ex¬ 

posure 

(sec) 

Effi¬ 

ciency 

(per 

cent) 

1/300 

5,000 

11.0 

1/455 

1/590 

77 

1/225 

2,500 

7.3 

1/340 

1/450 

75 

1/100 

2,500 

14.2 

1/175 

1/200 

88 

1/50 

1,250 

22.0 

1/57 

1/61 

93 


Table 15. Total exposure time (sec) at four set¬ 
tings for three modified Metrogon shutters. 


Shutter 

setting Shutter serial number 

(sec) 1684 1607 1643 


1/300 

1/455 

1/450 

1/475 

1/225 

1/340 

1/270 

1/245 

1/100 

1/175 

1/120 

1/167 

1/50 

1/57 

1/65 

1/60 


eral improved units were used on important 
missions in Italy. These improved units led to a 
redesign of the standard unit for later produc¬ 
tion. The success achieved also set a pattern 
for other types of shutter work. 


172 The Improved 24-in. K-17 Shutter 29 

Following the successful modification of the 
6-in. Metrogon shutter, work was initiated un¬ 
der Contract OEMsr-101 on similar improve¬ 
ments for the K-17 between-the-lens shutter 
for the 24-in. standard aerial lens. 

Whereas the study of the Metrogon shutter 
had revealed the cam to be a major limiting 
factor in speed, analysis of the 24-in. shutter 
showed that neither the cam nor the blades 
could be singled out for improvement. In addi¬ 
tion to a general improvement, it was believed 
that the most promising method of increasing 
the speed involved replacing the driving spring 
by a more powerful driving system. 


A triple spring was developed which can be 
installed readily, and which reduces the total 
exposure about 30 per cent. It was considered 
doubtful that the shutter blade assembly could 
withstand such an increase in speed for long, 
and that development of improved materials 
and construction was indicated. No work was 
accomplished along these lines. 

The triple spring system developed in proto¬ 
type comprised three 10-coil springs, each with 
a single winding. The driving torque was there¬ 
by tripled, relative to the standard 30-coil, 2%- 
turn spring drive, and the moment of inertia 
very much reduced. However, the total gain in 
speed was about 30 per cent, limited by the in¬ 
ertia of the other moving parts. 

Performance. Figure 114 shows a compari¬ 
son of three shutter drives as described. Table 
16 reproduces observed shutter speeds of the 
various drives. 

Discussion. Only about one-third of the total 
effective inertia of the system is due to the 
shutter blades. It was believed that any work 
on improved shutter blades should be in the 
direction of greater strength and durability for 
the same moment of inertia. A beryllium- 
aluminum alloy is recommended. 

An improvement in the cam similar to that 
found useful in the 6-in. Metrogon shutter 
would lead to an increase in shutter speed of 
only 15 per cent. Other improvements all along 
the line in bearings, spring clip, and safety col¬ 
lar, together would lead to a slight increase in 
shutter speed. 

It was believed that flutter of the shutter 
blades might deflect the blades at the center of 
the exposure out of line and thereby retard the 
closing action. Metal guides were added to re¬ 
strain the open leaves to one plane. A prototype 
showed no improvement, however, in shutter 
speed. 

Durability. Two new K-17 shutters were 
modified by means of the 10-coil triple spring 
drive. No other changes were made. 

One of the shutters was operated a total of 
2,725 exposures, the other 225. In both cases 
the tests were stopped when the shutter blades 
cut into each other on closing, a frequent cause 
of breakdown of the standard shutter. 

The bearings of the operating links showed 













SHUTTERS FOR AERIAL CAMERAS 


133 


noticeable wear in the first of the shutters, but 
this fact did not prevent satisfactory operation. 
Other parts of the shutter assembly and cam¬ 
shaft showed no evidence of unusual wear. 

Some difficulty was experienced with failure 
of the shutter to release. This failure is fol¬ 
lowed immediately by a further winding of the 
spring, which results in the breaking of a pin 
in the camera, necessitating repair. 


Figure 112. Left, new cam with bearing assem¬ 
bled. Center, new cam, including spring con¬ 
nector. Right, standard cam. 

In conclusion, although the triple spring it¬ 
self can be installed very simply, it probably 
cannot be used successfully without further 
changes in the assembly. The tests that have 
been made indicate that these changes include 
a minimum strengthening of the tripping mech¬ 
anism and some modification of the shutter 
leaves, which will prevent breakdown. The 
shutter-blade links and their bearings may also 
need strengthening. 


17 3 The Multiple Slit Focal Plane Shutter 

Under Project AC-29, NDRC was asked to 
develop a multiple slit focal plane shutter of 
the type proposed by Langer in the hope that 
distortion, nonuniformity of exposure, and 
limited life ordinarily associated with focal 
plane shutters might be reduced. 

The proposed type of shutter took on two 
forms. The first of these was developed at the 
Mount Wilson Observatory under Contract 
OEMsr-101, 29a and the second at the Techni¬ 
color Motion Picture Corporation under Con¬ 
tract OEMsr-710. 30 Both types of shutter pro¬ 


duce the exposure by moving a primary screen 
with parallel slots. A secondary screen with 
wider slots, but the same center to center spac¬ 
ing, protects the film from light fog, before and 
after the exposure, and moves during the ex¬ 
posure in such a way as to permit light to pass 
through slots in the primary screen. 

There is inevitably a narrow strip at the 
boundary between the regions exposed by adja- 



Figure 113. Shutter housings, with blade as¬ 
sembly and connecting link removed. Standard 
cam in position at left, new cam at right. 

cent slots which receives exposure from both 
slots. Although the time interval between the 
two exposures can ordinarily be held below 
0.001 sec, perceptible doubling of the image 
may exist in these boundary regions. The com¬ 
bined area over which this doubling can occur 
can be held to less than 10 per cent of the total 
area of the photograph, and in any case the 
doubling is noticeable only when image motion 
is considerable. It should be noted that blurring 
of almost the same amount occurs over the en¬ 
tire photograph with present between-the-lens 
shutters operating at y 150 sec. Actually, 
doubling may be preferable to blurring, since 
it has less effect on recognition of detail. 

The two types of multiple slit shutters differ 
in their mechanical motions. 

The Langer Shutter 

The primary and secondary slotted screens 
are linked together and travel with a fixed ratio 
of speed (approximately 2/1). As a result/the 
film is uncovered gradually, and there is a 
region of appreciable width within which there 
is double exposure. The mechanical linkage is 
very simple, and strips of uneven density can 




134 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


easily be reduced to an unobjectionable level 
by contact printing through a compensating 
screen. 


Table 16. Observed shutter speeds. 


Drive 

Total 

exposure 

(sec) 

Effective Effi- 

exposure ciency 

(sec) (per cent) 

Standard 
spring and 
winding 

0.0067 or 1/150 

0.0044 or 1/230 

66 

10-coil spring 
wound 1 
turn 

0.0071 or 1/140 

0.0048 or 1/210 

67 

wound IV 3 
turns 

0.0062 or 1/160 

0.0042 or 1/235 

68 

8-coil spring 
wound 1 
turn 

0.0071 or 1/140 

0.0051 or 1/195 

72 

Triple spring 
10-coil 
wound 1 
turn 

0.0048 or 1/210 

0.0035 or 1/285 

73 

wound 1 Vz 
turns 

0.0043 or 1/230 

0.0032 or 1/310 

74 

Triple spring 
8-coil 
wound 1 
turn 

0.0050 or 1/200 

0.0036 or 1/280 

72 


Type 1 . Analytically, the simplest arrange¬ 
ment is for the secondary screen to be held 
stationary while the primary slots are being 
uncovered and again while they are being cov¬ 
ered. Figure 115 shows the situation sche¬ 
matically at selected moments during an ex¬ 
posure. At the right of Figure 115 is given for 
each position the distance that each screen has 
moved from its initial position (1) and the 
time elapsed since the start of the exposure 
when the screens were at (2). The shaded areas 
below the line indicate the amount of exposure 
at each point on the film from the beginning of 
the exposure until the moment illustrated. To 
the left of the heavy broken line is shown the 
ideal case in which all light is incident normal 
to the film. To the right of the broken line is 
indicated the more realistic case of a lens of 
//3 aperture. 

It is clear that for this motion of the screens 
the area of overlap is caused entirely by the 
finite solid angle subtended by the lens at the 
film. The width of overlap is proportional to 
lens diameter, and to the distance h of film 


screen, and is inversely proportional to focal 
distance. It does not depend on dimensions of 
either the primary or secondary slots. 

Type 2. In Type 2 the secondary screen is in 
uniform motion. Construction of the operating 
mechanism is simplified in this case, provided 
the screen velocities have a fixed ratio. Figure 
116 shows the progressive exposure effects of 
this type of multiple slit shutter. 

Although motion in Type 2 gives more over¬ 
lap than Type 1, there are considerable prac¬ 
tical advantages involved. Since the screens 
are always positively linked together, a single 
source of power is sufficient, and there is noth¬ 
ing to cause irregularity in the motion. Once 
the blades are correctly assembled on the lever 
system required, there is no reason for their 
getting out of adjustment. If errors of adjust¬ 
ment do occur, they cause less noticeable ex¬ 
posure differences in the regions of overlap 
than do equal errors with motions of Type 1. 



Figure 114. Shutter tests—comparison of springs. 

Figure 117 shows a prototype of Type 2 con¬ 
structed at Mount Wilson. The shutter is in¬ 
tended to cover a film 6^x8^ in., the slots run¬ 
ning the short dimension. The eighteen slots in 
each screen are 6% in. long and have a spacing 
between centers of d = % in. The width w p of 
the primary slots is y 16 in. and that of the sec¬ 
ondary slots w s , in. The screens are 7%xl0i4 
in. in size and are of Duralumin sheet, 0.015 in. 
thick. 

The shutter is driven by a coiled spring, ex¬ 
erting a force of 12 lb at the beginning of the 
stroke. To give an effective exposure time of 
0.001 sec, the velocity of the primary screen 









SHUTTERS FOR AERIAL CAMERAS 


135 


must be 62.5 in. per sec, reached by a uniform 
acceleration of 870 ft per sec 2 (27 g). 

Figure 118 reproduces a typical photograph, 
made with a Cooke Aviar of 20-in. focus at 
//5.6. The automobile was moving at 30 ft per 
sec at a distance of 120 ft from the camera, 


ticeable doubling is confined to strips less than 
0.05 in. wide, or 10 per cent of the total picture 
area, and (3) the standard 24-in. shutter has 
a shutter speed of only 1 / 150 sec. 

The lower print of Figure 118 shows the effect 
of the compensating screen on the bars of dou- 



TOTAL SCREEN TIME FROM 
MOTION PO SITION (2) 
Xp x s t 

OO- 


d-w s -w p 

2 


O 


O 


d-w s +w p 




d + w p 

“2VT 


3d-w s -w p 

2 


d-w s 


d 

V P 


3d-w + w d + w D 

—g «-w 5 —7-^ 

2d-w s d-w s — 


Figure 115. Shutter screen motion, Type 1. 


corresponding to ground-speed motion of a 
plane flying at 1800 ft altitude at 300 mph. The 
exposure of 0.001 sec was fast enough to stop 
the motion, although imperfect focus prevented 
the test from being critical. The doubling of the 
image is seen at the lower edge of the front 
mudguard and the upper edge of the wind¬ 
shield. 

In judging the importance of this doubling it 
should be remembered that (1) it is roughly 
15 times as great under the conditions of the 
test as in normal aerial photography, (2) no¬ 


ble exposure. The improvement is marked. It 
should be pointed out, however, that the shutter 
development, as such, was not carried to ulti¬ 
mate possibilities, and that further develop¬ 
ment work would lead to negatives showing less 
double exposure, followed by still better com¬ 
pensation on printing. 

The Technicolor Multiple Slit Shutter 3051 

Following the developments described above, 
work proceeded at the Technicolor Motion Pic¬ 
ture Corporation on an improved version of 































































136 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


Type 1 described above. In the Technicolor 
form the primary screen containing narrow 
slots (% 6 x9 in.) moves over the stationary sec¬ 
ondary screen containing wide slots (^x9 in.) 
until the narrow slots are in the centers of the 
wide slots and therefore exposing. At this point 
the two screens move together for a distance 
of % in., producing uniform exposure through 


case at a time when this blade is stationary. 
The primary screen merely determines the ex¬ 
posure time, and it stops after the secondary 
screen has stopped. 

2. Overlap is theoretically reduced to a very 
small amount, which depends only upon the 
distance of the secondary screen from the film 
and on the angular aperture of the lens. 


1 


2 


3 


4 


5 


6 


7 



d —- 

r-PRIMARY SCREEN 
\ ^SECONDARY 

\ \SCREEN 

• \ = 
\ 

1h 

^-FILM 

\ 

\ 

\ 

\ 

\ 


\ 

\ 

\ 

\ 

\ 



\ 



~V\ 

\ 

\ 

\ 

a \ 1 

“V v 

\ 

_ 


\ 

\ 

^ 

i 

\ 




\ 

\ 

\ 

\ 

\ 

s 

1 



\ 

\ 

\ 

\ 

1 \ 

ZERO APERTURE 


TOTAL MOTION, x p TIME 
OF PRIMARY SCREEN FROM 
GENERAL IF POSITION 
CASE w s = 2 


0 0 


(d-V"fc) d d 0 

2 w s 2 "" Wp 


(d-v^+w p )d 

2w s 

2 w § 


w p d 
V p w s 

(w 8 +Wp)d 

2V P w 8 



(d+w s -v^)d 
2 w s 



d_ 

v p 


(d+w s +w„) d 3d (w,+w p )d 

2"s 2 P “v^ 


dt 

*3 


2d 


MOTION OF SECONDARY SCREEN 



Figure 116. Shutter screen motion, Type 2. 


the narrow slots. At the end of this run, the 
secondary screen stops, and the primary screen 
keeps moving until the exposure stops alto¬ 
gether. 

The following points apply to the Technicolor 
version: 

1. The edges of the secondary screen (wider 
slots) limit the width of the exposure in each 


3. Streaks of unequal density are much more 
likely to occur with this design than with the 
original Langer shutter, due to difficulties in 
keeping the primary screen traveling at uni¬ 
form velocity, especially since the secondary 
screen must be started and stopped during ex¬ 
posure of the V 2 -in. strip. It is also essential 
that stresses due to acceleration shall not dis- 

























































































SHUTTERS FOR AERIAL CAMERAS 


137 


tort the primary screen if streaks are to be 
avoided. 

4. Greater mechanical precision in the posi¬ 
tioning of the secondary screen as well as avoid¬ 
ance of distortion of the edges of the slots is 
necessary, since accurate butting of the 
strip pictures is entirely dependent on the sec¬ 
ondary screen. 

Driving Mechanism. Four types were consid¬ 
ered : (1) modified Geneva movement, discarded 
because of limited throw of driving bell crank, 



Figure 117. Test model of multiple slit shutter. 

(2) oscillating cam mounted on the bell crank 
of the main drive, discarded because of hammer 
blows on the secondary screen, (3) a two-spring 
drive actually used, and (4) a pair of linear 
cams, one on either side of the secondary screen, 
used in the final version. 

A standard 9x9 magazine was modified to 
incorporate the multiple slit shutter very near 
the film plane for the purpose of increasing the 
efficiency of the shutter action. The final shutter 
was positioned only Vs in* from the film. 

Preliminary experiments were made with 
small blades made of % 4 Duralumin, 8 in. long 
by 2 in. wide. The primary blade placed closer 
to the film was pierced with a slot opening, 
y_6 x % in. The secondary blade was pierced 
with a slot opening, in. Preliminary tests 

with this shutter were used to work out the 
proper balance between the two driving springs. 
Exposure times as short as M.,500 sec were re¬ 
corded. 

Further tests were conducted on full 9-in. 


length blades, but with slots only Vlg in. wide 
by 1 in. long in groups of six, placed at the 
center and four corners of a 9x9 frame. The 
difficulties encountered in the testing of 8x2-in. 
blades again appeared. Over a period of two 
months approximately fifty tests were made on 
these blades to study the effects of shock and 
acceleration on the various parts. It was found 
that if Duralumin were used for the plates, it 
should be surrounded by a steel frame for 
strength and rigidity. 

Following the tests on the double spring 
drive, a double linear cam drive was designed 
and constructed. Although sensitive to errors 
of construction, this system of driving the 
blades is positive in its action. The prototype 
system functioned very well, but the negatives 
were afflicted with shadow lines, both bright 
and dark. A series of tests indicated that there 
appeared to be no optimum overlap for the 
shutter action. 

A complete shutter was made up making use 
of new cams, whose design was based on the 
experience of the previous tests, together with 
a sandwich type of blade design. In this form 
three screens were used. The first (primary) 
screen carried % 6 -in. slots and governed the 
exposure. The third was rigidly fastened to 
the same frame and carried slots Vs in. wide. 
The second screen moved on the first, and the 
first on the shutter frame. The advantages of 
this type of construction were (1) compactness, 
(2) reduction in friction between the first and 
second frames, (3) rigidity and less chance of 
distortion, and (4) frame under tension, not 
compression. The disadvantages were (1) com¬ 
plexity of design and assembly, (2) higher pre¬ 
cision of frame parts, and (3) necessity of 
weakening the base plate of the standard mag¬ 
azine. 

Summarizing the results of extensive tests 
made on the completed 9x9 sandwich shutter, 
we have the following data. 

The advantages of the multiple slit shutter 
over the existing types of shutters have been 
attained to a great extent in regard to: 

1. Shorter travel of blades— 1 % 6 in. instead 
of 9 in. 

2. Higher exposure speed— y 790 sec observed, 
instead of %oo as in the K-17 standard shutter. 


















138 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 



Figure 118 . Photographs made with multiple slit shutter. 










SHUTTERS FOR AERIAL CAMERAS 


139 


Speed up to M .,000 sec could be achieved in a 
production model. 

3. Closer approach to simultaneous exposure 
over the entire picture area. 

4. Longer life. With proper design and mate¬ 
rials a Langer-type multiple slit shutter could 
probably be made to operate for a long period 
of time with minimum maintenance. 

The above advantages have been offset to a 
large extent by lack of uniformity in the pic¬ 
ture area. This occurs in two or more places, 
and is produced by unrelated factors. The great¬ 
est nonuniformity is at the junction of the 
V^-in. strip pictures, and is caused mainly by 
the difference in the exposure gradient at the 
start and end of the y^-in. strip. This abutting 
of the pictures is a very critical matter, and 
considerable further experimentation would be 
necessary in order to improve this region to 
any noticeable extent. The second nonuniform¬ 
ity, which is not serious, is caused by the 
change in velocity of the ViQ-in. slot in the 
primary screen. This trouble could no doubt be 
improved in a production model by further de¬ 
velopment of the driving system. 


1,7,4 Miscellaneous Shutter Types 

No other shutter types actually reached com¬ 
pletion under NDRC, but some work was car¬ 
ried out on experimental models. Under Con¬ 
tract OEMsr-622 31 the Eastman Kodak Com¬ 
pany considered several types of unorthodox 
between-the-lens shutter designs and made test 
models of a few. In this work a goal was set for 
a 3.5-in. aperture and 0.001-sec exposure time. 

Bartol Model. Two model shutters made at 
the Bartol Research Foundation were turned 
over to Kodak for further development. In 
both models two sets of rotating disk-type 
blades with three blades in each set were used. 
The set of blades which actually determined 
the speed of exposure rotated at four times the 
angular rate of the second set and, conse¬ 
quently, could be timed to make approximately 
three and three-fourths revolutions before the 
openings in all of the blades coincided with the 
aperture. 

The first model was driven by an electric 


motor, but was not further used, owing to a 
request to avoid use of electric power. The sec¬ 
ond model utilized compressed air at 250 psi. 
An analysis of pictures made with this model 
showed a total open time of between % 50 and 
y 20 o sec, with an effective exposure time of 
y 3 00 sec. Instead of increasing the speed of this 
model, resort was made to explosive driving 
power by means of blank cartridges. These 
experiments were afflicted with breakdowns 
caused by the products of combustion of the 
explosions. No further work was carried out 
on this model. 

Design work was carried out on a similar 
shutter driven by explosive force, with better 
distribution of the forces involved. Computa¬ 
tions showed, however, that the design would 
lead to mechanical complications and excessive 
forces. Work on such shutters was stopped at 
this point. 

Vane-Driven Shutter. In view of the difficul¬ 
ties with the above types, it was decided to 
investigate other sources of power. One device 
proposed was a single-bladed shutter powered 
by an external wind-actuated vane. This vane 
might be compared to a weathervane suddenly 
reversing its direction. Computations indicated, 
however, that the size of vane required for the 
short exposure time desired would be imprac- 
tically large. It was estimated that a 6x6-in. 
vane at the end of a 1-ft arm would give an 
exposure time of only 0.01 sec, at a plane speed 
of 250 mph. No further work was carried on. 

Continuously Operating Blades. This design 
consisted of a single pair of differentially 
geared blades rotating in opposite directions 
at slightly different speeds. The exposure was 
made when apertures in the two blades finally 
came into line across the aperture. To provide 
control of the interval between exposures and 
to allow rewinding of the film, an auxiliary 
shutter was to be so connected that it could be 
operated when desired, would stay open for 
one exposure only, and would then close until 
tripped again. Such a continuously operated 
shutter would avoid the heavy accelerations of 
the other types of shutters. 

No actual shutter was constructed because of 
lack of need by the Services for fast, small 
shutters, and because of the impracticability 



140 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


of large disk shutters. A simple model to show 
the principle was made up, with 3% in. aperture 
and 55 per cent efficiency. 

Continuously Operating Focal Plane Shutter. 1 
It was proposed under the Harvard contract 
to construct a magazine and focal plane type 
shutter in which the shutter passed over 
the film plane, over rollers around and back of 
the film rolls, and over rollers to the film plane 
again. Such a shutter would carry the usual 
focal plane type slit and pass very near to the 
film plane for high efficiency. Moreover, con¬ 
tinuously operated, a rather high linear speed 
could be achieved without requiring unusual 
strain on the flexible curtain. It was thought 
that a slower between-the-lens shutter or cap¬ 
ping curtain could govern the single exposures 
and open before the slit reached the emulsion 
and close after its passage. The longer the travel 
of the continuously operating curtain, the more 
time would be available for the slower shutter 
operation. No working model was constructed, 
owing to the end of World War II. 


18 MISCELLANEOUS EQUIPMENT 
FOR AERIAL PHOTOGRAPHY 

181 A Method for Checking the Focus of 
Aerial Cameras (Technicolor) 32 

Under Projects AC-29 and AC-88, in studies 
of factors that limit resolution of aerial photo¬ 
graphs, it was decided to produce a device for 
testing the focusing of aerial cameras. Such a 
device serves the following purposes: 

.1. To determine for any lens, from a series 
of photographs taken at equally spaced focal 
settings, the accuracy with which the focus 
must be established in order to avoid any sig¬ 
nificant loss in resolution. 

2. To determine the accuracy of focal setting 
of service cameras. 

3. To determine the accuracy with which the 
optical axis of the lens is set perpendicular to 
the film plane. 

4. To set the focus of cameras at the best 
setting for test flights. 

Considerations of various possible methods 
led to the selection of a simple 35-mm camera 


attachment to the camera body. The special 
camera back required carried five ports, to any 
of which could be attached a Kodak 35-mm 
camera. The most important feature of the 
device was the micrometer adjustment whereby 
it was possible to obtain exposures by chosen 
increments at any focusing position inside or 
outside of focus. Figure 119 shows a view of 
the final device. The device was used extensively 
on the Mount Wilson optical bench for lens 
tests. 


182 A Collimator for Testing Focal 
Setting (Mount Wilson) 19 

A portable collimator was constructed at 
Mount Wilson for the purpose of field tests of 
Army aerial lenses. The collimator consisted of 
a 3.5-in. doublet lens of 30 in. focal length 



Figure 119. Focus testing device. 

mounted in a steel tube. A lamp house at the 
opposite end made use of a condensing lens, a 
diffusing screen, a two-filament headlight bulb, 
and a pair of inclined reticles. The reticles con¬ 
sisted of photographs on glass plates of ink 
drawings of eleven pairs of fine lines intersect¬ 
ing at small angles. These reticles were placed 
at the focus of the collimator at right angles 
to one another and inclined 16 degrees to the 
optical axis. Successive fine patterns were sep¬ 
arated in focal distance by 0.015 in., giving a 
total focusing range of about 0.15 in. The col¬ 
limator was used in testing the focus and off- 



MISCELLANEOUS EQUIPMENT FOR AERIAL PHOTOGRAPHY 


141 


axis resolution of a standard 24-in. //6 aerial 
lens. 


18 3 Exposure Meter (University of 
Michigan) 33 

In order to obtain satisfactory aerial pictures 
with standard lenses, usually troubled by vig¬ 
netting, it is necessary to be within one or two 
stops of the proper exposure to fit the lighting 
conditions. Otherwise, the pictures will either 
be heavy and of muddy resolution, or else so 



| CAMERA 

CAMERA LENS --< TT~> 

Figure 120. Schematic drawing of exposure 
meter and camera. 

light that the corners will fail to print out 
properly. Indeed, the rule-of-thumb methods 
used during World War II stressed a slight 
over-exposure in order to prevent total loss of 
under-exposed pictures. On the other hand, the 
exposure range encountered in flying weather 
under military conditions is very large. 

In order to provide some means of determin¬ 
ing the exposure level and to fill in the inev¬ 
itable lapse of time until suitable photoelectric 
automatic shutters or iris diaphragms might 
be developed, the University of Michigan was 
requested under Contract OEMsr-1245 to de¬ 
sign and make a prototype of a visual meter. 

Figure 120 shows a schematic view of the 
optical arrangement. The photometer cube con¬ 


tains a small annulus of comparison light super¬ 
posed on the general field in focus of ground 
light from the viewfinder. The viewfinder lens 
and ground glass are shown in the proper posi¬ 
tion for use in the air. Figure 121 shows one 
of the two prototypes built under the contract 
and its approximate size. 

1,8,4 Film-Flatness Tester (Harvard) 34 

An examination of numerous prints of aerial 
photographs shows the existence of random 



Figure 121. Exposure meter, Model lb, assem¬ 
bly view. 


spots of poor focus, particularly evident on 
9x18 photographs. Under Contract OEMsr-474, 
Harvard undertook to devise and construct a 
device for the purpose of measuring departures 
from flatness of film under service conditions. 

Figure 122 shows a sketch of the final ap¬ 
paratus constructed at Harvard. In essence it 
was planned to use sixteen separate Hartmann 
tests at uniformly spaced intervals over the 
area of a 9x9 picture. Plans were under way 
for construction of a similar device for the A-7 
and A-8 magazines which are designed for 9x18 
photographs. 

No final analysis was made at Harvard, 
owing to the termination of the contract. The 
report suggests that the equipment is not quite 
















142 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


complete for a service test and would profit by 
an automatic means for changing the film. It 
was planned to set the equipment up experi¬ 
mentally in a medium-sized cold and low-pres¬ 
sure chamber and to operate a succession of 
films under conditions likely to obtain in the air. 

For ease of use, the light source was to be a 
small sodium aperture at a distance of 8 ft 
along the central vertical of the picture. Each 
of the sixteen identical lens elements was cov¬ 
ered by an opaque brass disk perforated in a 
Hartmann pattern. The twelve holes of the 



Figure 122. Schematic view of film-flatness 
tester. 


pattern were round and of a diameter found 
by experimental testing to be optimum for the 
exposure time planned. The rays defined by 
each Hartmann hole on a single circle of proper 
diameter pass through a common focus about 
Vs in. in front of the film plane. The Hartmann 
pattern is therefore out of focus and photo¬ 
graphs as a miniature pattern of the original 
perforated plate. All of the lenses are adjusted 
separately to yield a common focus on a plane 
for a light source at the distance specified. The 
oblique projection causes an elliptical spread 
of the small Hartmann spots, but the important 
consideration is that the defining focal plane 
carries with it a specific diameter constant 
throughout the photographic run, and that vari¬ 
ations from frame to frame can readily be 
detected. 


If film is run through the device at inter¬ 
vals of 5 sec or so, the Hartmann patterns 
photographed will have a maximum diameter 
defined by perfect positioning of the film. If the 
film bulges forward, the Hartmann pattern will 
contract. By very elementary geometry, it is 
possible to determine from the contraction how 
much the film has bulged. Indeed, the method 
is so accurate in terms of thousandths of an 
inch, that it is necessary only to measure the 
separation of two spots across a diameter lying 
perpendicular to a line in the film plane run¬ 
ning from the center of the picture. For 
economy of reduction one can use a projection 
system and a direct reading scale showing de¬ 
parture from flatness. 

The Harvard report mentions that a number 
of magazines used in the Services were received 
and examined. Only two out of six were in 
first-class working order. Three of the other 
four had vacuum leaks to the extent that the 
film could not have been held flat. Another had 
been damaged mechanically. The report also 
mentions that on photographs obtained from 
time to time, quite far out of focus, there were 
spots here and there of great sharpness, indi¬ 
cating that the film had bulged forward through 
the focal surface. It is suggested that, based 
on this experience, a simple test could be ap¬ 
plied by any organization interested in flight 
testing its magazines. One would preferably 
use a hard focus lens at maximum aperture, 
and work approximately 1 mm out of focus. 
Spots of sharp focus are more easily found on 
soft photographs than are soft spots on sharp 
photographs. 

Discussion 

The film-flatness tester should be completed 
and used for extensive tests of a previously 
renovated A-5 magazine. Similar tests should 
ultimately be conducted for A-7 and A-8 maga¬ 
zines which are much more likely to have 
vacuum troubles. 

It would be very informative to examine the 
results obtained with a half-dozen magazines 
reclaimed from the Services in order to ascer¬ 
tain what difficulties were most usually en¬ 
countered, and to what extent magazine troubles 
limit the quality of Service photographs. 




























MISCELLANEOUS EQUIPMENT FOR AERIAL PHOTOGRAPHY 


143 


1,8,5 Cold Chamber for Camera Tests 
(Harvard) 35 

Under Contract OEMsr-474, Harvard con¬ 
structed in 1943 an elementary type of cold 
chamber for conducting tests on the thermal 
properties of the 40-in. telephoto and the flu¬ 
orite apochromatic lenses. Relative to the in¬ 
vestment, the cold chamber provided many use¬ 
ful results in guiding later work. 

In 1945, work was begun at Harvard on a 
much more analytical form of cold chamber, 
which was also to include pressure testing. The 
earlier cold chamber work made use of auto- 
collimation and a quartz flat. The later proposal 
was to use only a single passage of light as a 
better guide to image characteristics of lenses 
under extremes of cold and apparent altitude 
and to effects of thermal and pressure gradi¬ 
ents. 

The cold and pressure chamber was to consist 
of a pipe 26 in. in diameter and nearly 4 ft 
high. A smaller pipe was to be welded to a 
base plate at the bottom of the larger pipe and 
was to extend 30 in. more into the basement 
below. This smaller pipe was to contain cells 
at the upper and lower ends for holding two 
10-in. diameter optical windows of BSC-2 glass, 
nearly 2 in. thick. 

The light path planned was to start at the 
focal plane of the lens under test within the 
chamber. A resolution target or any other form 
of target would be fastened securely in the 
focal plane of the camera under test. The light 
would then pass downward through the rear 
element and become more or less collimated by 
the camera lens. The emerging nearly parallel 
beam would then pass through the first window 
placed at the level of the first floor and base 
ring of the vacuum chamber. The space be¬ 
tween the two windows, nearly 30 in. in extent, 
was to have been evacuated in order that heat 
waves in the transition region between extreme 
cold and the room warmth would have no effect 
whatsoever on the optical path, and in order 
that condensation be eliminated. The light 
would then pass through the 30-in. vacuum, 
through the second window into the basement 
air, down to a parabolic mirror below the base¬ 
ment floor, and then to an observing microscope 


by way of a very small diagonal mirror 
cemented to the lower face of the lower window. 

Irrespective of the design details, it is ap¬ 
parent that such a cold and pressure chamber 
would afford the greatest comfort to the ob¬ 
server and would at the same time simulate 
conditions in the plane in every respect except 
vibration. The vertical position of the camera 
is desirable. Changes of focus are measured 
by the observing microscope under a longitu¬ 
dinal magnification varying as the ratio of the 
squares of the focal lengths of collimator and 
camera lens. 

The cooling effect of the apparatus on the 
room will produce a slow shift in zero point 
during a run intended to measure change of 
focus of the lens within the cold chamber. For 
establishment of a zero it is expedient to use a 
Gaussian arrangement in the observing micro¬ 
scope which sends a beam of light through the 
objective from its own focal plane, to the col¬ 
limating paraboloid, up to the lower face of the 
first window, and back again to the observer’s 
eye. 

The windows will act as hot radiators to the 
cold chamber, permitting much heat to enter by 
direct radiation. It would be nearly ideal to 
use a thin layer of gold on the lower face of 
the lower window. The gold film will reflect 
practically all the heat rays but will transmit 
very well in the visual. Such a film could have 
a thickness sufficient to return the reference 
beam of parallel light to the measuring micro¬ 
scope, enhanced relative to reflections from the 
other windows. 

It is recommended that such a cold and pres¬ 
sure chamber be carefully designed and con¬ 
structed for use in further testing in the lab¬ 
oratory. All wartime results point to the im¬ 
portance of knowing thermal gradients as well 
as temperature. Consequently, it would be of 
the greatest importance to include electric re¬ 
frigeration and heating controls for the estab¬ 
lishment of any arbitrary cooling curve desired. 
The effect of heating on position of focus, fol¬ 
lowed by rapid cooling, should be studied in 
an attempt to simulate thermal and military 
conditions in all parts of the world. Some 
thought should be given to installation of cir¬ 
culated air in an attempt to imitate the slip 



144 


equipment for aerial photography 


stream and its cooling action which was shown 
in the Harvard test flights to cause significant 
changes in the position of focus of large lenses. 


186 Aerial Camera Stabilizer 

A project was initiated in Division 7, NDRC, 
for the purpose of developing a gyroscopic 
mechanism for stabilizing large aerial cameras. 
Tests of the device finally constructed were 
confined to a wooden mockup of an aerial cam¬ 
era and conducted on a rocking platform. 

Whereas the usual type of stabilizer in effect 
couples the camera to the plane through the 
servo mechanism, which is generally too slow 
to respond to the higher frequencies and trans¬ 
mits them, the proposed type of design involves 
a pneumatic piston-operated actuator working 
at low pneumatic pressures. It was believed 
that such a system would permit the applica¬ 
tion of restoring forces through the actuator. 
The system is therefore analogous to one em¬ 
ploying weak restoring springs that in them¬ 
selves would be unable to transmit high-fre¬ 
quency vibrations to the camera. 

For a reference system, it was decided to 
make use of an angular rate gyro with pneu¬ 
matic pick-off and flexure gimbal bearings al¬ 
ready developed for other purposes. The reader 
is referred to the NDRC Division 7 report 86 
for engineering details of the coupling. In prin¬ 
ciple, the gyro is counterbalanced continuously 
by pneumatic forces in pick-off cups, which are 
in turn translated into corresponding differen¬ 
tial pressures. A pressure amplifier is included 
which converts the differential pressure into 
twentyfold amplified actual pressure in a cylin¬ 
der fastened to the air frame. A piston in this 
cylinder is attached to the camera. Any pres¬ 
sure in the cylinder, proportional to the original 
differential pressure, moves the piston and 
therefore the camera with respect to the air 
frame for maintenance of the gyro-controlled 
vertical. 

Test results showed that for a 1.5-sec period, 
and for an amplitude of approximately 10 
degrees, the angular departure of the camera 
from a fixed position was about one-fifth of 1 
angular mil. When the camera was forced off 


the true vertical and allowed to return, the 
time elapsed for it to come within one-third of 
its original tilt was 20 sec. In the absence of 
platform rocking, the camera was observed to 
come within a 6-mil hysteresis band of the true 
vertical. Any rocking of the platform, however, 
caused the camera to come within a fraction of 
1 angular mil from the true vertical, and more 
nearly simulated conditions in the plane. 

For application to an actual aerial camera 
mount, it would be necessary to add a second 
axis of stabilization not present in the working 
model in order for the camera to find a true 
vertical. Moreover, it is likely that the tests to 
date have not fully simulated aircraft condi¬ 
tions with respect to range of frequencies and 
amplitudes, and that time lag in cross product 
with natural frequencies and sensitivity of the 
gyro control may actually introduce small scale 
motions likely to upset the quality of the aerial 
photograph. Any stabilizing device working on 
detection of rate must be watched for this 
“three walnut game” to ascertain that the qual¬ 
ity of the photograph is maintained. It is also 
necessary to incorporate a sweep mechanism 
and preferably an antivibration mount in the 
stabilization device. 

19 RECOMMENDATIONS BY NDRC 
191 Lenses 

1. The development of high-resolution lenses 
of long focal length should be continued, em¬ 
phasizing high resolution scores based on area, 
wide angular coverage, preservation of micro¬ 
scopic contrast, maximum speed, and minimum 
vignetting. 

2. The following lenses should be given spe¬ 
cial attention: 

a. 36-in., //8, 9xl8-in. anastigmat, to re¬ 
place the 36-in., //8, 9xl8-in. telephoto, 
with better resolution over the entire 
area. 

b. 60-in., //5, 9xl8-in. telephoto, with 
sealed construction to permit evacua¬ 
tion. 

c. 72-in., //8, 9x36-in. anastigmat, for the 
combination of maximum resolution, 
focal length, and angular coverage. 





RECOMMENDATIONS BY NDRC 


145 


d. 100-in., //10, 9x18 in. This lens should 
be equipped with an antioscillation 
mount and should then be fully tested. 

3. Special lenses for extremely wide angular 
coverage should be developed. The same result 
may be achieved by using twin camera mounts 
or by using an optical device to aim the camera 
successively in two or more directions. The 
Harvard-NDRC 6-in., //2.85, 120-degree lens 
should be mounted and tested. If the value of 
this lens is demonstrated in a spherical shell 
camera, an automatic mechanism for changing 
shells should be developed. It will, however, 
probably be entirely practical to change shells 
by hand. 

4. All long-focus lenses should be hermet¬ 
ically sealed and thermostated so that they may 
give full performance. Automatic focusing is 
not necessary if the lens is provided with man¬ 
ual adjustment for approximate altitude. 

5. New optical materials, particularly flu¬ 
orite, barium fluoride, spinel, and some of the 
alkali halides should be developed further and 
applied to the design of long-focus lenses for 
high resolution, particularly for color photog¬ 
raphy. The 48-in., //8 glass-fluorite lens should 
be completed. 

6. Lenses and cameras for night photography 
should be developed further, particularly the 
Harvard-NDRC 8-in., //1.3 Schmidt camera 
for 35-mm film, to be used with Edgerton 
flashes, and the University of Rochester-NDRC 
6-in., //1.0 lens with curved field. Methods 
should be investigated for reducing scattered 
light when using Edgerton flashes, perhaps by 
employing sources rich in red light or by using 
polarized light. 


1,9,2 Shutters 

1. Focal plane shutters, with speeds up to 
Vi ,500 sec, with high efficiency and smoothness 
of operation, and with minimum vibration, 
should be developed. High speed offers the only 
hope for improving resolution markedly before 
better mounts are available. Higher speed can 
probably be achieved better with focal plane 
shutters than with between-the-lens shutters. 


It is essential that the shutter blind be located 
extremely close to the film, as the result of spe¬ 
cial design of shutter and magazine. It may not 
be possible to use standard magazines. 

2. A continuously operating blind for a focal 
plane shutter should be developed if possible to 
reduce vibration and to equalize exposure over 
the entire film. 

3. Further studies of the Langer shutter 
should be made to determine whether this type 
of shutter has useful applications. If so, the 
direct linkage of the blinds, suggested at Mount 
Wilson, should be investigated. 


193 Mounts 

1. Changes in the Eastman-NDRC mount 
should be made if indicated by carefully planned 
tests which should be conducted at the earliest 
possible moment. 

2. Other types of high-frequency filtering 
mounts should be investigated and developed 
if found to be promising. 

3. Development should be made of a mount 
for several cameras on a single rigid frame, 
with the maximum attainable moment of in¬ 
ertia and the longest possible period, supported 
at the center of gravity of the system, with 
damping and restoring force, and with a sweep 
mechanism. Such a unit would be entirely prac¬ 
tical in a bomber and offers one of the most 
promising outlooks for reducing angular mo¬ 
tion. 

4. The Harvard-NDRC center - of - gravity 
mount should be studied and developed further, 
with provision for releasing the camera at a 
moment when the angular rate is low, or else 
with provision of restoring force and damping. 
Experiments should be tried with a weak re¬ 
storing force and no damping, with provision 
for caging the camera when making turns. 

5. Studies of angular vibration data of the 
fuselage in various types of aircraft should be 
made to determine the best combination of nat¬ 
ural period, restoring force, and damping, in 
order to give the minimum angular velocity to 
the camera. 



146 


EQUIPMENT FOR AERIAL PHOTOGRAPHY 


19,4 Specially Engineered Lens-Camera 
Systems 

1. A 36-in. focus camera, with 9x18 in. cov¬ 
erage should be developed using the best avail¬ 
able design for the lens and shutter, and with a 
mechanical design which emphasizes lightness 
and dependability. This camera would be very 
useful in single-seater reconnaissance planes, 
including jet planes. 

2 . As soon as flight test data have led to a 


definite understanding of the factors which 
limit resolution in aerial photography, a project 
should be initiated to combine in one complete 
practical camera system all of the knowledge 
and technological skill available, based at the 
start on a 12-in. focal length camera, with a 
goal of 40 lines per mm resolution. Photographs 
as sharp as this would be equivalent to present 
photographs taken with cameras having more 
than twice the focal length. Experience gained 
should next be applied to longer focal lengths. 



Chapter 2 

RESOLUTION IN AERIAL PHOTOGRAPHY 

By Duncan Macdonald, Theodore Dunham, Jr., and James G. Baker 1 


21 INTRODUCTION (HISTORICAL) 

T he importance of aerial photographic re¬ 
connaissance has become generally acknowl¬ 
edged and assigned a high priority among the 
techniques of modern warfare. Aerial photog¬ 
raphy provides a large part of the information 
about enemy territory, about the identification 
and dispersion of men and materials, and about 
the damages inflicted by offensive action which 
serves as the basis for further military oper¬ 
ations. 

The usefulness of an aerial photograph de¬ 
pends to a large extent on the detail which it 
records. In general the ability to distinguish 
and identify objects on the ground increases 
very rapidly with increasing quality of the 
photograph, so that even a moderate gain in 
quality yields valuable additional information 
for the interpreter. The value of high-resolution 
aerial photographs in peacetime is equally great 
for precision mapping, and for agricultural and 
geological surveys, in which the maximum at¬ 
tainable detail can be used to great advantage. 

As the importance of high-quality aerial pho¬ 
tographs was well recognized, consequent pro¬ 
vision was made in the early stages of World 
War II to support investigations directed 
toward improvement of resolution in aerial 
photography. In October 1941, the Army Air 
Forces requested Section D-3 (Instruments) of 
NDRC to undertake Project AC-29, covering a 
variety of investigations directed toward this 
end. In February 1943, AC-29 was expanded 
to include the development of certain specific 
lenses. Since then the scope of AC-29 was fur¬ 
ther increased to include the development of a 
40-in. //5 lens and of a wide-held lens that had 
been started at Harvard in April 1941, under 
a direct Army contract. In March 1942, NDRC 

a The material in this chapter has been compiled by 
Dr. Duncan Macdonald (Boston University), with the 
collaboration of Dr. Theodore Dunham, Jr. (Chief, 
Section 16.1, NDRC), and Dr. James G. Baker 
(Harvard College Observatory). 


asked the Eastman Kodak Company to investi¬ 
gate the entire subject and to design a lens 
having the highest possible resolution, even if 
it should be necessary to limit the useful field 
severely. The workers at Eastman became con¬ 
vinced, however, that more would be gained 
from reducing vibration than from increasing 
inherent lens resolution, and accordingly, con¬ 
centrated their first efforts on developing an 
improved antivibration mount. Somewhat later, 
Eastman did develop two lenses of very high 
quality, but these developments were not car¬ 
ried out under NDRC. 

The Mount Wilson Observatory was asked 
by NDRC to develop methods for producing 
Schmidt correcting plates of high accuracy 
which would be suitable for production in case 
it were found desirable to employ these cameras 
in aerial photography. In 1942, the Observatory 
undertook the development of a 30-in. 2-mirror 
Schmidt camera 1 to evaluate the usefulness of 
this type of camera for precision photography 
from high altitudes and to determine whether 
the various mechanical difficulties connected 
with the design could be overcome. The experi¬ 
ence at Mount Wilson shows that for large 
Schmidt cameras these difficulties are so con¬ 
siderable that lens cameras are much to be 
preferred if they can be designed to give com¬ 
parable resolution. At the beginning of World 
War II it was uncertain whether long-focus 
lenses could compete with Schmidt cameras in 
giving precision resolution. But by 1944 it had 
been clearly demonstrated that lenses having 
resolution at least comparable with that of 
photographic emulsions over very considerable 
fields could be designed and, what is equally 
important, that they could be made under 
production conditions without sacrificing qual¬ 
ity. This demonstration of the capabilities of 
lenses reduced the need for pushing the de¬ 
velopment of long-focus Schmidt cameras for 
daytime reconnaissance. 

The development of special lenses at Harvard 


147 



148 


RESOLUTION IN AERIAL PHOTOGRAPHY 


University was transferred to Section D-3 of 
NDRC in September 1942. This project was 
assigned to Section 16.1 of NDRC in December 
1942. During the next three years the scope 
and the available facilities at Harvard were 
steadily increased, until the ending of World 
War II made it necessary to terminate the NDRC 
program. Several long-focus lenses of unusually 
high performance were developed. 2 ’ 3> 4> 5 These 
included lenses containing single fluorite ele¬ 
ments, which are nearly free from color aber¬ 
ration, and lenses with very wide angular cov¬ 
erage. Important improvements were made in 
methods for mounting lens elements with the 
required precision and for protecting the opti¬ 
cal systems against invasion by moisture and 
fungus, which often has a disastrous effect on 
otherwise excellent lenses, particularly in the 
tropics. 

In addition to this development work, Project 
AC-88 was established in May 1944 to cover 
the need for a systematic study of the factors 
that limit resolution in aerial photography, also 
offering provision for laboratory and flight 
testing of equipment developed under Project 
AC-29. Under Project AC-88 resolving-power 
targets were constructed at Wright Field, Day- 
ton, Ohio, by the Massachusetts Institute of 
Technology [MIT] in cooperation with the 
Photographic Laboratory of Wright Field. 6 

Flight tests using standard AAF equipment 
were conducted in the summer of 1944 at 
Wright Field over the resolving-power targets. 
These tests were conducted and assessed as part 
of a cooperative program by Mount Wilson, 
MIT, and the Photographic Laboratory. This 
program consisted of seven flights with a stand¬ 
ard 24-in. camera in the standard mount of an 
F-5E airplane. In addition to this program, 
fifteen other missions were flown, some at night 
over flashing lights, some in the daytime over 
the resolving-power targets, and others over 
sun-illuminated spheres. These missions were 
designated as vibration studies and the data 
interpreted in terms of the quality of antivi¬ 
bration performance of the camera mounts. 
This latter program extended into the spring 
of 1945. 

Early in 1945 it became obvious that progress 
in evaluating the factors which limit resolution 


could only be made rapidly enough to lead to 
improvements in equipment which might yet 
have an effect on the war if an intensive flight 
testing program were undertaken in close con¬ 
junction with one of the NDRC laboratories, 
with a special airplane and crew assigned for 
the purpose. At that time, an expansion of the 
staff and facilities of the Harvard laboratory 
was being considered in order to expedite com¬ 
pletion of new lenses which were urgently 
needed. Section 16.1 of NDRC regarded a 
knowledge of the factors limiting resolution 
as essential for realizing the potential advan¬ 
tages of precision long-focus lenses, but recom¬ 
mended that the laboratory and shop facilities 
at Harvard should be expanded only if an air¬ 
plane were made continuously available at Bed¬ 
ford for flight tests. The Army Air Forces 
supplied a B-17 airplane late in June 1945, and 
this airplane or another of the same type was 
available without interruption through Novem¬ 
ber 1945. 

An extensive program was planned at Har¬ 
vard to investigate all factors likely to affect 
the resolution of photographs. The termination 
of World War II a few weeks after the program 
was initiated prevented carrying out more than 
a small part of this program. After V-J Day 
the principal aim was changed from evaluation 
of limiting factors to that of achieving best 
possible resolution with available equipment. 
In spite of the sudden interruption which the 
project necessarily suffered, many extremely 
effective methods were developed and a large 
amount of very significant data was obtained 
on the fifty-five flights flown out of Bedford. 
It has not been possible to reduce and discuss 
more than a part of this data in the OSRD 
reports. However, the continued reduction of 
this data and the completion of the flight pro¬ 
gram are now being undertaken under the 
auspices of the Boston University Optical Re¬ 
search Laboratory and the Wright Field Photo¬ 
graphic Laboratory. 


2 2 STATEMENT OF THE PROBLEM 

The resolution obtained on an aerial photo¬ 
graph is the result of the combined effects of 




RESOLVING-POWER TARGETS 


149 


a number of factors. An upper limit of aerial 
resolution is determined by the performance of 
the lens in the laboratory, or more exactly, lens 
plus film laboratory resolution. When in the 
air, the peak resolution of a camera-film com¬ 
bination (laboratory performance) may be lik¬ 
ened to a position of unstable equilibrium where 
the influence of ground speed, vibration, air¬ 
craft rotatory motions, shutter performance, 
haze, focal setting, temperature, air turbulence, 
the presence of a photographic window, and 
other factors add together to pull resolution 
down to a lower, more stable level. In addition, 
exposure level and photographic processing 
techniques have considerable influence on the 
observable detail in the finished negative or 
print. Here, then, are really two closely re¬ 
lated problems: (a) a comparison of laboratory 
and in-the-air performance of both standard 
AAF and NDRC equipment, and (b) an evalu¬ 
ation of the individual factors that add together 
to cause the gap between laboratory and in-the- 
air performance, with the aim to minimize those 
factors found to be most detrimental. In the 
following sections there will be presented a de¬ 
scription of the methods and a discussion of 
the results of the studies by Section 16.1 of 
NDRC related to resolution in aerial photog¬ 
raphy. 


2 3 RESOLVING-POWER TARGETS 

In a program directed at the evaluation of 
in-the-air performance of photographic equip¬ 
ment, it is of course necessary to provide re¬ 
solving-power targets on the ground to enable 
a quantitative scoring of performance of this 
equipment. Four sets of targets were con¬ 
structed for use in the resolution program. 

In 1942 the Mount Wilson Observatory con¬ 
structed four circular painted canvas patterns, 
each approximately 30 ft in diameter and con¬ 
sisting of 32 sectors alternately light and dark. 7 
Each pattern was of different contrast. Al¬ 
though preliminary tests of equipment were 
conducted over these targets, rapid accumula¬ 
tion of dirt on the canvas indicated the need for 
an improved type of target construction. 

Four parallel-line resolution patterns painted 


on masonite sheets were then constructed by 
Mount Wilson, each pattern of different con¬ 
trast, consisting of six pairs of 4x8 ft panels 
hinged together. The unit structure selected for 
this pattern was later adopted as an unofficial 
standard for all other resolution targets con¬ 
structed by Section 16.1, and is described in the 
following text. These masonite patterns were 
damaged by wind while in use at Blythe, Cali¬ 
fornia, in connection with tests of the 40-in. 
//5 telephoto lens developed at Harvard. 

It became apparent that for a long-range 
testing program, permanent resolution targets 
were required, targets that would not demand 
constant attention and which would retain their 
photometric characteristics. 

With the establishment of Project AC-88, the 
first set of these permanent targets was con¬ 
structed at Wright Field. It was agreed, after 
considering several types of installation, that 
patterns painted on concrete slabs would prove 
to be the most satisfactory type. The Army Air 
Forces then installed five concrete slabs along 
the E-W runway at Wright Field, and MIT 
sponsored the painting of the thirteen parallel 
line resolution patterns on these slabs. 6a 

This project required the development of 
optically neutral paints with a high degree of 
color permanence, good weathering qualities, 
and strong adherence to concrete. This was 
done at MIT by first modifying a white (titan¬ 
ium oxide) highway paint to conform with the 
mechanical requirements of the problem and, 
second, by determining percentage weight com¬ 
position of this paint and two paints of similar 
composition but containing carbon and ferrous 
oxide pigments to give optical neutrality at any 
desired reflectance level. 

The choice as to types of resolution pattern 
for the targets soon was limited to two—radial 
or parallel line. In the evaluation of a photo¬ 
graph in terms of resolution alone, these de¬ 
signs seem to offer as simple and as complete a 
result as possible. The radial pattern possesses 
the obvious advantages of giving resolution at 
all azimuths and avoiding discrete steps. On 
the other hand, the method of assessing parallel¬ 
line patterns can be made simple and the per¬ 
sonal factor minimized. Moreover, in a parallel¬ 
line pattern, the ratio of line length to line 



150 


RESOLUTION IN AERIAL PHOTOGRAPHY 


width, and its consequent effect on resolution, 
is constant. These advantages, coupled with the 
fact that parallel-line patterns were in general 
use, led to the adoption of this type of pattern. 
The disadvantage of discrete steps was over¬ 
come in part by the adoption of a small step 
factor (^2 = 1.26). To determine resolution 
both parallel to and perpendicular to the line of 
flight, two sets of lines were provided, one in 
the direction of the flight course, the other 
normal to this direction. The range of line 
widths (width of a single line) was from 1 to 
30 in., covered in sixteen steps. 

The length of each line was five times its 
width; the space between lines was equal to 
the line width. Resolving power is increased by 
increasing the ratio of line length to line width. 
At a line length to width ratio of 5/1, resolution 
approaches that value obtained for infinitely 
long lines while the overall dimensions of the 
target do not become too great. It is felt that 
resolving power as measured by such a pattern 
corresponds closely to the resolution of detail 
in aerial photographs. 

It was found that three lines separated by 
two spaces was the ideal unit. Each unit there¬ 
fore forms a square. If but two lines are used, 
spurious resolution may be assigned as true 
resolution. Also, from the point of view of the 
film observer, a criterion of resolution can be 
established for the three-line pattern that is 
more nearly independent of the personal equa¬ 
tion. The criterion adopted was that a unit to 
be resolved must be seen as three lines sep¬ 
arated by two spaces of uniform density re¬ 
gardless of the degree of contrast between lines 
and spaces. Thus the observers looked for two 
spaces rather than three lines. The use of more 
than three lines is unnecessary and would re¬ 
sult in less correlation between observers, due 
to the difficulty that the human eye encounters 
in focusing on each of several uniformly spaced 
steps. The resolving power attained on a film 
was then defined as the reciprocal of the sep¬ 
aration (lines per mm) on the film of the 
centers of the smallest bright lines of the pat¬ 
tern that were acceptable under the criterion 
set up to define a resolved unit. Resolving power 
in the line of flight was defined by the pattern 
lines normal to this direction. 


The choice of pattern reflectivities was based 
on the fact that the landscape has an average 
reflectivity of the order of 16 per cent. This 
figure was then taken as an approximate geo¬ 
metric mean of the line-background pattern 
reflectivities. Two patterns were located on each 
of the five targets at Wright Field, one pattern 
of high contrast, the other of low contrast. In 
one target, three additional patterns were 
painted, one of the maximum attainable con¬ 
trast with the construction method used, the 
other two of low contrast with one of high re¬ 
flectivities and the other of low reflectivities. 
Unusual weathering conditions, due primarily 
to assorted dusts of various construction jobs, 
changed the target reflectivities from time to 
time. The proposed and initial reflectivities, and 
the change of reflectivities are shown in Table 1. 


Table 1. Pattern reflectivities. 




Reflectivities in per cent 


Pattern 

Pro- 

Initial 





posed 7/22/44 8/10/44 

9/2/44 

11/6/44 

High contrast 






Line 

50 

46.5 

40.9 

43.9 

32.1 

Background 
Low contrast 

5 

3.0 

3.6 

6.1 

6.0 

Line 

24 

25.0 

20.5 

27.9 

22.6 

Background 

12 

11.5 

10.5 

16.5 

12.9 

Maximum contrast 





Line 

80 

80.5 

70.4 

60.3 

47.5 

Background 
Low contrast of 

5 

3.0 

3.6 

6.1 

6.0 

high reflectivities 





Line 

80 

80.5 

70.4 

60.3 

47.5 

Background 
Low contrast of 
low reflectivities 

40 

39.0 

34.1 

35.3 

27.9 

Line 

10 

11.5 

10.5 

16.5 

12.9 

Background 

5 

3.0 

3.6 

6.1 

6.0 


In addition, three photometric areas each 
40 ft square were located on the largest target. 
These areas were of proposed reflectivities—50, 
16, and 6. A small area of each target (16x40 ft) 
was devoted to a 14-ft diameter, 64-sector radial 
pattern, a set of solid color circles, and several 
crosses. This area was to be devoted to use in 
studies of aircraft motions. 

These targets, and an additional high-con¬ 
trast parallel-line pattern and 40-ft diameter, 
32-sector radial circle painted on one of the 
runways as a preliminary test of the painting 







RESOLVING-POWER TARGETS 


151 


method, were employed throughout the summer 
of 1944 on the NDRC flight testing program. 

When the aerial photographic testing pro¬ 
gram was consolidated at Harvard in the spring 
of 1945, another set of targets was constructed 
at the Orange, Massachusetts airport. 8 The air¬ 
port has three little used 5,000-ft asphalt run¬ 
ways. Arrangements were made for use of one 
of these (the NE-SW) runways, and four res¬ 
olution targets were then painted on this run¬ 
way. 

On the basis of experience gained through 
use of the Wright Field targets, and particu¬ 
larly from the British, a smaller step factor ^/2 
was employed in the parallel-line pattern. This 
eliminated the necessity for observers to inter¬ 
polate between units in determining limiting 
resolution. The range of line widths on the 
Orange targets was from 2 ft to 1% 6 in. The 
unit structure, which had proved itself to be 
most satisfactory, was identical with that of 
the Wright Field patterns. 

It had been found that local defects in the 
emulsion often affect the selection of the last 
resolved unit when working at a high level of 
resolution. To eliminate this spurious loss of 
resolution, the eight smallest units were dupli¬ 
cated on these targets. 

The plans for the target layout at Orange 
called for a single standard resolution pattern 
at each target. Because of this feature the 
choice of contrast and reflectivities was impor¬ 
tant. The analysis of the Wright Field tests 
had employed only the readings from the high- 
contrast target images. On the other hand, 
British and Canadian researches were mostly 
done on low-contrast targets. It is true that 
aerial reconnaissance, as used in military oper¬ 
ations, deals for the most part with low-con¬ 
trast objects, and a low-contrast target would 
therefore be most representative of actual con¬ 
ditions encountered in the field. The British 
point out that higher resolutions assigned with 
use of high-contrast patterns are not true rep¬ 
resentations of resolution of detail in the aver¬ 
age military photograph. 

On the other hand, any factor that enters to 
limit resolution will lower a high resolution 
number more than it will lower a low resolution 
number. It is desirable, then, in a study of fac¬ 


tors that limit resolution to work at high levels 
of resolution where the numbers are more sen¬ 
sitive to change. This implies use of a high- 
contrast target for such studies. 

The British have also pointed out that devel¬ 
opment of equipment may be carried farther 
than is practical through use of high-contrast 
studies; the argument being that equipment 
that will improve high-contrast resolution will 
not necessarily improve the low-contrast reso¬ 
lution encountered in reconnaissance. This may 
be true, but given ideal atmospheric conditions 
on any day, equipment tuned to give maximum 
resolution at high contrast will outscore equip¬ 
ment that has been tuned to give maximum 
resolution at low contrast. Also, it is true that 
low contrast and the effects of varying con¬ 
trasts can be studied better under laboratory 
conditions where control factors are more 
easily imposed. 

Because the measurement of resolution be¬ 
comes less critical at lower contrasts, and be¬ 
cause emulsion properties themselves, partic¬ 
ularly graininess, are predominant factors in 
low-contrast resolution, it was felt that the 
standard pattern for the investigations should 
be of such contrast as to yield high resolution 
numbers. A 4/1 reflectivity ratio was proposed. 
The reflectivities of the finished patterns were 
7.5 per cent for the background and 46 per cent 
for the lines. 

In addition to this standard pattern, each 
target contained a small, high-contrast pattern 
(line reflectivity 88 per cent, background 7.5 
per cent). Three of the targets possessed 40-ft 
diameter, 64-sector radial patterns and the 
fourth target an 80-ft diameter, 128-sector 
radial pattern. This latter target also contained 
five 40-ft square photometric areas of reflec¬ 
tivities 88, 46, 80, 10, and 4.0 per cent and three 
other resolution patterns of the same dimen¬ 
sions on the standard pattern. These three pat¬ 
terns were: 

1. A reverse standard pattern, dark lines of 
7.5 per cent reflectivity on a light background 
of 46 per cent reflectivity, used to investigate 
the effect of scattered background illumination. 

2. A low-contrast pattern of low-exposure 
level, line reflectivity 30 per cent, background 
reflectivity 7.5 per cent. 



152 


RESOLUTION IN AERIAL PHOTOGRAPHY 


3. A low-contrast pattern of high-exposure 
level, line reflectivity 88 per cent, background 
reflectivity 43 per cent. 

These latter two patterns were designed for 
studies involving contrast and exposure level. 
This target also contained two other small pat¬ 
terns devoted to special tests. One pattern con¬ 
sisted of a series of dark lines on a white back¬ 
ground, all lines of the same length but of 
widths varying by a factor of y2. This pattern 
was used for measuring loss of contrast with 
decreasing line width. The other pattern con¬ 
sisted of nine sets of solid dark circles. In each 
set the diameters varied by a factor of y2. 
Three sets were painted on each of three dif- 


The choice of the standard resolution pattern 
has proven to be satisfactory in all respects. In 
addition, there was no measurable change of re¬ 
flectivity of the Orange targets during the time 
that they were employed. Recent observation 
has indicated that the paints have endured a 
New England winter without noticeable de¬ 
terioration or change in reflectivity. 


24 PERFORMANCE OF STANDARD 
EQUIPMENT 

It is natural to inquire what level of resolu¬ 
tion is achieved at the present time under repre- 



Figure 1 . Large target at Orange. 


ferent backgrounds, each set in a given back¬ 
ground of a different reflectivity. This pattern 
was to be used to study effects of scattered 
light and contrast in limiting the size of detail 
observed. Two of the other runway targets also 
contained three 40 ft square photometric areas. 

A small masonite parallel-line target was 
installed alongside the runway at the Orange 
Airport. This contained colored targets, yellow 
on gray, gray on yellow, red on gray, and gray 
on red. It was used in studies of resolution of 
colored objects and to compare the relative 
color corrections of various lenses. 

Figure 1 is an enlargement of the large tar¬ 
get at Orange showing the design of the stand¬ 
ard pattern, the target layout, and the special 
patterns. This photograph was taken from 10,- 
000 ft with the 40-in. //5 Harvard-NDRC lens. 


sentative operating conditions with standard 
equipment and regular personnel. It is ex¬ 
tremely difficult to obtain anything approaching 
quantitative information on this point because 
it has obviously been impractical to install reso¬ 
lution targets in regions where military opera¬ 
tions were being conducted. It would, however, 
be entirely practical to install test targets near 
training centers for instruction in aerial pho¬ 
tography. Much valuable information would be 
gained about the performance of the equipment 
used and about the effectiveness of the tech¬ 
nique used in its operation. It would be desir¬ 
able to install such targets as soon as possible 
at training centers. 

The level of resolution of photographs taken 
under operating conditions could be estimated 
to a reasonable degree by making a number of 













PERFORMANCE OF STANDARD EQUIPMENT 


153 


photographs of a region containing a variety of 
common objects (trucks, airplanes, railroad 
tracks, buildings, ships, etc.) and also resolu¬ 
tion targets. A series of photographs could then 
be selected which would represent known levels 
of resolution (5, 10, 15, 20, etc., lines per mm). 
By comparison with this test series, the resolu¬ 
tion of any photograph could be estimated. The 
comparison should be made preferably between 
the negatives rather than between prints. 

Extensive quantitative data on resolution 
with standard equipment was obtained on the 
NDRC-Wright Field Photographic Laboratory 
flight program using a standard 24-in. camera 
in the standard mount in an F-5E airplane. 
Seven missions were flown under this program 
during the summer of 1944. These tests, made 
over the Wright Field resolving-power targets, 
were flown at 25,000 ft. 

Laboratory tests conducted at the Mount Wil¬ 
son Observatory had previously shown that this 
lens was capable of resolving 25 lines per mm 
when used in conjunction with Super-XX 
film. 70 ’ 9 Analysis of the flight data at the Mount 
Wilson Observatory showed median in-the-air 
resolutions of 11.9 lines per mm across the line 
of flight and 11.4 lines per mm in the line of 
flight. This result was based on a study of 1,245 
target images. 

Although this result lumps together the data 
gathered using different /-stops and exposure 
times, and with no regard to the distance off 
axis, it is restricted to positions near the best 
focal setting and therefore may be regarded as 
a good indicator of overall average perform¬ 
ance. 

On the basis of these tests, the best perform¬ 
ance of the 24-in. lens under ideal weather con¬ 
ditions is obtained at aperture //11, exposure 
of % 5 o sec. Longer exposure times increase the 
effect of image movement while the increased 
aperture required by shorter exposure times 
offsets the effects gained by use of this expo¬ 
sure. A summary of the results is shown in 
Table 2. 

The results listed in Table 2 are based on se¬ 
lected target images, the selection rule eliminat¬ 
ing from consideration all images obviously 
blurred by motions of the aircraft. 

Although the probability of obtaining resolu¬ 


tion in the air at half the value of laboratory 
performance is slightly less than one-half, in¬ 
dividual photographs occasionally did show 25 
lines per mm. It may be estimated that 2 per 


Table 2. Average resolutions obtained with 24-in. 
standard camera at Wright Field, 1944. 


Exposure 
time (sec) 

Aperture 

Resolution 
in the line 
of flight 

Resolution 
across the line 
of flight 

1/150 

//16 

14.6 

14.7 

1/350 

//11 

15.3 

15.8 

1/800 

//6 

12.8 

14.0 


cent of the aerial photographs taken under the 
conditions of this test reach laboratory per¬ 
formance. This suggests that with standard 
equipment on fine days, haze, air turbulence, 
and other extraneous factors contribute rela¬ 
tively little to reduce resolution below the labo¬ 
ratory level of 25 lines per mm. Vibration is 
probably the chief offender. If this is correct, it 
is to be expected that laboratory resolution 
should be attained at occasional instants when 
the camera is momentarily free from angular 
motion except that which happens to compen¬ 
sate for aircraft translation. 

The results of these tests are in good agree¬ 
ment with data gathered at Harvard 10 and by 
the Eastman Kodak Company 11 on similar tests 
at lower altitudes, using a B-17 bomber and an 
F-2 airplane with standard mounts. This indi¬ 
cates that the degree of departure of in-the-air 
performance from laboratory performance of 
the standard equipment is little affected by 
choice of aircraft, standard mount, or altitude. 


2,4,1 Comparison of Performance of 
Standard Lenses with NDRC Lenses 

The flight program conducted at Harvard in 
the summer of 1945 offered a comparison of in- 
the-air performance of four lenses. 100 Figure 
2 shows the frequency distribution of resolu¬ 
tions obtained with these four lenses: the 24- 
in. Aero-Ektar, the 36-in. //8 fluorite apochro- 
mat, the 36-in. wide-angle telephoto, and the 
Harvard-NDRC 40-in. // 5 telephoto. Only 
target images recorded within 3 in. of the 







154 


RESOLUTION IN AERIAL PHOTOGRAPHY 


center of the film were used in the data shown 
by these curves. This restriction limits the field 
to within 4.3 degrees of the optical axis for the 
40-in. lens, 4.8 degrees for the 36-in. lens, and 
7.2 degrees for the 24-in. lens. All the distribu¬ 
tion curves have been normalized to 100 for 
maximum frequency; the number of observa- 


the distribution of resolution in the line of 
flight, the right-hand curves that across the 
line of flight. The full line contour used in the 
plot of resolutions of the 40-in. lens is derived 
from all flights with this lens over the resolu¬ 
tion targets; the dashed lines represent the re¬ 
sults of all flights except the last three of the 


o 

cr 

.< 

CM 




0 5 10 15 20 25 30 35 4 0 0 5 10 15 20 25 30 35 40 
LINES PER MM 

24-INCH f/6 IN A-11 MOUNT. BETWEEN-THE-LENS 
SHUTTER AT 1/150 TH. ALL SUPER-XX OBSERVATIONS 
AT MIXED APERTURES. (f/8 to f/11). 


o o 

o 

cr cr 

cr 


< 

CVJ O 


<J- CM 

CM 


o 

cr 

< 



0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 
LINES PER MM 

36-INCH f/8 FLUORITE APOCHROMAT IN NDRC MOUNT. 
NO SWEEP. PANATOMIC-X OBSERVATIONS. FOCUSING 
RUNS INCORPORATED BODILY. 


oo 

o 

o 

cr 

cr 

cr£ 

< 

< 

«< 

: 


s 'CM 

0) 

> 

O <t 
if) 

CM 




0 5 10 15 20 25 30 35 40 O 5 10 15 20 25 30 35 40 


LINES PER MM 

36-INCH f/8 TELEPHOTO FOR K-18. A-8 MOUNT. 
1/150TH. ALL SUPER-XX OBSERVATIONS. FOCUSING 
RUN INCORPORATED BODILY. 



0 5 10 15 2025 30 35 40 0 5 10 15 20 25 30 35 40 
LINES PER MM 


40-INCH TELEPHOTO FOR K-22. MOSTLY NDRC 
MOUNT, WITHOUT SWEEP. FOCUSING RUNS INCORPORATED 
BODILY. ALL PANATOMIC-X OBSERVATIONS. 


Figure 2. Frequency distribution of resolutions obtained with four lenses. 


tions used in the determination of each curve 
is given just above the location of maximum 
frequency on the abscissa. The angular resolu¬ 
tion noted at the peaks of the curves is taken 
as the angle subtended at 10,000 ft, the alti¬ 
tude of the flights, by the center-to-center dis¬ 
tance of adjacent white lines in the resolution 
targets. The left-hand curves for each lens show 


testing program. On these three flights the 
lenses were apparently distorted by a clamping 
device and results were of decidedly poor qual¬ 
ity. It is felt that the dashed contour gives the 
more accurate picture of performance of the 
40-in. lens. 

For comparison purposes the angular resolu¬ 
tion corresponding to the most probable per- 
































PERFORMANCE OF STANDARD EQUIPMENT 


155 


formance of the 24-in. lens has been scaled to 
the corresponding focal lengths and indicated 
on each of the curves. Thus, in the line of flight 
the 24-in. gives about 42 sec of arc resolution, 
as compared to 16 sec for the 40-in. Much of 
this difference arises from the exposure times. 
Across the line of flight, the 24-in. gives 29 
sec of arc, compared to 10 sec for the 40-in. 
Again some of the difference can be ascribed 
to the exposure times used. If the 24-in. were 
used entirely at f/S with yellow filter and % 50- 
sec exposure, its most probable resolution 
might reach 16 lines per mm or 22 sec of arc 
(see Table 2). The 40-in. at //5 with Super-XX 
should yield about 22 lines per mm or 11 sec of 
arc. (See Chapter 1, Table 7.) Thus, the 3 to 1 
difference in angular resolution might fall to 
2 to 1 under circumstances more favorable to 
the 24-in. lens. Peak resolution, however, would 
still differ by more than 3 to 1. The //5 working 
aperture of the 40-in. would then make this lens 
more versatile at high resolution than the 24- 
in. lens. 

Results with the 36-in. telephoto and K-18 
between-the-lens shutter in the line of flight 
confirm earlier predictions that unless sweep 
mechanism is provided, or unless exposure time 
is shortened, no appreciable gain in angular 
resolution over the 24-in. standard lens has 
been achieved, except for scale. Across the line 
of flight, the difference is about 2 to 1, whereas 
the focal lengths are only 1.5 to 1. The 36-in. 
would lose little on printing. With the 36-in. 
fluorite lens, the difference in angular resolu¬ 
tion was about 2 to 1 in both directions. It 
should be pointed out, however, that photo¬ 
interpreters regard a small decrease in graini¬ 
ness as a significant improvement in the quality 
of reconnaissance photographs. The increased 
scale of the 36-in. in effect gives this result by a 
reduction in the ratio of grain size to detail size. 

In considering the other types of test pat¬ 
terns in the Orange installation, it is observed 
that the performance of the 24-in. Aero-Ektar 
lens is at about the same level of resolution for 
the standard pattern, the low-contrast targets, 
and the reversed standard pattern. On the other 
hand, the 40-in. telephoto gives lower resolution 
on the other patterns than on the standard pat¬ 
tern. This result is due to the larger amount of 


image flare present when using the 24-in. lens 
which results in less contrasty photographs 
than those taken with the 40-in. lens. For this 
reason, the performance in lines per millimeter 
of the 40-in. lens is more seriously impaired at 
peak resolution by the presence of haze and 
other factors that tend to lower contrast than 
is the 24-in. lens, but except under extreme 
conditions may be expected to yield superior re¬ 
sults. It is to be expected that on the same 
emulsion, the difference in resolution, as re¬ 
corded by two different lenses, becomes less and 
less with decreasing target contrast, and in 
fact the resolution numbers will converge at 
very low contrasts. At this point the problem 
becomes one of film properties rather than lens 
quality. 

2 4 2 Comparison of Laboratory and 
In-the-Air Performance 

The starting point of any study of the factors 
influencing resolution is to determine for any 
camera the gap between resolution in the lab¬ 
oratory and in the air. With a high-contrast 
target imaged on Super-XX film, the standard 
24-in. lens resolves between 22 and 25 lines per 
mm in the laboratory. 71 " This lens cannot be 
used with finer grain emulsions because of 
speed limitations. Since Super-XX film is ca¬ 
pable of resolving about 50 lines per mm at 
high contrast, it is evident these lenses must 
limit resolution to a serious degree. On the basis 
of the results of NDRC tests of standard equip¬ 
ment at Wright Field and Harvard, it was 
found that resolution of the standard 24-in. 
lens across the line of flight was about 50 per 
cent of its laboratory performance. (See Sec¬ 
tion 2.4.) Resolution in the line of flight was 
at a considerably lower level, due to the trans¬ 
lational motion of the aircraft. The results of 
these two programs indicate that in-the-air 
performance is at a level comparable to labora¬ 
tory performance less than 2 per cent of the 
time. 

Laboratory tests at Harvard with the Har- 
vard-NDRC 40-in. //5 lens showed this lens to 
resolve at least 40 lines per mm on Super-XX 
film and 45 lines per mm on Pan-X film, using 
high-contrast targets. (See Chapter 1.) With 



156 


RESOLUTION IN AERIAL PHOTOGRAPHY 


near perfect conditions, this lens can appar¬ 
ently reach the limiting resolution of the film, 
but it is felt that these conditions are not com¬ 
parable with those encountered in flight test. 
The average resolution across the line of flight 
with the 40-in. lens in the Eastman-NDRC 
mount, at 10,000 ft over the Orange targets, 
was 22 lines per mm on Super-XX film, and 27 
lines per mm on Pan-X film. This again is at a 
level of little better than 50 per cent of the 
laboratory performance, whereas this lens at 
no time attained laboratory performance in the 
air. Twice, however, 40 lines per mm were ob¬ 
tained on Pan-X. It is likely that haze and re¬ 
sultant loss of contrast, which proved unimpor¬ 
tant for the 24-in. lens at 22 lines per mm, be¬ 
comes a limiting factor for the 40-in. at 45 
lines per mm. 

There seems to be a striking gap between 
laboratory and in-the-air performance which 
must be explained and, if possible, eliminated. 
The study of factors which cause this gap and 
thereby limit resolution is complicated by the 
fact that many of these factors are operating 
simultaneously. As a result, special methods 
must be devised for isolating and evaluating 
each factor. Several factors have been consid¬ 
ered and studied. There follows a discussion of 
these studies and the methods employed. 


2 5 STUDY OF FACTORS LIMITING 
RESOLUTION OF AERIAL PHOTOGRAPHS 

2,51 Lens Performance 

• 

On the basis of the discussion in the preced¬ 
ing paragraphs, although it is apparent that 
standard lenses do limit resolution, it seems un¬ 
likely that the resolution of the best available 
lenses is now an important limiting factor in 
aerial photography. While further improve¬ 
ment in lens design is to be desired, other fac¬ 
tors are probably more serious in limiting pres¬ 
ent day resolution. (See Chapter 1.) 

2,5 2 Translational Motion of the Aircraft 

Reference to Figure 2 will indicate the differ¬ 
ence between resolution in the line of flight and 


across the line of flight as determined by the 
Harvard tests. This discrepancy is due to image 
movement in the focal plane during the expo¬ 
sure, caused by the translational motion of the 
aircraft. Tests made by Eastman representa¬ 
tives 11 " 1 at Harvard have shown clearly that 
resolution in the line of flight can be made of 
the same order as that across the line of flight 
by employing a sweep mount to compensate for 
image movement. In general, the performance 
of the Eastman-NDRC antivibration mount 
raised the overall level of resolution in aerial 
photographs by use of the sweep mechanism. 
The improvement in resolution across the line 
of flight by use of the sweep method of image 
movement compensation is shown in the plot 
(Figure 3) of the results of one of the tests 
conducted at Harvard. 

Other methods have been devised to compen¬ 
sate for translational image movement. Har¬ 
vard developed a unit consisting of two 10-in. 
diameter objective prisms rotating in opposite 
directions, but this device has not been flight 
tested. 8 " 1 The British as well as Wright Field 
have developed a moving film magazine and also 
a rotating plane-parallel glass plate inside the 
camera. The performance of the sweep mount 
has, however, proved highly satisfactory in the 
compensation of ground movement. It would 
seem that this may offer a simple solution to 
the problem of image movement compensation. 


2S - 3 Vibration 

Apart from the effect of translation in the 
line of flight, vibration is probably the most 
serious factor in limiting resolution in aerial 
photography. The presence of vibration re¬ 
quires that short exposure times be used in all 
aerial photographs. 

Such a requirement is generally met by using 
high-speed lenses and/or high-speed emulsions. 
There are obvious practical limitations in the 
construction of high-speed lenses of long focal 
lengths. However, the 40-in. //5 telephoto lens 
might be considered as an approach to this 
method of solution and did, in turn, yield com¬ 
paratively good results when used in conjunc¬ 
tion with Pan-X film. The standard 24-in. lens 






FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


157 


shows considerable loss in resolution when used 
at or near maximum aperture. The results of 
Table 2 show that this factor masks out any 
gain due to decreased exposure times. Further, 
this lens is not in itself fast enough to allow for 
the general use of any emulsion slower than 
Super-XX, in which case grain size becomes an 
important factor. 

This combination of factors, the large-aper- 


ard equipment and NDRC mounts, and (c) the 
problem involved in design and construction of 
a stabilized camera mount for aerial mapping. 

The method employed in evaluation of the 
magnitude of vibration was developed at MIT. 12 
Trails of flashing lights were recorded on film 
at night by flying over a line of flashing lamps 
with open shutter. Early studies were made 
over neon lamps operating from a 120-c a-c line. 





NO. TARGETS 12 23 9 

STANDARD DEVIATION 

IN FLIGHT DIRECTION 2.1 7.8 5.1 

-L TO FLIGHT DIRECTION 2.8 5.1 3.7 


IN FLIGHT DIRECTION - 

-L TO FLIGHT DIRECTION- 


Figure 3. Effect of sweep on resolution. 


ture lens and coarse-grain emulsion, demanded 
by the exposure limitations seriously limits the 
quality of aerial photographs. If, on the other 
hand, antivibration mounts are developed to 
allow for longer exposure times, present lenses 
in conjunction with fine-grain film would yield 
considerably higher quality results than those 
now obtained. 

A study was made of the motions of aerial 
cameras in flight to evaluate: (a) the frequen¬ 
cies and amplitudes present in actual flight con¬ 
ditions, (b) the relative performance of stand- 


Later work was done over special strobolux 
lamps designed to give flashes of }4ooo-sec 
duration spaced y 60 sec apart. These lamps im¬ 
pressed a time scale in the form of a trail of 
dots over the film; the neon lamp images were 
separated by % 2 o sec in time and successive 
strobolux images by % 0 sec. With this time 
scale, it was possible to interpret the data re¬ 
corded on the film in terms of angular veloci¬ 
ties of the camera in three coordinates—roll, 
pitch, and yaw. Straight level flight would be 
recorded as a trail of evenly spaced dots lying 







158 


RESOLUTION IN AERIAL PHOTOGRAPHY 


in a straight line over the length of the film. 
Roll of the camera system would displace the 
dots, normal to the direction of flight (adopted 
as the y axis); pitch of the camera would cause 
irregularities in the spacing of the dots along 
the length of the trail (along the x direction), 
or in other words departures of instantaneous 
Az’s from the average Ax; while yaw would 
cause the line connecting the image of two 
lamps (widely separated on the ground but im¬ 
pressed on the film at the same time) to change 
its slope with respect to the coordinate system. 

It was found that several characteristically 
different frequencies were always present in 
the trails. Arbitrarily, the long-period motions 
were assigned as overall aircraft motion. These 
were taken to include all motions with periods 
greater than 1 sec. Periods less than 1 sec were 
assigned to the camera-mount system. 

Fifteen missions were flown at Wright Field 
in connection with the first vibration program 
and later fifteen other missions were flown out 
of Bedford on the Harvard project. Five of 
these Wright Field tests were over flashing 
lights, as were twelve of the Harvard tests. 
The rest of the missions were devoted to com¬ 
paring mount performance over resolution tar¬ 
gets. 

Sixty-four night films were analyzed for vi¬ 
bration characteristics. The results of the first 
Wright Field test over the flashing lights are 
not representative of comparative mount per¬ 
formance due to the difference of location of 
the equipment in the aircraft, for on this mis¬ 
sion seven different cameras were used in seven 
different mounts in seven different locations in 
a B-17. There is, however, considerable evi¬ 
dence on the basis of this test, in comparison 
with other tests, that the vibrational charac¬ 
teristics of the aircraft vary considerably from 
point to point within the ship. The periods were 
found to be of the same order over that portion 
of the ship tested, but the amplitudes of these 
motions were nearly twice as great in the bomb 
bay as in either the radio room or the nose of 
the aircraft. 

With data from photographs taken with 
cameras located only in the standard position, 
the average median value of aircraft motion for 
the three B-17’s used in the Bedford tests was 


found to be 3.7 mils per sec for the roll velocity 
and 1.9 mils per sec for the pitch velocity. These 
results come from the analysis of twenty-five 
films and are of the same order of magnitude 
from film to film, aircraft to aircraft, and day 
to day. Presumably these values represent mo¬ 
tion of the fuselage itself. 

Tests made at Wright Field with a fighter- 
type aircraft indicate that the average median 
roll velocity of the F-5E may be taken of the 
order of 6 mils per sec, while the corresponding 
pitch velocities are of the order of 3 mils per 
sec. Yaw motions act through a small radius on 
the film as compared to roll and pitch motions 
and hence are less effective in influencing reso¬ 
lution. The aircraft yaw velocities for both B-17 
and F-5E are of the order of 3 mils per sec. 
On the basis of these results, it is concluded that 
aircraft yaw is relatively unimportant in limit¬ 
ing resolution in aerial photography. 

On the other hand the median velocities of 
aircraft in roll and pitch are of such order as to 
impair the level of resolution of present stand¬ 
ard equipment. As might be expected, the evi¬ 
dence points to the fact that lighter aircraft un¬ 
dergo more violent motions than do heavy 
bombers. In addition, the amplitudes of the mo¬ 
tions of both aircraft are great enough to im¬ 
pose restrictions on mapping problems. It 
should be pointed out that in the event that vi¬ 
brationless aircraft are employed in photo¬ 
graphic missions, it will not eliminate this prob¬ 
lem of overall aircraft motion. 

For reference we should note that the angu¬ 
lar motion of about 1 mil per sec will cause a 
10 per cent loss in resolution with the 40-in. and 
24-in. cameras at a % 5 o-sec exposure or that 
angular rates of 2.25 mils per sec with the 24- 
in. and 2.90 mils per sec with the 40-in. will 
produce a 25 per cent loss in resolution at this 
exposure. Bearing in mind that rates of motion 
are greater 50 per cent of the time than the 
average median velocities given above, it then 
appears highly desirable to develop a stabilized 
camera mount. The magnitude of the effect of 
aircraft motions will be even more serious with 
cameras of longer focal length. 

Similar work on vibration carried on by the 
Sperry Gyroscope Company, using a gyro re¬ 
corder, substantiates the above results. Gyro 



FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


159 


records made of the flights of a B-24, B-29, and 
a DeHavilland Mosquito bomber indicate that 
the B-24 behaves similarly to a B-17, a B-29 is 
more stable, and the DeHavilland bomber less 
so. British work shows the Wellington and the 
Sterling to be very stable aircraft, more so than 
our bombers. 

In all cases, rolling motions are found to be 
more serious than the pitching of the aircraft, 
while yaw is negligible in its effect. Limited 
data point conclusively to the fact that manu¬ 
ally controlled aircraft are subjected to lower 
angular velocities than are aircraft set on auto¬ 
pilot. 


Table 3. Average median angular velocities 
(mils per second) of the camera-mount system. 


Mount 

Lens 

(in.) Camera 

Roll 

Pitch 

Approximate 
number of 
seconds of 
performance 
analyzed 

A-ll 

24 

K-17 

2.97 

1.56 

18 

A-11PM* 

24 

K-17 

2.9 

1.97 

6 

A-llBMf 

24 

K-17 

3.4 

2.6 

3 

A-8 

24 

K-17 

1.79 

1.51 

15 

A-27A 

24 

K-17 

4.2 

4.6 

6 

Eastman 

antivibration 

24 

K-17 

4.42 

4.42 

6 

A-ll 

40 

K-22 

1.88 

1.66 

3 

A-11BM 

40 

K-22 

2.6 

1.6 

3 

A-8 

40 

K-22 

2.75 

2.79 

15 

Eastman 

antivibration 

40 

K-22 

4.02 

3.00 

21 

Center of 
gravity 

40 

K-22 

0.99 

0.67 

12 


* A-11PM indicates the standard A-ll mount with plywood 
spacers replacing the Lord pads. 

t A-11BM indicates the standard A-ll mount with balsa-wood 
spacers replacing the Lord pads. 

Table 3 summarizes the analysis of the short- 
period velocities as assigned to the camera- 
mount system. These results were obtained 
from analysis of films taken from a B-17. In 
each case, the camera was located in the same 
position, the outstanding variables from mis¬ 
sion to mission being weather and pilot. Com¬ 
parison of long-period aircraft motions from 
night to night indicate that the behavior of the 
plane was nearly the same on all missions. On 
this basis, it is felt that these results give a good 
indication of the average rate of travel of an 
image on the film when the mount is supporting 


the type of camera indicated. However, further 
studies should be carried on to expand the quan¬ 
tity of the statistical data and to measure the 
consistency of mount performance. 

In determining the velocities assigned to the 
camera-mount system, the motions previously 
assigned to the aircraft have been taken out. 
In plotting the amplitudes of these vibrations, 
it is clearly seen that the high-frequency cam¬ 
era-mount vibrations are superimposed on long- 
period aircraft motions. The determination of 
total velocity at any point on the film, by the 
simple reduction method suggested in the pre¬ 
ceding text, involves the sum of these two 
effects. To arrive at the pure camera-mount per¬ 
formance, the effect of the long-period motions 
must be removed. To do this, smoothed long- 
period curves were fitted to the amplitude plots. 
This smoothed curve was assigned as aircraft 
motion and subtracted from the total velocity. 
The residual was assigned to the camera-mount 
system. 

The evidence indicates that the A-8 mount, 
of the mounts now available, gives the best per¬ 
formance for the support of 24-in. K-17 cam¬ 
eras and probably all 24-in. cameras except the 
K-18. When a heavier camera load (40-in. cam¬ 
era) is imposed on the mount, it appears, based 
on limited data, that the A-ll mount of the 
standard mounts gives best performance. 

There is little difference in the performance 
of the A-ll mount whether Lord pads or solid 
plywood blocks are used as spacers. 

Limited data available from the B-17 test 
made at Wright Field indicates that, in sup¬ 
porting a 24-in. K-18 camera, the A-ll and 
A-8 mounts both allow about twice the velocity 
on the film as when supporting 24-in. K-17 or 
K-22 cameras. 

The performance of all mounts except the 
A-8, A-27A, and the Eastman mount is better 
in the direction of flight than across the direc¬ 
tion of flight. The A-8, A-27A, and Eastman 
mount performed equally well in both direc¬ 
tions. 

With the selection of the proper mount for a 
given camera, it is evident that the long-period 
motions of the aircraft are more effective than 
those of the camera-mount system in impairing 
resolution. 







160 


RESOLUTION IN AERIAL PHOTOGRAPHY 


It is now possible to make a tabulation of the 
expected average resolution if mount vibrations 
were the only factor limiting resolution. (See 
Table 4.) The following tabulations are based 
on the assumption that the 24-in. Aero-Ektar 
lens will resolve 25 lines per mm on Super-XX 
film, and that the 40-in. //5 lens resolves 40 lines 
per mm on that film. The data is arrived at by 
taking the reciprocal of the sum of the recip¬ 
rocal of laboratory resolution and the travel 
in millimeters due to vibration during the ex¬ 
posures. This is, in effect, the addition of 
angular resolutions. 


we should expect mounts, such as the antivibra¬ 
tion mount, so damped as to transmit only low- 
frequency vibrations, to produce some pictures 
without trace of vibration, while nearly all pic¬ 
tures taken with a mount that transmits high- 
frequency vibrations will exhibit evidence of 
motion. This will be so even if both mounts 
have the same mean rotational velocity as de¬ 
termined by the above tests. It should therefore 
be pointed out that median angular velocity on 
the film should not be regarded as the only cri¬ 
terion for judging mount performance. 

As is now planned, a very thorough investi- 


Table 4. Expected average resolutions if mount vibrations were the only limiting factor. 


Focal 

length 

(in.) 

Mount 

Exposure 1/150 sec, 
resolution in lines 
per mm 

In line Across 
of line of 

flight flight 

Exposure 1/350 sec, 
resolution in lines 
per mm 

In line Across 
of line of 

flight flight 

Exposure 1/800 sec, 
resolution in lines 
per mm 

In line Across 
of line of 

flight flight 

24 

A-ll 

19 

22 

22 

24 

24 

24 

24 

A-8 

21 

22 

23 

24 

24 

24 

24 

Anti¬ 

vibration 

17.5 

17.5 

21 

21 

23 

23 

24 

Center of - 
gravity* 

22 

23 

24 

24 

24 

24 

40 

A-ll 

27 

28 

33 

33 

37 

37 

40 

A-8 

23 

23 

30 

30 

35 

35 

40 

Anti¬ 

vibration 

19 

22 

27 

30 

35 

35 

40 

Center of 
gravity 

32 

34 

36 

38 

38 

39 


* Based on vibration data for 40-in. camera. 


On the basis of this tabulation, it is seen that 
mount vibration alone will not explain the ob¬ 
served loss of resolution, and, in fact, will not 
seriously impair performance at the faster ex¬ 
posure times now in general use. 

There has been no adequate comparison of 
mount performance by resolution tests. One 
mission was flown by Eastman representatives 
while at Harvard. The conclusions of the report 
discussing that mission 11 indicate that the East- 
man-NDRC antivibration mount gave better 
performance than the A-ll mount, contrary to 
the results listed in Table 3. Additional confir¬ 
mation of this is found in further analysis of 
Harvard resolution tests which showed that the 
A-ll mount gave more consistent performance 
than the Eastman-NDRC mount but at a 
slightly lower level of resolution. It is true that 


gation of relative mount performance over 
resolution targets will be undertaken at the 
Boston University Optical Research Laboratory 
during the summers of 1946 and 1947. 

As is seen in Table 3, the performance of the 
center-of-gravity mount is far superior to that 
of any other mount tested. In the arrangement 
of this mounting, two 40-in. K-22 cameras were 
mounted symmetrically with respect to the cen¬ 
ter of gravity (see Section 1.6.6). Particular 
attention focused upon having focal plane shut¬ 
ter recoil and film winding opposed to one an¬ 
other in each camera. Such arrangement in¬ 
sured cancellation of shutter recoil effects dur¬ 
ing a photographic exposure and left the center 
of gravity of the system constant regardless of 
the amount of film used. The entire mass of the 
system was supported by a Yg in. diameter 






FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


161 


hardened steel ball resting in a cup of matching widely employed in the problem of mounting 
radius in hardened steel. Figure 4 shows this aerial cameras. There is no reason to suppose 
mount supporting two 40-in. K-22 cameras. The that the quality of performance of this type of 



Figure 4. Center-of-gravity mount supporting two 40-in. //5 K-22 cameras. 


results obtained by the use of this mount in the mounting would change with the type of cam- 
vibration study confirms the thought that the eras employed. It has been stated that the rela- 
center-of-gravity mounting principle should be tive position of the center of gravity and effec- 




162 


RESOLUTION IN AERIAL PHOTOGRAPHY 


tive point of support do not appear to be as im¬ 
portant as the natural frequencies and damping 
of the mounting. It has been shown, however, 
that any mounting will be improved by bring¬ 
ing the camera center of gravity to the effective 
point of support. The results of the NDRC tests 
point out that the best camera mounting for 
minimizing the velocities transmitted to the 




Figure 5. Plot of the first 20 harmonics of F-5E 
standard mount-pitch vibrations. 


film is at the present time a point suspension 
at the center of gravity of the camera system. 

An attempt was made to compare the anti¬ 
vibration characteristics of the two center-of- 
gravity mounted cameras on the same path over 
the night targets. One camera exhibited twice 
as large median angular velocities as the other 
on the same pass. Assuming that the cameras 


were rigidly mounted so that they vibrated as 
a unit, the difference might then be ascribed to 
the vibration of lens elements in their cell or of 
the focusing bellows. There is, however, no 
definite evidence that this is a serious factor, 
but the usual clearance in a standard lens 
mount suggests that this factor may warrant 
further study. The magnitude of the entire 
effect is small and hence is not regarded as a 
limiting factor, being of the order of 0.5 mil 
per sec. 

It was observed that the periods and ampli¬ 
tudes of the F-5E aircraft motions were dis¬ 
tributed over a wide range, implying that the 
behavior of this type of aircraft is highly er¬ 
ratic. On the other hand, the heavier B-17’s, on 
the basis of thirty-six films observed, exhibited 
very consistent characteristics, roll frequencies 
lying between 0.1 and 0.4 c (with one excep¬ 
tion) with half amplitudes less than 6 mils 
(two exceptions) and pitch frequencies of the 
order of 0.3 c and half amplitudes of 1 mil with 
equally small scatter. It would, therefore, seem 
to be more simple to design a stabilized mount 
for use in a heavy bomber than in a fighter-type 
aircraft. 

The characteristics of the high-frequency 
camera-mount vibrations were more difficult to 
determine due to the presence of overtones, but 
analysis clearly showed that the Eastman- 
NDRC antivibration mount transmitted the 
lowest frequency of all the mounts tested. These 
characteristics varied for the different types of 
cameras used in each mount, and in addition 
were further obscured by impressed beat fre¬ 
quencies attributed to the nonperfect synchro¬ 
nization of the engines of the aircraft, and the 
harmonics transmitted by the mount itself. 

Additional work was carried out on large 
scale plots of the amplitudes of the roll, pitch, 
and yaw motions, both by harmonic analyzer 
and visual inspection to determine the ampli¬ 
tude frequency characteristics of these vibra¬ 
tions. As is clearly shown in Figure 5, the 
standard mount in an F-5E aircraft transmits 
to the camera two large amplitude pitch vibra¬ 
tions with periods of 0.13 and 0.06 sec. The 
press of time prevented a complete harmonic 
analysis, and, as it was found that the frequen¬ 
cies of the main harmonics could be determined 









FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


163 


by visual inspection to about as high a degree 
of consistency as that given by the harmonic 
analyzer, influenced as it is by the arbitrary 
choice of a fundamental, the further work was 
done by visual inspection. The quantity of data 
reduced has not been sufficient to yield other 
than the most general conclusions on this 
matter. 

Another form of vibration present is that im¬ 
parted to the camera by the shutter. Vibrations 
imparted by the focal plane shutter have been 
studied at Eastman, and by laboratory tests at 
Mount Wilson and flight tests at Harvard. In 
the Mount Wilson test, 70 cameras were tested by 
taking photographs with the focal plane shut¬ 
ter and comparing these photographs with simi¬ 
lar photographs made with external shutters, 
shutters completely detached from the cameras. 
Exposures with focal plane shutters gave the 
same resolution as exposures with external 
shutters. Exposures with between-the-lens 
shutters gave resolutions consistently less than 
exposures with external shutters, although the 
differences were small. In the Eastman tests, llb 
loss of resolution was observed when either 
type of shutter was used. The Eastman results 
are shown in Table 5. In-the-air investigation 
made by comparing resolution obtained on film 
exposed with the focal plane shutter traveling 
first in the line of flight and then across the 
line of flight give indication of a recoil ve- 
locity. 10b Shutter recoil velocities necessary to 
explain observed losses in resolution in the air 

Table 5. Effect of shutter vibration on resolving 

power. 

Relative resolving power 
Horizontal lines Vertical lines 


Between-the-lens shutter 


1/50 sec 

83 

71 

1/150 sec 

81 

73 

External shutter 

100 

100 

Focal plane shutter 

1/150 sec 

92 

82 

1/350 sec 

95 

85 

External shutter 

100 

100 


are shown in Table 6. The magnitude of these 
results are in good agreement with the results 
of the Eastman tests. 

Further study of this factor and incidently 


the ultrahigh frequency vibrations of the cam¬ 
era system was done by employing a rotor of 
twenty plain mirrors so oriented as to throw 
flashes along a row of fifty convex mirrors. The 
convex mirrors were placed to direct these 
flashes vertically up across the line of flight. 
By this device it was possible to impress dots 
on the film separated in time by %o,ooo sec. 


Table 6. Shutter recoil velocities calculated from 
the observed resolutions. 



Recoil velocity 
in mils per sec 

Camera rotated through 90° 

Resolution across the line of flight 
drops from 24.0 to 20.5 lines per mm 

2.5 

Difference between high and low ten¬ 
sion shutters 

With image movement compensation, 
resolution in the line of flight drops 
from 23.0 to 19.5 lines per mm 

2.8 


These flashes were thrown across the line of 
flight and gave a record of angular excursion 
of the camera during the interval of exposure. 
The mass of data gathered through the use of 
this equipment has not been analyzed but the 
reduction of four photographs indicate a very 
significant amount of vibration due to the shut¬ 
ter and perhaps other factors during the ex¬ 
posure. The results yielded by this method are 
in good agreement with the values shown in 
Table 4. Figure 6 is a plot of image travel in 
microns as a function of time during ordinary 
exposures made using a focal plane shutter. 
These curves were determined from the mirror 
images. 

Additional data were recorded on the night 
films to interpret loss of resolution as a func¬ 
tion of angular velocity on the film. These data 
have not been reduced. A target: used in this 
case consisted of opal-glass slits so oriented as 
to have their lengths correspond to widths of 
the lines of the standard resolution pattern. 

By flying over these with open shutter, these 
slits were trailed along their narrow width 
and the entire length of the film gave in effect 
an infinitely long resolution unit. Excursions 
of the camera across the line of flight would 
tend to mar this resolution. By identifying cor¬ 
responding points on the film, it would then be 











164 


RESOLUTION IN AERIAL PHOTOGRAPHY 


possible to evaluate loss of resolution directly 
in terms of the angular velocity at that point. 

As is indicated by the data on vibration, the 
resolution of cameras in available mounts is 
definitely improved by reducing the exposure 
time as much as is permitted by the density re- 



10 1 i i I i i i i i i l I i I I i I i I l H 



,0 pMIAl I IT I' I I I l M l l I l l 
-io I i i i i i i i i i i mVmj i i 

0 I 2 3 4 S 6 7 8 9 10 tl 12 13 14 15 16 17 18 19 20 21 2 2 
MIRROR NUMBER 
' • AX ROLL CURVE 

X -x Ay PITCH CURVE 

Figure 6. Image travel in microns during an 
exposure. 

quirements of the negatives and the aberrations 
produced by increased apertures. In Table 3 it 
was indicated that the 24-in. Aero-Ektar lens 
gave best performance on Super-XX film at 
f/11 and % 50 -sec exposure. Although a com¬ 
plete study to determine the optimum exposure 
with the Harvard-NDRC 40-in. lens was not 
undertaken, better results were obtained on 


Pan-X film at % 50 -sec exposure at //5 and 
//6.3 than with %oo- se c exposure. 


2,5,4 Focal Settings 

The stabilizing of focus of the 40-in. //5 
camera by use of bellows for automatic focus¬ 
ing for changing object-distance and changing 
index of refraction of air with altitude, and in 
addition a thermostat employed for tempera¬ 
ture control, is now paralleled by the use of 
heated and pressurized compartments in B-29 
aircraft. This use of pressurized and heated 
compartments should be generally adopted for 
aerial photography with careful attention paid 
to the problem of convection currents. It would 
seem that hermetically sealed cameras set at 
laboratory focus should prove an ideal solution 
to this problem. 

Cold-chamber tests at Harvard 2a * 4a have 
shown that thermostatic control is essential if 
lenses with focal length of 40 in. or more are to 
give their full resolution. This is even more im¬ 
portant when fluorite elements are used. With 
the present standard equipment, however, it has 
been found that laboratory setting of focal po¬ 
sition is quite adequate. It was found that the 
use of a specially constructed optically flat 
photographic window stabilizes the position of 
focus and for that reason seems desirable. If 
airplane windows are to be used, they should 
either be made for the purpose according to 
acceptable specifications, or else, if selected 
from plate glass, be graded and marked. It is 
essential that long-focus and large-aperture 
lenses be provided with windows of suitable 
quality. Laboratory tests of selected plate-glass 
windows for use in an F-5E showed that a high 
percentage of the windows selected by visual 
inspection were of such quality as to lower the 
performance of the 40-in. Harvard-NDRC lens 
to the same level as that of the standard 24-in. 
lens. 

A portable collimator with test target has 
been developed at Mount Wilson 7d and a 35-mm 
micrometer film holder has been developed at 
Technicolor . 13 The combination is intended for 
use in checking the focal setting of commercial 
cameras and in determining the mean variation 











FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


165 


of the setting from the optimum value under 
production conditions. This study could not be 
carried out under NDRC but would yield valu¬ 
able information. The equipment is also capable 
of showing how accurately the optical axis is 
set perpendicular to the film. 

There is evidence of local variation in resolu¬ 
tion on many films. Often small areas appear to 
be out of focus, and in viewing successive 
frames it is noticed that these regions shift over 
the focal plane. This suggests that the standard 
vacuum magazine does not hold the film in 
good contact at all points, particularly at high 
altitude. Equipment has been designed to study 
this effect. 14 Time has not permitted making 
tests with this equipment. 

The apparatus consists of sixteen identical 
simple lenses, mounted individually in focusing 
cells, symmetrically distributed over the 9x9 
area for the A-5 magazine. Each lens was 
focused individually for a standard sodium 
source to give a sharp image approximately % 
in. in front of the focal plane. In testing the film 
flatness, the sodium light falling on the film is 
in the form of a pattern of spots arranged 
around a circle of optimum diameter. Bulges in 
the film are manifest by smaller diameter 
circles. This Hartmann test forms a simple de¬ 
vice for determination of film flatness. 


2 ' 5,5 Air Turbulence 

Studies in 1941 at Mount Wilson Observatory 
were directed toward evaluating the effect of 
air turbulence on optical definition in air pho¬ 
tography. A high-intensity flickering light 
source on the ground was photographed from 
an aircraft. Comparison of the definition of 
images on the film made from an aircraft with 
those made with the same camera at rest was 
used to detect the effect of air turbulence. On the 
basis of this study, there was no indication that 
air turbulence was a serious factor. Additional 
evidence, such as visual tests made in a low 
velocity wind tunnel and the occasional report¬ 
ing of laboratory resolution on aerial photo¬ 
graphs, gives clear indication that air turbu¬ 
lence with present equipment is a second-order 
effect. 


With very long focus lenses, this may prove 
to be the limiting factor and hence warrants 
further investigation. Harvard tests of in-the- 
air performance of the 40-in. Harvard-NDRC 
lens, with and without an optically flat photo¬ 
graphic window, showed little change in reso¬ 
lution, yet turbulence was considered as being 
diminished when the window was in place. 

This test should be repeated using the 100-in. 
camera now being completed at Mount Wilson, 
for this camera should be more sensitive to air 
turbulence than any camera now in use. 

Additional tests of air turbulence have been 
proposed. These include a periscope device for 
conveying light into the side of a camera body 
from a condenser system or a Kodatron unit to 
a resolution target in the periscope tube. At 
the end of the tube in the camera was to be a 
right-angle prism which directed the light 
toward the lens, so oriented that the light from 
the resolution target completely filled the lens 
aperture. A quartz mirror was to reflect the 
light back to the lens, the lens serving as an 
autocollimator. The resolution target image was 
then to be recorded on the film. The periscope 
tube was to be on a focusing mount for labora¬ 
tory setting. The resolution target was to be 
stair-stepped in such a way that some one of 
the patterns would be in focus on the aerial 
film. The mirror to be used on this test was to 
be mounted in a streamline housing that could 
be lowered into the slip stream under the pho¬ 
tographic window opening during flight. The 
mounting to minimize mirror vibration was 
to consist of hollow tubular braces supporting 
the streamline housing with solid rods inside 
these tubes, yet free of them, supporting the 
mirror itself. The loss in peak resolution when 
the mirror was lowered into the slip stream 
to allow 2 or 3 in. of air to stream over the 
mirror, compared to that when the mirror was 
inside the aircraft, was to be attributed to air 
turbulence. 


2 5,6 Aerial Haze 

The effect of aerial haze was investigated at 
the Mount Wilson Observatory. 9a Photographs 
of distant ground targets were studied, and re- 



166 


RESOLUTION IN AERIAL PHOTOGRAPHY 


duction of photographic contrast of these tar¬ 
gets due to aerial haze was found to correspond 
closely to the reduction of visibility. The con¬ 
clusions of these reports were that, when haze 
is noticeable, the resolution of a standard 24-in. 
camera is seriously limited. 

Flight tests at Harvard, where loss of con¬ 
trast of ground objects was measured, show 
that aerial haze is most effective in lowering 
contrast in the first few thousand feet of air 
mass. This is in good agreement with researches 
carried on by the Army Air Forces Photo¬ 
graphic Laboratory at Wright Field. From 
the Harvard results, it was found that the mag¬ 
nitude of this effect is such that apparent target 
contrast may be lowered as much as 50 per 
cent at 10,000 ft. Above 5,000 ft, on the two 
missions undertaken, the loss of contrast drops 
off appreciably indicating that a layer of haze 
near the ground is most effective in lowering 
contrast and hence lowering resolution. Fig¬ 
ure 7 shows target contrast as a function 
of altitude as determined on these two mis¬ 
sions. 

The method employed at Mount Wilson to 
evaluate the haze factor was to photograph 
two distant targets over a period of time which 
included widely different haze conditions. One 
target was light gray, the other dark gray. 
Each 5-ft square was placed about 6,000 ft 
away from the camera. The evaluation of haze 
was done by setting up a scale of 10, in which 
condition (10) indicated extremely clear, (9) 
very slight haze, (8) slight haze, (7) percepti¬ 
ble haze, etc. The resulting loss of contrast 
with haze was plotted. It is evident, based on 
their conclusions, that haze is a far more im¬ 
portant factor affecting resolving power in 
aerial photography than is scattered light. If 
haze is sufficiently heavy, it becomes the domi¬ 
nant factor in limiting resolution in aerial pho¬ 
tography. The reduction of resolving power 
due to aerial haze corresponds closely to the 
reduction in visibility. 

Two other methods for the study of the haze 
factor have been suggested. Equipment was set 
up under the Harvard contract for these stud¬ 
ies but time did not permit carrying out the 
investigation as planned. The first proposed 
test was to make use of a masonite resolution 


pattern illuminated by an Edgerton flash lamp 
for night resolution studies. This same target, 
housed in a roofless building to afford deep 
shadow, was to be photographed similarly in 
the daytime. The comparison of day and night 
resolution was to be regarded as a measure of 
the haze factor. The duration of the Edgerton 
flash would render the effects of vibration and 



ALTITUDE IN FEET 

Figure 7. Target contrast as a function of al¬ 
titude. 

translation negligible. By this same method, a 
comparison of day and night resolution was 
proposed for shutter exposures of an opal-glass 
resolution target illuminated by transmission 








FACTORS LIMITING RESOLUTION OF AERIAL PHOTOGRAPHS 


167 


of the light from eighteen photo flood lamps 
set up for test. One night mission was flown 
over this target but target images were heavily 
overexposed and it was impossible to assess the 
film. It is now planned to continue these studies. 

In daytime photography, one finds not only 
the light reflected from the target reaching the 
photographic plate, but also light scattered by 
particles in the atmosphere lying between the 
ground and the aircraft. This latter source of 
actinic energy is called “aerial haze.” The night 
photographic methods proposed in the preced¬ 
ing paragraph are therefore free of this type 
of haze. As this light is scattered in accordance 
with Rayleigh’s criterion, we find most of its 
energy concentrated in the blue region of the 
spectrum. Consequently, a considerable portion 
of the haze effects can be eliminated by use of 
yellow or red filters. 

The effect of aerial haze is to lower the con¬ 
trast of ground objects. This loss of contrast, 
in turn, results in a loss of resolution. As haze 
is a function of altitude, requirements are for 
higher contrast emulsions and development con¬ 
ditions the higher the altitude of the photo¬ 
graph. 

At low levels of resolution in the air or in 
the laboratory, haze does not affect resolution 
as markedly as at high levels of resolution. 915 
Thus, the 24-in. Aero-Ektar lens which gives 
less contrasty pictures than does the 40-in. 
//5 due to its large amount of flare has its 
level of resolution less affected by the presence 
of haze or low-target contrast than does the 
40-in. lens. This was shown on the Harvard 
tests by comparing resolution of the lenses as 
obtained on the high-contrast and low-contrast 
patterns. The 24-in. lens gave nearly the same 
resolution for all contrast targets while the 
40-in. lens, particularly on high-resolution pho¬ 
tographs, gave considerably lower resolution 
values on the low-contrast target than on the 
standard pattern. Use of a black line target 
with white background found the performance 
of the 24-in. lens little affected by this reversal 
of line background reflectivities. The 40-in. lens 
gives lower resolution values on this pattern 
than on the standard pattern due to the scat¬ 
tering of background light into the line image, 
again confirming that the lens responds more 


sensitively to atmospheric haze than does the 
24-in. lens. 

The British have pointed out 15 that graini¬ 
ness is the most important factor limiting res¬ 
olution of detail at low contrast in that resolu¬ 
tion tends to the same value for a given film 
whatever the lens. As haze manifests itself by 
loss of contrast on the emulsion, it was con¬ 
sidered important that all other factors pro¬ 
ducing this same or similar effects should be 
investigated. This was proposed, but time did 
not permit the carrying out of laboratory tests 
to evaluate these factors contributing to loss 
of contrast. The imperfections of the lens, lens 
flare, color aberrations, and poor focus, in addi¬ 
tion to irradiation losses in the emulsion, con¬ 
tribute to this. Microdensitometer tracings, 
made of the photographic image of a ground 
target of eight equally spaced lines of decreas¬ 
ing line widths, showed an almost linear de¬ 
crease of contrast (line to background) with 
line width. As this occurs quite markedly even 
for the high-quality images of the 40-in. lens, 
one may infer that resolution of fine detail not 
only demands high angular resolution but also 
must overcome the additional effect of loss of 
resolution due to the lowering of contrast of 
this fine detail on the emulsion. 

This loss of contrast introduced by the film 
is a minimum for low exposure levels and for 
fine-grain emulsions. Employment of both these 
conditions markedly improves the level of reso¬ 
lution at the sacrifice of detail in the shadows. 

The sharp qualities of image recorded by the 
40-in. lens is a feature that does not manifest 
itself by comparative resolution figures. The use 
of the Harvard-NDRC 40-in. lens tends to de¬ 
crease the magnitude of the scattered light 
thrown into the image compared to the per¬ 
formance of standard equipment. Thus the 
40-in. lens shows greater contrast at all times, 
yet is subjected to greater relative losses of 
contrast by the presence of aerial haze than is 
standard equipment. 


2 5 7 Scattered Light in Lens and Camera 

Measures made at Mount Wilson 9c indicate 
that the effect upon target contrast of light 



168 


RESOLUTION IN AERIAL PHOTOGRAPHY 


scattered from the,camera lens was of minor 
importance under normal conditions. Light 
scattered from the interior of the camera was 
found to be of no importance in the reduction 
of target contrast. In conjunction with these 
studies, the Mount Wilson Observatory made 
tabulations of resolving power as a function 
of target contrast. The conclusions from their 
work with the 24-in. Aero-Ektar lens are shown 



Figure 8. Resolving power as a function of 
target contrast. 


in Figure 8. This experiment was made by 
using targets of different contrast. The con¬ 
clusion from their work was that the amount 
of light scattered into the central part of the 
field by the 24-in. Aero-Ektar lens and the 
resulting reduction in resolving power is of 
minor importance under normal conditions. 


25 8 Photographic Emulsions and Photo¬ 
graphic Processing Techniques 

For a given lens, the quality of a photograph 
is limited by the properties of the photographic 
emulsion. The standard practice of employing 
high-speed emulsions is the result of limitations 
of the light-gathering power of standard lenses 
and results in a relatively low level of perform¬ 
ance. The practice of employing Pan-X, with 
fine-grain development process, in combination 
with the 40-in. //5 lens has yielded a consid¬ 
erably higher level of resolution than has been 
heretofore obtained. As a result, it implies that 
fine-grain emulsion should be adopted for gen¬ 
eral use but the exposure requirements of such 
films require improved mountings and image 
movement compensations. Resolutions greater 
than 40 lines per mm have been obtained with 


the 40-in. with exposures of 345 0 sec at //5 
and //6.3. 

The density of the negative affects resolution 
to a significant degree. Studies should be made 
to determine whether an exposure meter could 
be used to control exposure not only to improve 
resolution but also to include high lights and 
shadows within the range of exposure. Depar¬ 
tures from standard processing techniques, par¬ 
ticularly rapid development, will cause serious 
loss in resolution. On the basis of Harvard 
studies, it is recommended that a continuous 
processing development method be adopted. The 
uniformity of development by this method is 
highly desirable and will become more neces¬ 
sary when finer grain films come into general 
use. Although Smith tank processing gives no 
evidence of serious Eberhard effects, contrast 
is a function of distance from the end of the 
film in this method of processing. 

With present methods of reconnaissance, it 
becomes necessary to make prints from the 
photo negatives. Investigations of the printing 
method 100 ’ 16 are in good general agreement. It 
was found that there is considerable loss of 
resolution on prints made with the standard 
diffuse printing box. Point-source printers em¬ 
ployed at Harvard showed marked improve¬ 
ment in the level of print resolution, as did a 
parallel-light printer employed by the British. 
The diffuse printers were found to eliminate 
from the negative almost all the gain obtained 
by using better lenses and better mounts. 

Loss of resolution by printing is probably 
the most serious obstacle today to the marked 
improvement of the level of aerial photographic 
reconnaissance. Limit of overall print resolu¬ 
tion obtained by diffuse printing methods of 
aerial negatives seems to be of the order of 20 
lines per mm. This would imply that the quality 
of prints from negatives resolving 22 and 45 
lines per mm would be about the same. The 
maximum resolution obtained from a perfect 
negative by a diffuse printing process is about 
30 lines per mm. b 

An immediate study of this problem is in- 


b More recent studies at Boston University indicate 
that resolution up to 60 lines per mm can be achieved 
under the most favorable conditions with the diffuse 
printing process. 






















EVALUATION OF FACTORS 


169 


dicated since improvements may offer an easy 
way to raise the level of resolution of present 
aerial photographic positives. Consideration 
should be given to the use of positive films to 
be viewed as transparencies, which would re¬ 
cord a wider range of densities than is possible 
with paper prints. The method of precision 
enlargements of 1.5 to 2 diameters rather than 
direct contact prints from high-quality nega¬ 
tives would also seem to be capable of improv¬ 
ing the level of performance. In this connection, 
it would seem that a great deal is lost if no 
assessments are made from the original nega¬ 
tive of present high-quality photographs. 


2 6 EVALUATION OF FACTORS 

Approximate numerical determinations of 
the final resolution of a system are made by 
adding the reciprocals of the resolutions of each 
component of the system and taking the re¬ 
ciprocal of the sum. This method is used to 
weigh the factors that limit resolution in aerial 
photography. In addition to the limitation of 
lens-plus-film laboratory resolution, in-the-air 
resolution is affected by: angular motion effects 
that increase linearly with altitude; haze which 
introduces effects that increase with altitude 
but nonlinearly; translation which introduces 
an effect that varies inversely with the altitude, 
air turbulence, shutter inefficiency, effect of the 
photographic window, poor focus, and film 
processing which are considered as independent 
of altitude. Taking the average values of fac¬ 
tors as given in Section 2.5 we calculate the 
resolution in the air. The adopted values are: 
(1) for the 10-in. lens, 45 lines per mm labora¬ 
tory lens-film resolution on Pan-X film, an ex¬ 
posure of %oo sec, altitude 10,000 ft, translation 
of 200 mph, pitch of aircraft of 1.9 mils per sec, 
pitch of camera-mount 3 mils per sec, shutter 
recoil 3 mils per sec in flight line, shutter vibra¬ 
tion across the flight line of 1 mil per sec, roll 
of aircraft 3.7 mils per sec, roll of camera- 
mount 4 mils per sec, lens vibration 0.5 mil per 
sec, yaw 3 mils per sec, haze unknown, and 
other factors negligible; (2) for the 24-in. lens, 
25 lines per mm laboratory lens-film resolution 


on Super-XX film, an exposure of y ± 50 sec, other 
conditions as above. We then tabulate the travel 
on the film during the exposure, add the rms of 
those terms that are regarded as introducing 
random motions, and add in the reciprocal of 
lens-film resolution. 

From these data we can weigh the limiting 
factors. The good agreement of observed and 
calculated results should not be construed to 
imply that other factors, haze primarily, are 
negligible. Haze, when present, is a fir*t-order 
limiting factor but its effects are marred by 
the presence of the other factors. Eventually, 
methods must be developed for minimizing haze 
effects. An evaluation of the average haze fac¬ 
tor cannot be made on the basis of our present 
data, but an estimate of the weight of its effect 
is included by assuming that the difference be¬ 
tween observed and calculated resolution across 
the line of flight is due to haze. The average 
photographic window, not used on these in-the- 
air tests, was taken to be as effective as haze in 
limiting resolution, probably a conservative 
estimate. 

The data of Table 9 are to be regarded as giv¬ 
ing only an indication of the order of magni¬ 
tude of these factors that limit resolution. It 
should be viewed with consideration of the 
stated conditions of lens, aircraft exposure 


Table 7. Prediction of effect of factors in limiting 
resolution of the 40-in. lens at 1/800 sec exposure. 



In flight line 
(mm) 

Across flight 
line (mm) 

Lens plus film 

0.0222 

0.0222 

Translation 

0.0375 

0.0000 

Aircraft motion 

0.0024 

0.0046 

Camera-mount motion 

0.0038 

0.0050 

Yaw (mount plus aircraft) 0.0004 

0.0004 

Lens vibration 

0.0006 

0.0006 

Shutter vibration 

—0.0038 

0.0012 

Total effect 

0.0604 

0.2910 

Predicted resolution 

16.6 lines/mm 

34.5 lines/mm 

Observed average 
resolution 

16 lines/mm 

27 lines/mm 


Taking the same value for the factors, predicted resolution for 
the 24-in. lens at 1/150 sec exposure is tabulated. (See Table 8.) 


time, and altitude before being employed to 
consider the relative magnitude of factors 
tinder other conditions. 







170 


RESOLUTION IN AERIAL PHOTOGRAPHY 


2 7 RECOMMENDATIONS BY NDRC 

Studies made during World War II on reso¬ 
lution of aerial photographs resulted in general 
improvement in equipment and in the photo¬ 
graphs which were obtained. The entire subject 
can be placed on a sound quantitative basis as 
the result of complete studies of data accumu¬ 
lated under NDRC and as the result of further 
carefully planned studies. 


Table 8. Prediction of effect of factors in limit¬ 
ing resolution of the 24-in. lens at 1/150 sec ex¬ 
posure on Super-XX film. 



In flight line 
(mm) 

Across flight 
line (mm) 

Lens plus film 

0.0400 

0.0400 

Translation 

0.1190 

0.0000 

Aircraft motion 

0.0077 

0.0147 

Camera-mount motion 

0.0112 

0.0160 

Yaw (mount plus aircraft) 

0.0013 

0.0013 

Lens vibration 

0.0019 

0.0019 

Shutter vibration 

—0.0122 

0.0037 

Total effect 

0.2089 

0.0621 

Predicted resolution 

4.8 lines/mm 

16.1 lines/mm 

Observed average 
resolution 

7.5 lines/mm 

12.0 lines/mm 


Aerial tests at the end of World War II 
showed that the best lenses then available, when 
used in the best available mounts, attain a 
resolution which is only about 50 per cent of 
that attained in the laboratory. This striking 
fact presents a challenge of great significance 
to all users of aerial photographs, and partic¬ 
ularly to the Army and the Navy. There can be 
no doubt whatever but that a carefully planned 
program for improving resolution in aerial pho¬ 
tographs will more than pay for itself in re¬ 
sults, since at present it is necessary to use 
cameras with twice the focal length of those 
that could be used to give the same results if 
lenses, mounts, windows, and other accessories 
were brought up to adequate specifications. 
When each of the many factors which are now 
responsible for reducing resolution has been 
quantitatively evaluated, it will be possible to 
take stock of the entire situation, to state just 
what is required to reduce each factor to a 
specified extent so that overall performance 
may reach a certain level, expressed in terms 
of the resolving power, and finally to decide 


whether this result is worth the effort and ex¬ 
pense involved. A compromise may well be in¬ 
dicated at a level somewhat below what is 
actually attainable, in view of the fact that the 
perfection of equipment and the cost involved 
rise rapidly as performance is improved. 

Laboratory tests should be used to provide 
as large a part of the basic information needed 
as possible, so as to reduce to a minimum the 
need for flight tests. Flight tests are not well 
suited for isolating variables in a complex sit¬ 
uation owing to the great variety of conditions 
which are inevitably encountered within any 
series of flights. Only in the laboratory can 
other conditions be held constant while one 
factor is studied. On the other hand, certain 
factors can be studied only in the air, and for 
this reason it is essential that facilities be avail¬ 
able for extensive flight testing as part of the 
program. 


Table 9. Relative weight of factors taken indi¬ 
vidually. 



40-in. lens 
1/800 sec 
exposure 
in B-17 at 
10,000 ft 

40-in. lens 
1/800 sec 
exposure 
in F-5E at 
25,000 ft 

Translation 

10.0 

10.0 

Lens plus film 

5.6 

9.3 

Aircraft roll 

1.2 

3.2 

Aircraft pitch 

0.6 

1.6 

Aircraft yaw 

0.1 

0.2 

Center-of-gravity mount roll 

0.3 

0.5 

Center-of-gravity mount pitch 

0.2 

0.3 

Center-of-gravity mount yaw 

0.1 

0.1 

Standard mount roll 

1.0 

1.6 

Standard mount pitch 

0.6 

1.0 

Standard mount yaw 

0.1 

0.2 

Shutter recoil 

1.0 

1.6 

Vibration introduced by shutter 0.3 

0.5 

Air turbulence (est.) 

0.0 

0.0 

Average haze (calc.) 

2.0 

4.2 

Average selected photographic 



window (est.) 

2.0 

3.3 

Zero-power window 

0.0 

0.0 

Lens vibration 

0.2 

0.2 


Every effort should be made to attain accu¬ 
racies in laboratory tests which are one whole 
order of magnitude beyond the probable effects 
of the factors concerned in the air. In the case 
of air turbulence, for example, it would be 
much more useful to know that at 300 mph, 
when photographs are being taken through a 












RECOMMENDATIONS BY NDRC 


171 


certain window in the fuselage, resolution defi¬ 
nitely cannot be expected to exceed 1 sec of arc 
because of turbulence, than merely to know 
that no effect as large as 5 sec of arc can be 
observed. Again, it would be much better to 
know that shutter vibration amounts to an 
amplitude of 0.002 mm during the exposure 
than to find merely that no effect is observed 
as great as 0.010 mm. Quantitative measure¬ 
ments of small effects are important because 
there are many such effects which are simul¬ 
taneously at work to reduce resolution and 
these effects are additive. 

A program of research in this field should 
include the points discussed in the following 
sections. 


2 71 Laboratory Tests 

1. Lens tests should be continued and ex¬ 
tended in scope. They should be put on a strictly 
standardized basis after careful study of pres¬ 
ent results and after full discussion with all 
who are likely to contribute useful ideas. Paral¬ 
lel-line resolution targets should be used, and 
also a method which employs a neutral wedge 
to give a photographic record of the energy 
distribution in the image of a line source. Ex¬ 
periments should be undertaken to find out 
whether a useful test of resolution can be based 
on photographs of parallel-line patterns with 
a wedge placed in front of an image enlarged 
with a microscope objective. The lines would 
need to be longer in proportion to their width 
than those ordinarily employed. This method 
would yield energy distribution curves for the 
images of parallel-line patterns, from which 
it should be easy to determine, quite imperson¬ 
ally, the resolving power on the basis of a 
definition based on the contrast in the image 
of high-contrast parallel-line patterns of vari¬ 
ous spacings. The contrast used for defining 
resolution might be 10 or 20 per cent, for ex¬ 
ample. This method would eliminate the present 
uncertainties involved in reading resolution 
patterns, without adding any complex equip¬ 
ment for testing. 

2. Resolution studies should be continued, 
based on accurate standardization of photo¬ 


graphic procedures, absolute exposure times, 
a range of contrasts, and evaluation of lens 
performance by area. 

3. Accurate studies of microscopic contrast 
as a function of line spacing should be initiated, 
making use of microphotometer methods, stand¬ 
ardization of exposures, and intensity curves 
replotted on an enlarged scale. 

4. Photographic emulsions should be studied 
extensively under the above conditions, to deter¬ 
mine resolution with a perfect lens, with tar¬ 
gets of various contrasts, and at various densi¬ 
ties. These measurements should be made with 
various relative apertures. The results will 
make it possible to discuss on a quantitative 
basis the performance of lenses in the labora¬ 
tory, and to infer the resolution that should be 
attained in the air in the presence of haze, if 
other factors did not tend to reduce resolution 
further. 

5. Studies should be made to determine the 
effect on resolution of image movements of dif¬ 
ferent rates. This should be done separately 
for each lens. The resulting data will show just 
how much angular motion can be tolerated in 
the air without reducing performance to a sig¬ 
nificant extent. This data is fundamental to 
the entire program, and present information is 
very limited. 

6. Printing methods should be thoroughly 
investigated. Improved methods should be de¬ 
veloped and introduced into general use. No 
method should be regarded as acceptable unless 
it retains in the print more than 90 per cent 
of the resolving power in the negative. An im¬ 
provement at this part of the process is one 
which is least expensive to make, and can yield 
large returns. The effectiveness of a IV 2 /I pre¬ 
cision enlarger for making paper prints with 
minimum loss of resolution should be investi¬ 
gated. Tests should be made of the performance 
of transparent positive film which may be 
markedly superior to paper for retaining reso¬ 
lution. 

7. All promising shutters should be ade¬ 
quately tested for speed, efficiency, acceleration, 
recoil, and vibration. 

8. Vibration studies should be made on all 
equipment that is normally used in close prox¬ 
imity to cameras to determine possible effects 



172 


RESOLUTION IN AERIAL PHOTOGRAPHY 


on resolution. This should include magazines, 
sweep mechanisms, and servo equipment. 

9. Mounts should be tested for transmissivity 
at all frequencies in the range that is encoun¬ 
tered in aircraft for rotational and linear vi¬ 
brations in each of three coordinates both sep¬ 
arately and simultaneously. 

10. Stabilized mounts should be tested for 
performance in restoring the camera axis after 
displacements of various degrees. Measure¬ 
ments should be made to show, after a dis¬ 
turbance, the angular rate of restoration rela¬ 
tive to the true vertical as a function of time. 
Such tests should be made for disturbances of 
various angular rates. The accuracy with which 
the vertical is maintained should also be meas¬ 
ured when the servo mechanism is attached to 
a vertical-seeking element. Although the main¬ 
tenance of a true vertical is not necessary for 
improving resolution, it is very much to be 
desired for mapping purposes. 

11. The focal settings of samples of commer¬ 
cial cameras should be measured when they are 
first delivered and again after various intervals 
of time in service to determine how accurately 
the settings are made at the factories and also 
the extent of changes that take place later. 

12. Vacuum magazines should be tested under 
conditions simulating those encountered in 
service by using the device described in an 
OSRD report 14 at low pressure and at low tem¬ 
perature. If the film does not lie accurately in 
contact with the platen over its entire area at 
high altitudes, glass defining plates should prob¬ 
ably be substituted. 

13. Tests should be made of the effect of in¬ 
troducing glass windows of various optical 
qualities into the beam in front of lenses of 
various focal lengths as the basis for establish¬ 
ing a specification for quality. 

14. The extent to which scattered light in 
various cameras reduces contrast, and thereby 
reduces resolving power, should be accurately 
measured. The effect of a lens cone and of coat¬ 
ing the elements should be investigated. The 
effect on resolution of considerable dirt on the 
front surface should also be investigated to 
guide maintenance regulations. 

15. The effect of air turbulence on resolution 
should be investigated by making experiments 


with optical glass windows in a wind tunnel. 

16. The most favorable exposure level for 
maximum resolution over the entire negative 
should be determined, taking into account vig¬ 
netting and the range of intensity within the 
image. 

17. Cold- and pressure-chamber tests should 
be carried out to determine effects on focus, 
quality of image, and operation of equipment. 

18. A device should be developed, for use 
with standard camera bodies, to permit preci¬ 
sion focusing in the air. 

2 7 2 Flight Tests 

1. The frequency spectrum of vibration of 
the fuselage at the camera station should be 
determined by using a rigid mount (a) carrying 
a camera which photographs strobolux lights, 
and (b) carrying an optically recording free 
gyro. This is one of the most important parts 
of the program, since it provides data for lab¬ 
oratory tests of mounts. 

2. From records of the angular motion of the 
fuselage, which is necessarily impressed on the 
camera mount, in each coordinate (determined 
as suggested under item 1), calculations should 
be made to predict the distribution of angular 
rates which would result if the mount were to 
have various periods and various amounts of 
restoring force and damping. The result of this 
investigation will show whether it is possible 
for a simple gimbal or spring mount with the 
most suitable values of period, restoring force, 
and damping, to reduce the mean angular rate 
of the camera, when exposed to the existing 
angular vibration of the fuselage, to a level 
which will not impair the resolving power of 
photographs with a lens of predetermined focal 
length. Unless the angular rate can be reduced 
by a simple mount to an innocuous level, there 
is need for developing a gyrostabilized mount. 
The mathematical study suggested is of funda¬ 
mental importance in planning the entire pro¬ 
gram on camera mounts, and should be pushed 
through as fast as possible. 

3. The angular vibration performance of 
various camera-mount combinations should be 
determined, using strobolux lights and also 




RECOMMENDATIONS BY NDRC 


173 


resolution targets. The targets should include 
not only the usual painted parallel-line targets, 
varying in dimension by steps of >g/2, but also 
trailed resolution targets at night having the 
same steps in size. Provision should be made for 
varying the intensity of the trailed targets to 
give optimum density. 

4. The ultrahigh frequency spectrum of 
angular vibration should be determined by 
means of the rotor and row of convex mirrors, 
described in an OSRD report, 8 to evaluate the 
effect of exposure time on resolution and also 
the effect of shutter and other vibrations dur¬ 
ing the time of exposure. 

5. The resolution of the standard 24-in. cam¬ 
era and of the Harvard-NDRC 40-in. camera, 
both in the standard A-ll mount and in the 
Eastman-NDRC mount, should be fully es¬ 
tablished by averaging the results of numerous 
flights. The results are of fundamental impor¬ 
tance, since they will show how much gain in 
angular resolving power can be achieved by 
improving the camera and the mount very con¬ 
siderably, both separately and together. This 
result will do much to guide further efforts. 

6. The extent to which resolution is im¬ 
proved with the 24-in. and with the 40-in. cam¬ 
era as a result of using the Eastman-NDRC 
mount without sweep, the same mount with 
sweep, and as a result of using the best stabil¬ 
ized mount should be determined. 

7. Test patterns of various types should be 
compared from the point of view of significance 
and reproducibility of reading, as well as from 
the point of view of determining microscopic 
contrast in the image by using a micropho¬ 
tometer on the negatives. 

8. The resolution given by strip cameras 
should be determined, using automatic rate 
control. 

9. The extent to which haze reduces contrast, 
and thereby reduces resolving power, should be 
determined as a function of wave length and 
altitude on some of the best and some of the 
worst days for photography. 

10. A study should be made of the correla¬ 
tion of resolution and of measured microscopic 
contrast as a function of line spacing with the 
ability of photointerpreters to identify objects 
on the ground placed close to the test patterns. 


Ordinary objects of various sizes should be 
used on normal backgrounds. Inexperienced ob¬ 
servers should be compared with trained inter¬ 
preters in these tests. 


2 7 3 Specifications 

1. Specifications for lens performance should 
be based on the results of further tests of reso¬ 
lution patterns. 

2. Specifications for photographic windows 
should be based on tests made to show what 
quality is necessary for lenses of various aper¬ 
tures and focal lengths to prevent any detect¬ 
able loss of resolution. 

3. Specifications for camera mounts should 
be based on standardized tests on specified 
shake tables. The maximum transmissivity for 
vibrations of various frequencies, in each co¬ 
ordinate, should be specified. 


274 General Recommendations 

1. Studies should be made to establish the 
most effective combination of lens, aperture, 
shutter, exposure time, filter, emulsion, and 
mount for each focal length, using equipment 
now available. 

2. On the basis of tests of the 100-in. lens, 
careful consideration should be given to the de¬ 
sirability of developing lenses with still greater 
focal length, employing folded optical paths. 

3. A full study should be made of all Service 
problems, and equipment should be developed 
specifically to meet these needs. 

4. Test targets should be installed at training 
centers to show what resolution is ordinarily 
attained with present equipment, procedures, 
and training. Examination of films showing 
these targets would serve as a stimulus to bet¬ 
ter work on the part of both staff and students, 
and would show the extent to which further de¬ 
velopment of equipment is indicated. 

5. A special group should be trained in the 
technical details of operating and servicing 
precision photographic equipment. This group 
should have special squadrons of aircraft, with 



174 


RESOLUTION IN AERIAL PHOTOGRAPHY 


precision equipment of the latest type, for its with laboratories where development work is in 
own exclusive use over special targets. They progress. In response to special requests, such a 
should report directly to a high level in the group could undoubtedly provide military in- 
Army Air Forces, and should be in direct touch formation of great importance. 



Chapter 3 

MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 

By Robert Singleton a 


U NDER PROJECT NA-124 investigations of 
equipment and processes for mapping 
from oblique aerial photographs were carried 
on. Two levels of accuracy were considered: 
reconnaissance sketching and large-scale topo¬ 
graphic mapping. For various reasons the use 
of oblique photographs became necessary in 
World War II, but existing oblique reconnais¬ 
sance methods gave insufficient accuracy, and 
no methods existed for large-scale topographic 
mapping from oblique photographs. 

Instruments and methods were devised for 
using oblique photographs as desired, and were 
proved experimentally, but they had not been 
developed to the point of practical use in pro¬ 
duction when the war ended. 

31 ORIGIN OF THE PROJECT 

On December 11, 1942, the Hydrographer 
recommended to the Coordinator of Research 
and Development, U. S. Navy, that the National 
Defense Research Committee [NDRC] be re¬ 
quested to proceed with the development of the 
Miller stereoscopic plotting instrument and the 
Miller single eyepiece plotter. These were in¬ 
struments for mapping from aerial photo¬ 
graphs, which had been planned and partially 
developed several years earlier by the Amer¬ 
ican Geographical Society. 

Endorsed by the Bureau of Aeronautics, a 
request for Project NA-124 was forwarded to 
NDRC, which accepted it and assigned it to 
Division 16. Direction of the work was given to 
Section 16.1. In initial investigations by mem¬ 
bers of Section 16.1 it became apparent that 
broader views should be taken of the whole 
problem of mapping from aerial photographs, 
both near-vertical and high oblique. 

On May 15, 1943, Contract OEMsr-1087 was 
concluded with Merrill Flood and Associates 
[MFA] of Princeton, New Jersey, to make a 
fundamental study of the problem. On June 1, 
a Aero Service Corporation. 


1943, Contract OEMsr-1039 was concluded 
with Aero Service Corporation, Philadelphia, 
Pa., as a companion contractor to provide lab¬ 
oratory and other facilities and to perform ex¬ 
perimental work. 

At the time of the Hydrographer’s request, 
the Hydrographic Office faced two problems in 
the use of oblique photographs auxiliary to 
their regular mapping from vertical photo¬ 
graphs. These were the extension of control 
through island groups and occasional plotting 
of shoreline information. See Sections 3.3.4 and 
3.3.5. Although instruments and methods 
existed for handling both these problems, none 
was satisfactory for both precision and opera¬ 
tional facility. 

The best existing instrument for control ex¬ 
tension was the photo-alidade, but its operation 
was cumbersome and required extensive hori¬ 
zontal ground control, which was a decided dis¬ 
advantage. For shoreline delineation either the 
oblique sketchmaster or the Canadian grid 
method could be used, and had been used, but 
neither possessed the required precision and 
facility. 

The stereoplanigraph, a German instrument, 
could handle both problems satisfactorily, but 
none was available. The only stereoplanigraph 
in the United States was owned by Fairchild 
Aerial Surveys in California, and it was in use 
continuously. From its plans the Miller stereo¬ 
scopic plotter appeared to be satisfactory for 
both problems and to possess the precision and 
operational qualities desired. To a lesser extent 
the single eyepiece plotter also appeared to be 
satisfactory. No other equally useful instru¬ 
ments had been suggested, and hence the Hy¬ 
drographic Office requested the construction of 
these two. 

32 NAVAL MAPPING REQUIREMENTS 

From the conferences and preliminary 
studies undertaken in getting the project under 


175 



176 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


way it became evident that the work could not 
be confined simply to constructing the instru¬ 
ments requested by the Hydrographer. Two 
facts were involved in this decision. 

1. The development of the Miller stereoscopic 
plotter would be difficult because of the compli¬ 
cated mechanism involved and the precision re¬ 
quired. Both it and the single eyepiece plotter 
appeared less suitable for the problems occa¬ 
sioning the request for reasons discussed in 
Section 3.3.2. Other approaches to solution of 
those problems promised to be more satisfac¬ 
tory. These are described in Sections 3.3.4, 
3.3.5, and 3.3.6, and need no further attention 
in this section. 

2. The Navy’s mapping problems appeared 
to be much more fundamental than merely the 
use of oblique photographs. Activity in the 
Pacific was increasing rapidly, and most of that 
area was insufficiently mapped. To supply maps 
needed for the operations being undertaken, in 
time to be useful, was a major problem. It was 
later solved to a considerable extent by the cap¬ 
ture of Japanese charts. 

This problem became a major concern of the 
project from the start, but it was much more 
difficult to approach than were purely technical 
problems. While the studies of methods for con¬ 
trol extension and delineation were going on, 
investigations of the type of maps needed and 
the problems of their supply were made. The 
activities in this connection occupy the rest of 
this section. 


3,2,1 Brief Description of Naval 
Mapping Organization 

The Hydrographic Office is the Navy’s map¬ 
ping (or charting) organization. It is concerned 
with other aids to navigation besides charts, 
and its extensive mapping activities include re¬ 
sponsibility for charts of all water except the 
coastal waters of the United States. The latter 
region is the responsibility of the U. S. Coast 
and Geodetic Survey. 

For its new mapping program the Hydro- 
graphic Office uses standard mapping methods, 
including ground surveys and photogrammetry 
with vertical photographs. Occasionally oblique 


photographs are used with the Canadian grid 
method. Before World War II, almost the entire 
map production consisted of navigation charts, 
and almost no topographic maps were made. 
During World War II the Hydrographic Office 
was called upon to produce large quantities of 
topographic maps for operation in the Pacific 
in addition to increased demands for navigation 
charts and new types of graphical presenta¬ 
tions for landing operations, aircraft approach, 
and others. It expanded its facilities to provide 
for this new type of work and met the demands 
successfully. 

The Hydrographic Office is a permanent or¬ 
ganization in Washington. The Navy has no 
other mapping units located elsewhere, and no 
mobile trained mapping crews except a small 
number of survey ships which are primarily 
field survey parties operating under the Hy¬ 
drographic Office. 


322 Organization Requirements in 
World War II 

In the exploratory investigation made at the 
beginning of this project many groups in the 
Navy were visited including the Hydrographic 
Office, Bureau of Aeronautics, Photographic 
Interpretation Center and Special Devices Di¬ 
vision. The Army Engineer Corps was also 
visited. From discussions with the Washington 
personnel and with men recently returned from 
the Pacific it was learned that in many opera¬ 
tions the maps were quite inadequate. 

The chief reason given for this lack in some 
operations was the fact that there were no 
naval field mapping units. If an Army Engi¬ 
neer mapping unit were assigned to the opera¬ 
tion, it could produce the necessary maps, pro¬ 
vided photographs were available. If no such 
unit was assigned, as happened most often 
when the operation was chiefly naval, there was 
no naval counterpart to produce maps. There 
were almost no previously existing maps of the 
areas except, as discovered later, Japanese 
maps. 

The second reason assigned was the frequent 
lack of vertical photographs, and the difficulty 
of obtaining them because of enemy action and 




NAVAL MAPPING REQUIREMENTS 


177 


local clouds over the Pacific islands. Many 
oblique photographs were available, but there 
were no methods for mapping from them. 

Attempts were being made to improve these 
conditions by teaching some elementary map¬ 
ping to the naval photographic interpretation 
units and developing a crude method of sketch¬ 
ing in plan from oblique photographs, which re¬ 
quired no special instruments. While this cer¬ 
tainly aided the photointerpreters to sketch the 
appearance of the target for briefing in air 
operation, for example, it made no contribution 
toward preparing the accurate maps needed for 
artillery bombardment and infantry operation. 
Accurate mapping techniques cannot be learned 
as a sideline. 

The exploratory investigations uncovered a 
problem. In an attempt to learn precisely the 
requirements in maps (scale, accuracy, infor¬ 
mation to be represented, etc.) and the condi¬ 
tions of their use for Pacific operations so that 
recommendations could be made, several con¬ 
ferences were held through the Coordinator of 
Research and Development, attended by repre¬ 
sentatives from many service branches includ¬ 
ing those previously mentioned, amphibious 
and air operations groups in the Navy, the 
Marine Corps, and the Army field artillery. 

At these conferences it was learned that, for 
artillery use, the horizontal accuracy required 
is the same as that for a chart on the 1/25,000 
scale, i.e., 33 ft, and the vertical accuracy re¬ 
quired is that of one-half a contour interval on 
a similar chart, i.e., 10 ft. For landing opera¬ 
tions, underwater topography and large-scale 
topographic maps of the beach back to 500 ft 
from the water showing beach' egress were es¬ 
sential. However, the requirements of such 
maps could not be given in detail. Accuracy of 
beach gradients, for example, and the critical 
beach gradients under various soil conditions 
for different vehicles, were not stated. It ap¬ 
peared that, although in Washington the prob¬ 
lem was known to exist, sufficient information 
on which to base recommendations could not 
be learned. 

To obtain this information, a party consist¬ 
ing of one representative of the Hydrographic 
Office and two from the Office of Field Service 
was sent to the Pacific. They went to Hawaii in 


May 1944, and learned that all new mapping 
for the Pacific Ocean area was done there under 
the authority of the Joint Intelligence Center, 
Pacific Ocean Area [JICPOA]. The cartogra¬ 
phy was customarily done by the 64th Engi¬ 
neers Topographic Battalion, located at Hawaii. 
Their method consisted chiefly of laying un¬ 
controlled or partially controlled mosaics from 
vertical photographs and making planimetric 
maps at 1/20,000 from the mosaics. Informa¬ 
tion was added from time to time as more was 
obtained from later photographs. If contours 
were included they were compiled from other 
maps, estimated by aid of stereoscopes or 
drawn with Fairchild stereocomparators. 

The accuracy required of maps for naval 
gunfire and inshore navigation was at least five 
times as great as that obtained. This was 
caused in part by the unsophisticated method 
used and in part by the photographs, which 
were taken primarily for intelligence purposes 
rather than mapping. The solution to these 
problems consisted of applying methods and 
knowledge already well known in the art of 
mapping. 

One additional problem was found. At the 
time of the field trip, trimetrogon camera in¬ 
stallations were being widely introduced in 
order to reduce the flying required for complete 
photographic coverage. In the trimetrogon 
camera installation three 6-in. cameras are 
mounted in an airplane so that one points ver¬ 
tically downward and the other two point out 
sideward at 30-degree angles of depression. No 
satisfactory method existed for using the two 
lateral (or “wing”) pictures for large-scale 
mapping. 

The findings 1 of the field party were, in sum¬ 
mary, that the photographs, maps, and charts 
then obtained for the Pacific Fleet were suffi¬ 
cient for assault operations, except for naval 
and field artillery gunfire, navigation of landing 
craft, and troop operations throughout the ini¬ 
tial engagement. They recommended that: 

1. A program be instituted to consist of two 
parts which would increase mapping accuracy: 

a. Certain immediate changes in present 
methods. 

b. The prompt development and adoption 
of a mapping procedure specially de- 



178 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


signed for the requirements in the Pa¬ 
cific, with special equipment designed 
to fit such a procedure. 

2. More details be included on the maps 
which aid troop operation and navigation. 

3. An immediate effort be made to find the 
best method of determining the depth of water. 

Thus, the major cause of the inadequacy of 
maps, at least in the Pacific operations, was a 
lack of organized and trained personnel at the 
right places. The organization existed in Wash¬ 
ington at the Hydrographic Office, but the time 
schedules of operations would not permit pho¬ 
tographs to be sent to Washington for maps to 
be made there. One or several “Engineering 
Squadrons,” fully equipped and trained in ex¬ 
isting mapping methods, and located at the 
bases of operations, were required. Without 
such units no improved methods or equipment 
would be useful. With such units, headed by in¬ 
telligent engineers, reasonably adequate maps 
could have been made from even the rather 
faulty material available. 


3 ' 2 ' 3 Technical Developments Required 

While many technical developments are 
needed in photogrammetric mapping generally, 
very few can be characterized as having been 
especially needed during World War II. The ex¬ 
isting methods of mapping from aerial photo¬ 
graphs were for the most part adequate for 
producing the maps needed, where applied by 
trained technical personnel. Three specific de¬ 
velopments, however, which would have eased 
materially the problems encountered during 
World War II, are: aerial triangulation, photo¬ 
grammetric sounding, and large-scale wide- 
field photogrammetry. 

While these particular developments stood 
out, because of the geographic conditions en¬ 
countered in World War II, as will be evident 
in the discussion which follows, they are no less 
important now that the war is ended. In the 
mapping that must now be done of the less 
well-developed areas of the world, the same 
conditions exist and the same problems must 
be solved if this mapping is to be done accu¬ 
rately and efficiently. 


Aerial Triangulation 

For mapping at large scales, the existing 
methods of photogrammetry require rather 
dense ground control. In the United States the 
ground control is provided by ground survey. 
In mapping such regions as the Philippines, the 
Pacific islands, and China, very little ground 
control existed, and there was almost no oppor¬ 
tunity to send out field parties to provide con¬ 
trol. The accuracy of the maps produced suf¬ 
fered thereby. 

A method of providing accurate ground con¬ 
trol from the air was required. How such a de¬ 
velopment should best proceed cannot be stated. 
One method would be the development of spe¬ 
cial photographic equipment of high precision 
and resolution, including cameras, photo¬ 
graphic materials, and camera mounts. 

No work was done directly on this problem 
during World War II, but the developments for 
stabilized camera mounts and improved aerial 
lenses carried on for other purposes might con¬ 
tribute to this development. 

Photogrammetric Sounding 

The necessity of knowing inshore water 
depths with great accuracy for amphibious op¬ 
erations is obvious. Only a few spot depths were 
known before World War II, and until near the 
end of the war the only methods available for 
obtaining depth were the usual ship sounding 
methods. Had aerial methods been available for 
obtaining accurate and complete soundings 
such losses as occurred at the landing on 
Tarawa might have been lessened. 

The most promising methods for sounding 
are with low-altitude flying (about 500 ft alti¬ 
tude) using photography, as with the strip 
camera, or some variation of the radio altim¬ 
eter giving reflections from both the top and 
bottom of the water. A satisfactory method 
employing the strip camera was developed at 
the Photographic Interpretation Center, and 
another was proposed by this project. They are 
described briefly in Section 3.3.8. Both came too 
late to be of much value. 

Wide-Field Photogrammetry 

Standard methods of photogrammetry use 
only vertical photographs for large-scale map- 




TECHNICAL INVESTIGATIONS 


179 


ping. While high oblique photographs (and 
such photographs rectified) are dismissed as 
unusable, the historical trend in vertical photo¬ 
graphs has ever been toward a wider field until 
now a 90-degree lens field is common. These two 
attitudes are inconsistent, since the inner re¬ 
gion of a rectified oblique photograph is identi¬ 
cal with the outer region of a vertical photo¬ 
graph, both taken from the same point in the 
air. The attitude toward oblique photographs 
most probably arose from the lack of any meth¬ 
ods of handling them in quantity in the past. 
Obviously the criterion of utility should refer 
to the vertical angles of photographic images, 
rather than to the tilt of the camera at the time 
of exposure. Experiments made in this project 
indicated that at least a 120-degree field, sym¬ 
metric about the vertical, could be used for 
large-scale mapping, and even greater fields for 
lesser accuracy. 

The advantages of employing wider fields 
are: substantial reduction in flying, since each 
strip covers a greater area, and reduction in 
control necessary, since each photograph covers 
a greater area. Subsidiary advantages are the 
possibilities of control extension in island areas 
and photographing cloud-covered islands, men¬ 
tioned in Section 3.2.2 and discussed in Sections 
3.3.4 and 3.3.5. 

Two ways of increasing the useful field are 
by developing aerial lenses of wider fields and 
by developing methods of mapping at large 
scales from oblique photographs so that multi¬ 
ple camera photography such as trimetrogon 
photography can be used. Work on wide-field 
lenses for aerial photography was done by Har¬ 
vard University under another NDRC project. 
The development of methods for handling 
oblique photographs was carried on by the Geo¬ 
logical Survey at Clarendon, Virginia, in devel¬ 
oping the multiplex for using oblique photo¬ 
graphs; by the Engineer Board, Fort Belvoir, 
Va. in the multiplex development and in recti¬ 
fication; and by this project of the NDRC in 
rectification and plotting. 

The developments in this project are de¬ 
scribed in Sections 3.3.6 and 3.3.7. They came 
too late to be of use, but the multiplex develop¬ 
ment, although not fully completed, was used 
extensively by the Geological Survey in making 
maps of the Philippines and other regions. 


3 3 TECHNICAL INVESTIGATIONS 

Definitions and Fundamental 
Geometry 

Nomenclature 

The terminology and relations used in the 
technical exposition are given here. Figure 1 
shows two views in elevation of an aerial photo¬ 
graph at the moment of exposure, and other in¬ 
formation. The position shown is that of the 
negative. The salient geometric quantities are 
indicated thereon, and their symbols and names 
are shown. 

Definitions 

Plane I is horizontal (axiom). 

Line LN is perpendicular to plane I. 

A is the distance LN. 

Line Lp is perpendicular to plane II. 

/ is the distance Lp. 

t is the angle between lines Ln and Lp. 

Line Lj is the bisector of angle nLp. 

Plane LH is parallel to plane I. 

Plane P is the plane of L, n, and p. 

Intersection of planes P and II is the “prin¬ 
cipal line. ,, 

The “scale ,, at a point such as b is the dis¬ 
tance Lb divided by the distance LB'. 

The scale of the photograph is f/A. 

Properties 

An aerial photograph is a perspective projec¬ 
tion (perspection) of ground points, e.g., B, 
onto the plane of the photograph, as at b, ex¬ 
cept for distortion. That is, B, L, and b are col- 
linear, except for small displacements of b due 
to many causes called “distortions.” 

A pair of lines lying in plane I, intersecting 
at J and including some angle a would be 
imaged on II as intersecting at j and including 
the same angle a. That is, j is a conformal point 
of the perspection (isocenter), and is unique 
unless I and II are parallel. 

Scale at j = f/A. 

Scale at H = 0. 

Scale at £ = 1. 

Distance jH = / esc t. 

Distance jS = distance JS. 

If / and the location of p are known (from 



isocenter 


180 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 



& 


c 

4 





z 


s' 

S' 

s' 


s' 

s' 




Figure 1. Geometry of aerial photographs. 






















TECHNICAL INVESTIGATIONS 


181 


camera calibration) and if the locations of any 
three points on the ground and their images on 
the photograph are known, all other quantities 
in Figure 1 may be determined. 

If / and p are not known, the locations of 
five ground points and their images (not co- 
planar, and subject to conditions of noncol- 
linearity) will determine the perspection, but 
in practice this determination is very weak, 
since the differences in elevation occurring on 
the ground are small, and it is not used. In fact, 
if the ground were a plane, e.g., coincident 
with I, the perspection would be indeterminate 
without knowledge of / and p, a linearly infinite 
set of perspections all giving the same image on 
the photograph. To determine this infinite set 
of perspections, four points are sufficient, and 
these determine the quantities / esc t and A 
esc t, but /, A, and t are individually indetermi¬ 
nate. 

Plane III represents a plane perpendicular to 
Ln. The image on III from a perspective pro¬ 
jection of II through L onto III is called a “rec¬ 
tified photograph.” It is apparent in Figure 1 
that a rectified photograph looks the same, ex¬ 
cept for distortion differences, as would a ver¬ 
tical photograph (t = 0) taken from the same 
point in space if the field of the camera lens 
were large enough. 

If the inverse perspection from III to II is 
considered in the same light as the perspection 
from G to II, then this is obviously equivalent 
to the case when the ground is plane. Thus, a 
linearly infinite set of perspections all give the 
same image. This is an important property 
and is used intensively in practical rectifi¬ 
cation. 

A map is a drawing on a reduced scale show¬ 
ing the orthographic projection of landscape 
features such as shoreline, rivers, roads, build¬ 
ings, and contour lines onto a horizontal plane. 
In mapping, the complete plotting operation 
requires the production of such a drawing from 
the representation shown on a photograph. 
For instance, an oblique photograph would 
show a vertical flag pole as a short line, but in 
an orthographic projection it should appear 
only as a point or small circle. The top of the 
flag pole is said to be displaced on the photo¬ 
graph because of relief. This displacement can¬ 


not be corrected from a single photograph but 
only by using two jointly. 


3 3,2 The Miller Stereoscopic Plotting 
Instrument and Single 
Eyepiece Plotter 

The essential parts of the stereoscopic plot¬ 
ting instrument as conceived are: 

1. Two goniometers for holding a pair of 
aerial photographs taken from different points 
in space and overlapping in their views of the 
ground. 

2. A pair of reference marks, which are 
points of light. 

3. Two telescope systems for viewing the 
photographs from their perspective points, for 
viewing the reference marks, and for melding 
the views of the reference marks and photo¬ 
graphs. As usual in stereoscopic instruments, 
each eye views one photograph and one refer¬ 
ence mark through one telescope. 

The goniometers have complete freedom of 
rotation about the perspective points. The pair 
of reference marks moves as a unit without ro¬ 
tation, but as a unit has freedom of motion in 
space. In addition, the distance between the 
reference marks may be changed, and the axes 
along which the marks move may be rotated 
with respect to the goniometers. All these mo¬ 
tions have scales and vernier settings for meas¬ 
uring, recapturing, and presetting positions. 

In operation a pair of overlapping photo¬ 
graphs is placed in the goniometers and ori¬ 
ented to bring the entire overlapping region 
into stereoscopic fusion at once, as viewed 
through the telescopes. The photograph orien¬ 
tations necessary to obtain fusion are unique 
except for rotation of the pair as a unit about 
the line joining the perspective centers. When 
fusion is reached, the view corresponds geo¬ 
metrically to a view of the ground with the eyes 
placed one at each of the points in space from 
which the photographs were taken. 

When the photographs are properly oriented 
the reference marks also fuse and appear as a 
single point of light moving three-dimension- 
ally in the space model of the photographs. The 
axes on which the reference marks move may 



182 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


now be oriented to the photographs to corre¬ 
spond in the stereoscopic model to a rectangu¬ 
lar space coordinate system on the ground. This 
operation requires the equivalent of at least 
three control points in the model whose coordi¬ 
nates in the ground coordinate system are 
known. The axes are oriented so that, when the 
fused reference mark is placed in coincidence 
with a control point, the scale readings corre¬ 
spond to the known ground coordinates. 

When this orientation has been achieved, the 
features of the stereoscopic model may be traced 
in horizontal projection by moving the fused 
reference mark along each feature, such as a 
road. The model may be contoured by keeping 
the fused reference mark at a fixed altitude and 
moving it so as to stay apparently always in 
contact with the ground. A stylus connected to 
the reference marks does the drawing. The 
stylus reproduces the horizontal movement of 
the reticle marks, but remains still when the 
reticle marks are moved vertically. 

Since the stereoscopic model is a true repre¬ 
sentation of the ground, at some small scale, 
the drawing produced is a map of a small sec¬ 
tion of the ground. Many such drawings, made 
from many pairs of photographs, may be as¬ 
sembled into a sizable map. 

There exist several instruments which per¬ 
form like the Miller stereoscopic plotting in¬ 
strument, although none is so well conceived 
geometrically. All are of foreign manufacture. 
Such instruments must be precise and are ex¬ 
pensive. Highly trained operators are neces¬ 
sary, requiring at least a year’s training before 
becoming efficient. The Miller instrument 
would be versatile, and is thereby well adapted 
to the use of oblique photographs where the 
demands are varied. On the other hand, it is 
expensive for only incidental use, and would 
probably not perform well unless used continu¬ 
ally, because of the operator skill required. 

The single eyepiece plotter amounts to one- 
half of the stereoscopic plotter, photographs 
being used in it singly. It consists of one goni¬ 
ometer, telescope, reference mark, and stylus. 
The precise relations necessitated in the stereo¬ 
scopic plotter by having these parts in pairs are 
not required and therefore construction and use 
are much easier. Correspondingly, however, the 


stereoscopic model is absent. Versatility is 
much reduced and operation is more tedious. 

The request by the Hydrographer, as a result 
of which this project was established, had been 
for the development and construction of one 
model of each of these instruments. The design 
and function of both were examined, and the 
use to which each might be put was investi¬ 
gated by conferences with the mapping and 
photographic branches of the Navy. As a re¬ 
sult of this study it was decided not to develop 
either instrument. 

The stereoscopic plotter was designed to be 
a precise instrument for making accurate topo¬ 
graphic maps. It was intended primarily for 
use with vertical photographs, but could have 
been adapted to oblique photographs without 
great difficulty. Under conditions existing at 
the beginning of the project, the Navy would 
have had little use for the stereoscopic plotter 
since almost no topographic mapping was done. 

The problems in the use of oblique photo¬ 
graphs facing the Navy at that time were (1) 
extension of control between islands, and (2) 
tracing of shorelines from oblique photographs 
when necessitated by lack of vertical photo¬ 
graphs. For neither of these problems would 
the stereoscopic plotter have been efficient. 
When, later, it was learned that topographic 
maps in large quantities would have to be made 
from trimetrogon photography, the method 
described in Section 8.3.7, employing rectifica¬ 
tion, was already well along in its development. 
Since that method promised greater efficiency 
and more rapid development, the stereoscopic 
plotter was not revived. 

The single eyepiece plotter was designed ex¬ 
pressly for tracing shorelines from oblique pho¬ 
tographs and was readily adaptable to extend¬ 
ing control from such photographs. Hence, this 
instrument was given serious consideration for 
these problems. Further analysis showed, how¬ 
ever, that the single eyepiece plotter possessed 
no inherent superiority in accuracy or efficiency 
over completely graphical methods for extend¬ 
ing control, employing only standard drawing 
equipment. As to the tracing of shorelines, ex¬ 
periments proved that greater accuracy and at 
least equal operational facility could be 
achieved with the pinhole rectifying camera, as 




TECHNICAL INVESTIGATIONS 


183 


described in Section 3.3.5. Development of this 
instrument, therefore, was also not undertaken. 


3 ‘ 3 ' 3 Study of Photographic Quality 

One of the first investigations to be under¬ 
taken in this project was a study of the charac¬ 
teristics and quality of oblique photographs. 
Almost all the photographs used by the Army 
and Navy in mapping were taken with cameras 
of short focal length (usually 6 in.) and wide 
field on Super-XX Aero Panchromatic film. 
Oblique photographs taken in this manner were 
characterized by excessive background haze, 
and their small scale made identification diffi¬ 
cult, especially for control purposes. Little con¬ 
sideration had been given to other promising 
emulsions and to cameras of long focal length 
for the special purpose of oblique photographs 
for mapping. 

Accordingly, a cooperative project was be¬ 
gun in August 1943 to obtain comparable pho¬ 
tographs with various cameras and films. After 
some intermediate changes of plans, the locality 
finally selected was the group of four islands 
off the coast of Santa Barbara, California. An 
F-2 airplane was borrowed from the Chief of 
Intelligence, Corps of Engineers. The Bureau 
of Aeronautics, Division of Photography, 
loaned one K-17 camera with 6-, 12- and 24-in. 
cones and three F-56 cameras with focal 
lengths 8*4, 20, and 40 in. Aero Service Cor¬ 
poration supplied a 4-in. wide-angle camera 
and a photographer, and supervised the experi¬ 
ment. 

The photographer made his headquarters at 
Mount Wilson Observatory, where photo¬ 
graphic facilities were made available to him, 
and also facilities for testing and focusing the 
cameras. Photography began about December 
15, 1943, and was finished the following Febru¬ 
ary. It was carried out under various light and 
weather conditions. The film was developed in 
California and sent to the Aero Service plant 
in Philadelphia for printing and study. 

Four Eastman films, Super-XX Aero Pan¬ 
chromatic, Shellburst Panchromatic, Infrared, 
and Microfile, were tried. The Shellburst and 
Microfile were specially coated for this experi¬ 


ment. The characteristics of Super-XX and 
Infrared are well known. Shellburst has finer 
grain, higher resolving power, and higher con¬ 
trast than Super-XX, but only about one-third 
the speed. Microfile is a very much slower film 
with very high resolution. It proved to be too 
slow to record more than very faint images in 
this experiment. 

The conclusions, from careful inspection of 
the photographs, but without quantitative 
analysis, were: 

1. The superior haze-penetrating ability of 
Infrared film and the lack of serious offsetting 
disadvantages provides greater accuracy than 
that obtained from Super-XX whenever the 
area above the principal point in high oblique 
photographs is to be used. 

2. The remarkable photographic quality 
given by Shellburst film merits its further con¬ 
sideration, especially for vertical photographs 
or foreground use of oblique photographs. 
Where background information is desired, it is 
inferior to Infrared. 

3. Long focal length photographs on Infra¬ 
red film can provide excellent control informa¬ 
tion, especially for extending position across 
water. 

4. Microfile film is too slow for present aerial 
photography. 

Some of the photographs were later used for 
testing the mapping method developed in the 
last phase of the project. 

This study has been reported in full. 2 


3 ' 3 ' 4 Control Extension 

In mapping island areas by means of vertical 
photographs it frequently happens that the 
water gaps between islands are too great to be 
included in a single vertical photograph. In this 
case, while each island may be mapped sepa¬ 
rately from vertical photographs, the islands 
cannot be related one to another and the scale 
of the individual island maps is unknown with¬ 
out extensive survey work on the ground. The 
water leaves gaps in the photography because 
the images of the water surface cannot be used, 
due to wave action and difficulties of identifica¬ 
tion. 



184 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


In such an area, however, many islands may 
be included in a single oblique photograph. 
With adequate methods, the maps of individual 
islands made from vertical photographs can be 
related by using several oblique views. This is 
called “controlling the map/’ 

One such area being mapped at the time by 
the Hydrographic Office was the Bahama 
Islands, and similar areas were expected in the 
Pacific. 

While graphical methods already existed for 
control by high oblique photographs, none of 
these methods gave as great accuracy as ap¬ 
peared to be inherent in the photographs, and 
it was believed that by measurement and com¬ 
putation greater accuracy might be obtained. 

Statement of the Problem 

Control points in mapping are points whose 
positions are known relative to some coordinate 
system. Extension of control means the deter¬ 
mination of positions for new points, which can 
then serve as control points, from existing con¬ 
trol points. The purpose of this investigation 
was to determine the feasibility of extending 
control by computational methods and to com¬ 
pare the accuracy of such methods with 
the accuracy of graphical and instrumental 
methods. 

For a computational method, coordinate systems are 
established on the ground and on the photographs. The 
general form of the transformation between points in 
the ground coordinate system and their images in the 
coordinate system of a photograph is assumed to be a 
perspective. The constants in the perspective trans¬ 
formation are parameters, varying with different 
photographs, and their numerical values for each par¬ 
ticular photograph are not known a priori. 

To determine the parameters numerically, control 
points must be used. Three control points are required 
to determine the transformation equations for each 
photograph. When this has been done, the equations 
may be used as computing formulas to compute the 
ground coordinates of other points from measure¬ 
ments of the photograph coordinates of their images. 

The transformation equations are well known, being 
essentially the transformations of plane projective 
geometry. They may be written in many forms which 
are all algebraically equivalent and lead to exactly the 
same result if three control points are used. However, 
in any practical method it is desirable to use more 
than the minimum number of control points and to 
determine the transformation equation coefficients by 


some averaging process in order that the effects of 
errors of identification and measurement may be 
minimized. In this case the form in which the trans¬ 
formation equations are phrased and the averaging 
process used become important. 

The problems in this approach are (1) to 
find a form for the transformation equations 
and an averaging process which lead to a suf¬ 
ficiently easy computation and to efficient esti¬ 
mates of the transformation coefficients, and 
(2) to determine by tests on photographs how 
large the errors of identification and measure¬ 
ment may be expected to be. 

Analysis for the first of these problems was 
begun about June 15, 1943, and was completed 
by August 15 in form sufficient for first tests 
to determine the size of error. 

A large number of photographs and other 
data, supplied by the Hydrographic Office, were 
selected for testing the method. Specifically, the 
material consisted of: 

1. Almost complete vertical photographic 
coverage at 15,000 ft, of upper Frobisher Bay 
in Baffin Island with 6-in. K-17 cameras. Very 
many islands were included. 

2. A triangulation net over the area, with 
stations identifiable on the photographs. 

3. Oblique photographs of the area, at ap¬ 
proximately 60-degree tilt, from 10,000 ft with 
6-in. K-17 cameras. 

It was necessary first to compute the coordi¬ 
nates of many points (minor control points) on 
the shorelines of islands, using the vertical pho¬ 
tographs and the known coordinates of the tri¬ 
angulation stations (primary control points). 
This was done essentially by triangulation. 
Minor control points were selected, and both 
minor and primary control points were marked 
on all photographs on which they appeared. Co¬ 
ordinates of all points were measured on the 
photographs. Angles at the principal points of 
the photographs, subtended by the control 
points taken in pairs, were computed and were 
assumed to be the same as the corresponding 
angles on the ground. (Note the definition of 
isocenter , Section 3.3.1, and the proximity of 
j and v when t is small in Figure 1.) By resec¬ 
tion and intersection, using these angles, it was 
possible to compute trigonometrically the co¬ 
ordinates of minor control points from the pri- 



TECHNICAL INVESTIGATIONS 


185 


mary control points. Many multiple determina¬ 
tions were included, so that an adjustment to 
reduce error could be made. 

Minor control points were then identified on 
five of the oblique photographs, coordinates 
were measured, and the perspectives of the 
photographs were determined. As mentioned 
previously, three control points are sufficient to 
determine the perspective of a single photo¬ 
graph. An easier method, lending itself also to 
more accurate determinations, makes use of the 
photographs in pairs. The one used was such a 
method. It may be described briefly as follows. 

From one photograph, suppose that lines are drawn 
through the lens from each of four image points. A 
four-sided solid angle results. Another such figure is 
formed from another photograph and the images on it 
of the same four ground points. These two figures may 
be oriented in an infinity of ways so that the edges of 
the solid angle of one intersect the corresponding edges 
of the other. For one of these orientations the points 
of intersection are coplanar. The complete figure can 
then be oriented so that the plane containing the points 
of intersection is horizontal. Note that the minor con¬ 
trol points are at sea level, and hence coplanar. This 
completes the orientation of both photographs. 

Four points provide the necessary number of con¬ 
ditions, but more than four points are desired to reduce 
error. As many as ten points were used in the test, 
with an averaging process. The general method itself 
is well known, but a unique feature of the particular 
procedure developed for this application was the com¬ 
putational ease of using excess points and averaging. 
The difficulty of computing increased in proportion to 
the number of points used rather than exponentially 
as is customary. A great many more points on each 
photograph were not used, so that the accuracy of the 
determination could be tested. 

Coordinates for the control points reserved 
for test were then computed. Two sets of coor¬ 
dinates resulted from each pair of photographs. 
The mean position was used and compared with 
the coordinates found by triangulation from 
the vertical photographs. It was believed that 
this double determination would reduce the 
error below that of more customary methods 
in which a single position is found for each 
point. 

Conclusions 

A theoretical analysis of error was then 
made, assuming systematic error in / and p 
and uniform accidental error in identification 


and measurement of points on the oblique pho¬ 
tographs. Interpreting the errors actually 
found in terms of the theoretical error analy¬ 
sis, the dominant source of error was found to 
be in identification of points on the photo¬ 
graphs, which had a probable error ±0.005 in. 
This is far greater than is customarily found 
with vertical photographs and is probably due 
to the extreme perspective. 

In summary it was concluded that: 

1. Since careful graphical work can be held 
to 0.005 in. accuracy, and is considerably easier, 
no advantage results from computation. 

2. The possible reduction in error from the 
double determination is more than counter¬ 
balanced by the great magnification of errors 
in a direction parallel to the principal line. 

3. Any method based on geometry similar to 
that used in this experiment would be unsatis¬ 
factory. 

4. The graphical intersection methods using 
lines passing near the nadir point, in which the 
effects of errors parallel to the principal line 
are reduced, embody the most satisfactory 
geometry. 

5. There is only a small possibility of such 
methods being improved much by substitution 
of computation for graphics. 


33 5 Delineation from Oblique 

Photographs 

In mapping from vertical photographs it is 
occasionally desirable to be able to draw shore¬ 
lines and other features with considerable ac¬ 
curacy from oblique photographs. For example, 
a gap in the vertical photography may be cov¬ 
ered in some incidental oblique photographs, 
or the most important portion of an area may 
have been covered with verticals, while out¬ 
lying islands may show only in oblique views. 
Both of these cases occurred at Frobisher Bay 
in Baffin Island, which was being mapped by 
the Hydrographic Office at the time of this 
project. 

Such circumstances occur more frequently in 
areas of military action than in friendly re¬ 
gions. The danger involved in repeat flights to 
complete coverage makes the full utilization of 



186 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


existing information of utmost importance. 
Fortunately, in areas of serious military action 
where incomplete vertical coverage occurs most 
frequently, oblique views taken for interpreta¬ 
tion and intelligence are more frequent. 

At the time of the investigation, the Hydro- 
graphic Office performed any necessary deline¬ 
ation from oblique photographs by use of 
Canadian grids, a process of drawing square by 
square from a perspective gridwork superim¬ 
posed on the photograph to a square grid on the 
drawing board. This procedure is tedious and 
is less accurate than plotting with a good in¬ 
strument. Hence a definite need existed for a 
satisfactory instrument to plot from single 
high oblique photographs with adequate accu¬ 
racy. 

For drawing shorelines and nearby features, 
locating islands, and sketching in the field with¬ 
out high accuracy, the relief displacement is 
not important. It is necessary only to remove 
the perspective distortion caused by the obliq¬ 
uity of the photograph. Hence, for this use it 
is sufficient to obtain from an oblique photo¬ 
graph a drawing such as might be made by 
tracing a vertical photograph taken from the 
same point in the air, if a vertical photograph 
of such great angular field could be made. 

Preliminary Experiments 

Starting in June 1943, investigations of pos¬ 
sible types of plotters were begun. The prin¬ 
ciples considered as possibilities for use in such 
a plotting instrument were: 

1. Photographic rectification, by projection 
of the oblique photograph onto photographic 
material 

a. through a lens, 

b. through a pinhole, 

c. from a point source of light. 

2. Direct drawing, by 

a. viewing photograph and drawing board 
simultaneously, using a half-silvered 
mirror, 

b. projecting a point of light situated on 
the tracing stylus through a lens or 
pinhole onto the photograph. 

3. Projection as in photographic rectifica¬ 
tion, but onto a drawing board for tracing, in¬ 
stead of onto photographic material. 


4. Mechanical devices for continuously 
changing scale in the appropriate manner as 
between a tracing point and a drawing 
point. 

The geometry is illustrated in Figure 1, 
where plane II represents the oblique photo¬ 
graph and plane III the required drawing. 

Pilot experiments were made with the vari¬ 
ous principles, and tentative designs for pos¬ 
sible instruments were tried in order to select 
the most promising. All the photographic rec¬ 
tification methods remained as possibilities. 
From the preliminary experiments it was 
learned that the major difficulty in rectification 
by projection through a lens lay in the great 
image distortion. With pinhole and point-light- 
source projection the questions were what 
image quality and speed could be achieved. 

For direct drawing, the. simultaneous view¬ 
ing principle had already been used in practice 
in the oblique “sketchmaster.” This device had 
many faults, including awkwardness of opera¬ 
tion, parallax, and great differences in focus 
between the photograph and drawing. It was 
being improved and developed by several or¬ 
ganizations, however, so no further attention 
was given it in this project. 

Other principles for direct drawing appeared 
to be impractical. Available practical light 
sources gave insufficient light for projection by 
any means except a lens. Lens projection was 
subject to the same difficulty of distortion that 
occurred in photographic rectification using a 
lens. 

It was finally decided to concentrate on the 
development of two rectifying cameras, one 
employing either a pinhole or a point source 
of light, whichever proved superior in subse¬ 
quent experiment, the other using a lens or 
lenses specially designed to have tolerable dis¬ 
tortion. 

In subsequent experiments with wooden 
mockups many light sources and many photo¬ 
graphic materials were tried. Reasonably good 
continuous tones and image quality could be 
obtained with the pinhole, certainly sufficiently 
good for tracing shorelines and major features 
near the shore, or for obtaining a quick sketch- 
map. The diffraction patterns in the point-light- 
source method were much more apparent and 





TECHNICAL INVESTIGATIONS 


187 


almost all detail was lost in solid blacks and 
whites. 

With the pinhole camera employing a 0.01-in. 
hole, a reasonably short exposure could be ob¬ 
tained only if the image was received on fast 
film and an intense light, such as a carbon arc 
was used. Under these conditions exposure ap¬ 
proximated ten minutes. The light distribution 
was far from uniform. 

The point-light-source camera was therefore 


medium-pressure mercury-vapor lamps. The open side 
has an opal glass diffusion plate. The top has a light- 
trapped chimney, and in the bottom are openings to 
admit a forced draft from the blower B. 

At C are two glass plates between which the nega¬ 
tive is held. The negative is taped to one glass plate, 
in a position as if it were being replaced in the aerial 
camera. The other plate which has cut corners is then 
placed over the negative and the two are pressed 
together to hold the negative flat. 

In the lens mount D is a pinhole of 0.01-in. diameter. 
The plane of the pinhole is perpendicular to the easel 



Figure 2. Aero Service pinhole rectifier 


dropped. One pinhole rectifying camera was 
constructed by Aero Service Corporation under 
NDRC, and another by the Hydrographic 
Office. These are described in text that follows. 
A lens rectifying camera, which received its 
impetus in this investigation, also was con¬ 
structed, but since it was ultimately put to 
more precise use it is described in Section 3.3.7 
in connection with the MFA mapping method. 

Pinhole Rectifying Camera 

The pinhole camera constructed by Aero 
Service Corporation is shown in Figure 2. 

Item A is the light box, containing two Mazda AH-2 


at the position shown in the figure. Its normal position 
is 6 in. from the negative, but this distance may be 
changed slightly by the wheel on the lens mount to 
match the focal length of the aerial camera if it is 
known more accurately. An 8^-in. cone is also provided. 

Item E is the easel, on which photosensitive material 
is taped. It represents the horizontal plane III in 
Figure 1. The easel moves on rollers F, is set by hand 
to the proper tilt as read on the scale G, and held by 
the clamp H. 

The easel unit moves along ways on the base, to 
provide an enlargement setting. It is moved by a rack 
and pinion operated by the wheel 7, set by use of the 
scale J, and held by the clamp K. 

Another wheel L actuates a lock to hold the pro¬ 
jection unit to the base after assembly. All parts are 
easily dismounted, and the camera is semiportable. Its 










MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


size may be judged from the scale on the base, the 
white portion of which is 31 in. long. 

A still simpler rectifying camera is shown in 
Figure 3. b 

The negative or a tracing is taped to the back of the 
cone A, which is fixed for a principal distance of 6 in. 
No illumination is provided in the camera and the sun 
is usually used. The tilt setting is made by tilting the 
cone and clamping it in position at D. 



Figure 3. Hydrographic Office pinhole recti¬ 
fier. (Courtesy Alexander Forbes, Commander, 
USNR, by direction of the Hydrographer.) 


The pinhole is at B. Paper or film is taped to the 
base C, and a cover is provided to exclude extraneous 
light during exposure. No enlargement setting is pro¬ 
vided, the perpendicular distance from pinhole to base 
being fixed at 2 in. Thus a reduction of 3X is imposed. 

In a second model of this instrument the 
pinhole was replaced by the simple thin-lens 
system described in Section 3.3.6. The lenses 
were designed and manufactured by the Opti¬ 
cal Research Laboratory, Harvard University 
(Contract OEMsr-474), especially for this use, 
and afforded a larger aperture and better image 
quality than did the pinhole. 

Performance 

Performance data for the Aero Service pin¬ 
hole rectifier and the first Hydrographic Office 
pinhole rectifier are lacking, since both these 
instruments were delivered to JICPOA without 
adequate preliminary tests. The second Hydro- 
graphic Office rectifier was retained at the Hy- 

b This was designed by Commander Alexander Forbes 
at the Hydrographic Office and built at the Naval Gun 
Factory, Washington Navy Yard. 


drographic Office and put to frequent use. The 
quality of the results is indicated by the fol¬ 
lowing quotation : 3a 

The result is very satisfactory. Nearly all the dis¬ 
crepancies of over 500 feet, except in the far distance, 
are matters of photo-interpretation, due to difficulty 
in locating high water mark. Even in the case of Monu¬ 
ment Island, which is 53,000 feet away from the Nadir 
in the photograph in which it appears, the distance is 
only 900 feet out. 

It should be noted that none of these photographs 
afforded a sea horizon, only distant land of unknown 
elevation. This introduces the possibility of tilt error. 
Also tracing the shoreline by eye involves the possi¬ 
bility of significant error at distances beyond 4 or 5 
miles. The results seem to show that the instrument is 
capable of mapping shorelines with fair reconnaissance 
precision very quickly. 0 


3 3 6 Lenses for Rectification 

The investigation of means for delineating 
from oblique photographs, described in Section 
3.3.5, led to the conclusion that photographic 
projection methods would be most accurate and 
easiest to use where standard darkroom facili¬ 
ties were available. The pinhole cameras were 
decided upon partly because of their relative 
simplicity in construction and operation and 
their freedom from distortion, and partly as an 
interim device to serve until a satisfactory lens 
rectifier could be built. These were intended 
only for reconnaissance, however, and for more 
precise work the greater speed and better 
image quality afforded by projection through a 
lens were essential. 

Early experiments with commercial lenses 
by MFA, Aero Service Corporation, and other 
agencies indicated that speed and image quality 
were satisfactory, but distortion was excessive. 
Harvard University (Contract OEMsr-474), 
then undertook to design a lens especially for 
the rectification of high oblique photographs, 
possessing satisfactory characteristics with re¬ 
spect to speed, resolution, and distortion. 

Method of Attack 

The following working specifications were 
adopted. 


c Information supplied by Alexander Forbes, Com¬ 
mander, USNR, by direction of the Hydrographer. 






TECHNICAL INVESTIGATIONS 


189 


1. The lens should be as fast as possible and 
should work at least at //50 on the axis. 

2. The lens should resolve 20 lines per mm, 
referred to the object plane. 

3. Distortion due to the rectifying lens 
should not exceed 0.5 mm on a rectified photo¬ 
graph at unit scale with respect to the oblique 
photograph/ 1 

The approach to the problem consisted of 
starting with the simple pinhole and testing 
successively more complicated lenses to find the 
simplest lens form that would satisfy the speci¬ 
fications. A test camera was constructed,-and 
four lens forms were tested as follows. 

Pinhole. Pinholes of diameters 0.010, 0.020, 
0.028, and 0.040 in. were tested in three orien¬ 
tations, such that the plane of the pinhole was 
inclined 0, 30, and 70 degrees to the image 
plane. 

Simple Lens. This lens actually consisted of 
two very thin plano-convex lenses cemented 
with the plane sides in contact, and with a 
central stop. Tests were made with 0.028-, 
0.040-, and 0.100-in. apertures, with the plane 
of the lens parallel to the negative, and with 
the plane of the lens passing through the inter¬ 
section of the object and image planes in the 
correct orientation for first-order geometric 
focus. One test was also made with the lens 
plane parallel to the negative and the plane of 
the aperture stop passing through the intersec¬ 
tion of the object and image planes. 

Spherical Lens. This was a centrally sym¬ 
metric spherical system with a central stop. 
Since a single spherical lens would have been 
bulky a lens was constructed with an outer 
spherical shell of EDF-3 and an inner spherical 
shell of C-l glass. It was tested with apertures 
of 0.020, 0.028, and 0.040 in., and in the two 
orientations described for the simple lens. 

Hypergon Lens. The Hypergon lens consists 
of two very steep thin meniscus lenses about a 
central stop. The optical data for the lens used 
in testing resolution (focal length 3.464 in.) 


were: 

Surface 

Radius 

Separation 

Index 

Glass 

1. 

0.3550 in. 

0.0834 in. 

1.62867 

DBC-1 

2. 

0.3565 in. 

0.574 in. 

1.00000 


3. 

—0.3565 in. 

0.0834 in. 

1.62867 

DBC-1 

4. 

—0.3550 in. 





d For definition of the scale of a tilted photograph 
see Section 3.3.1. 


Apertures of 0.050, 0.080, 0.100, 0.150, and 
0.200 in. were tested with the lens in the posi¬ 
tion and orientation required for first-order 
geometric focus. 

Resolution Tests 

For determining the resolution obtainable 
with the optical systems to be tested, a simple 
rectifying camera was constructed at Harvard. 
This camera was similar in conception to the 
Hydrographic Office pinhole rectifier illustrated 
in Figure 3. However, the angle between object 
and image planes was fixed at 60 degrees, and 
the point of projection was 6 in. from the image 
plane as well as from the object plane. 

Illumination consisting of a tubular mercury- 
vapor light source parallel to both object and 
image planes was provided and condensed on 
the projection lens by a cylindrical lens system. 
The light source and condenser were mounted 
on rocker arms pivoted on an axis passing 
through the projection lens. Thus, the illumi¬ 
nator could be moved to illuminate any part of 
the object, and the great variation in illumina¬ 
tion required at different portions of the object 
could be achieved by moving the illuminator 
rapidly near the isocenter and slowly near the 
horizon (respectively j and H, Figure 1). 

For determination of resolution, twenty-eight 
high-contrast resolution targets were mounted 
on a glass plate in the object plane, in seven 
rows of four each, running parallel to the hori¬ 
zon. The targets were about 14/2 in. apart in the 
rows, the first target in each row lying on the 
principal line. The first row passed through 
the isocenter, and the seventh lay at about 
75 degrees off the vertical. The positions and 
assigned members of the targets as projected 
on the image plane are shown in Figure 4. 

The projected images were received on blue- 
sensitive glass plates laid in the image plane. 
The resolution determined from the images was 
recorded as lines per millimeter on the object 
plane. The values of the best average resolution 
obtained with each of the four projection sys¬ 
tems tested are shown in Table 1. 

It is evident that the simple lens and spher¬ 
ical lens were better than the pinhole, that the 
spherical lens was no better than the simple 
lens, and that the Hypergon was far superior 
for uniformity of coverage and resolution. In 




190 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


fact, the performance of the Hypergon was 
adequate, and so no more complex lenses were 
designed. 

At targets 1 to 4, the projection was approxi¬ 
mately 1/1, and 23 lines per mm were resolved. 
When the resolution of the Hypergon as incor¬ 
porated in the two-stage rectifier (see Section 
3.3.7) was measured this result was verified. 
In that rectifier, however, the isocenter region 
was reduced 1.8X in each stage, and at that 
reduction ratio only 14 lines per mm on the 
object were resolved. In two stages, 8 lines 


tally at Harvard University, but it was com¬ 
puted on the basis of the lens design. By this 
computation the image distortion should not 
exceed 0.5 mm in rectification of a 60-degree 
oblique, 6-in. photograph at 1/1. The maximum 
amount occurs far out near the horizon, at 
about 78 degrees vertical angle. Thus, the 
Hypergon lens design meets the distortion speci¬ 
fications adequately. 

The first 4.402-in. Hypergon delivered to 
MFA was tested extensively for distortion in 
the two-stage rectifier. Distortion of 0.2 to 


Table 1 . Resolving power of four projection systems (lines per mm on the object). 


Target 


0.020 in. 
pinhole 


0.040 in. 
thin lens 

0.020 in. 
spherical lens 

0.10 in. 
Hypergon 

1 

1.1 

1.1 

5 

10 

5 

7 

23 

23 

2 

1.1 

1.2 

5 

8 

5 

5 

23 

23 

3 

1.1 

1.4 

3 

4 

4 

5 

20 

20 

4 

1.1 

1.7 

2 

2 

3 

3 

20 

17 

5 

1.3 

1.3 

7 

10 

7 

8 

23 

20 

6 

1.3 

1.3 

7 

8 

7 

7 

27 

27 

7 

1.2 

1.4 

4 

5 

5 

5 

27 

20 

8 

1.2 

1.4 

3 

2 

5 

3 

25 

17 

9 

1.4 

1.4 

8 

10 

8 

8 

30 

30 

10 

1.5 

1.5 

8 

10 

7 

7 

23 

23 

11 

1.4 

1.4 

7 

7 

7 

6 

27 

23 

12 

1.4 

2.0 

5 

2 

6 

4 

27 

20 

13 

1.9 

1.4 

10 

10 

8 

10 

33 

30 

14 

1.5 

1.4 

8 

9 

8 

7 

30 

27 

15 

2.0 

2.0 

8 

7 

7 

4 

27 

20 

16 

2.2 

2.2 

8 

2 

7 

3 

23 

20 

17 

2.0 

1.3 

10 

9 

8 

10 

30 

27 

18 

2.6 

2.0 

10 

10 

8 

8 

27 

27 

19 

2.6 

2.3 

10 

8 

8 

6 

23 

23 

20 

3.3 

2.3 

8 

2 

7 

4 

23 

14 

21 

4.0 

2.0 

10 

10 

8 

10 

23 

30 

22 

5.0 

2.0 

9 

10 

7 

8 

23 

30 

23 

4.0 

2.4 

10 

7 

7 

5 

20 

23 

24 

3.3 

3.3 

4 

1.5 

6 

5 

10 

17 

25 

3.3 

2.7 

8 

10 

7 

9 

17 

27 

26 

3.3 

2.7 

8 

10 

7 

8 

13 

30 

27 

3.3 

2.8 

4 

4 

5 

6 

13 

23 

28 

2.5 

2.5 

2 

2 

3 

4 




per mm on the first object were resolved on 
the second image. In the region near the hori¬ 
zon, 40 lines per mm on the object were re¬ 
solved. Thus, the final rectified print maintained 
approximately average resolution, and inspec¬ 
tion of the photographs showed no loss of qual¬ 
ity from original negatives photographed with 
a 6-in. Metrogon lens on infrared film. 

Distortion Tests 

The image distortion in rectification using 
the Hypergon was not determined experimen- 


1.0 mm was found in various images, although 
in this design a maximum net distortion of 
only 0.05 mm was expected. The variation in 
distortion was found apparently to have arisen 
from bending of the plates by the plate clamps. 
However, further tests indicated with reason¬ 
able certainty that distortion of at least 0.2 mm 
was caused by the lens. This was believed to 
be due probably to lack of perfect centering of 
the lens elements, which is critical with respect 
to the distortion properties. The second 4.402-in. 
lens was not tested. 







TECHNICAL INVESTIGATIONS 


191 


Since only a central strip of each rectified 
photograph was used in the mapping test the 
distortion was not troublesome. The distortion 
tolerance could well be relaxed for mapping 
under such conditions, but not when the entire 
area of the photograph is to be used. 

Manufacturing Problem 

Two simple lenses for rectification were made 
and delivered to the Hydrographic Office. Their 
focal lengths were 2.40 in., and their apertures 



0.015 and 0.030 in. These lenses presented no 
manufacturing problems. 

Four Hypergons were made: one with focal 
length 3.464 in., used for the tests and later 
delivered to the Engineer Board, Fort Belvoir, 
Va.; one with focal length 1.484 in. delivered 
to MFA for their one-stage rectifier; two with 
focal length 4.402 in., delivered to MFA for 
use in their two-stage rectifier. 

These lenses were somewhat difficult to man¬ 


ufacture. The edge thickness is only 0.013 in. 
in the 3.464-in. lens. Consequently it was nec¬ 
essary to support the edge throughout all oper¬ 
ations. This was done by first grinding and fin¬ 
ishing the concave internal curves of both 
elements, lapping the ring surrounding the 
concave face, edging the disks centered, and 
grinding the back side parallel to the lapped 
ring as measured by a 0.0001-in. dial gauge. 

A blocking tool was then made consisting of 
two identical glass hemispheres ground to fit 
the finished concave surfaces and of precise 
thickness. The two elements and the two hem¬ 
ispheres were then blocked with balsam to form 
a solid cylinder, and the blocked assembly 
ground and polished to a perfect sphere of 
appropriate diameter. 

It was found that silicon carbide leaves sub¬ 
surface bruises which weaken the thin edges, 
so that several lenses cracked. In the later work 
the last 0.005 in. was removed with No. 302 
emery. 

The first Hypergons were cemented into their 
cells, without retainer rings, because of the 
steepness of the curves. The second 4.402-in. 
Hypergon was mounted with retaining rings 
in a cell of special design, which included an 
aperture whose plane was inclined 60 degrees 
to the axis of the lens, and a similarly inclined 
barrel or hood. The hood and the walls of the 
aperture were threaded to reduce flare. 


a- 3 -? MFA Method for Mapping from 
High Oblique Photographs 

Upon their return from Hawaii in June 1944, 
the field party recommended the immediate 
development of a method for mapping at large 
scales from high oblique photographs. The 
value of oblique photographs as opposed to 
vertical photographs can be argued at length. 
The advantages of the large area included in 
each oblique photograph, with consequent re¬ 
duction in the amount of flying and ground 
control required, are offset by the great vari¬ 
ations in scale and resolution in different por¬ 
tions of the photographs and the obscuration 
which occurs frequently because of the angle 
of view. 











192 MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


For commercial and peacetime operations no 
valid general decision can be made because of 
lack of experience with oblique photographs. 
For military operations the reduction in flying 
and ground control assumes great importance, 
and the value of having available a method for 
using oblique photographs becomes apparent. 
However, more important as a reason for de¬ 
veloping such a method than any theoretical 
considerations was the fact that a great many 
high oblique photographs had been taken and 
were already available, and more would be 
taken in the trimetrogen photography with 
which the Army Air Forces had covered a large 
part of the Pacific area. These photographs in 
many cases included areas not otherwise cov¬ 
ered, and the cost in lives, time, and equipment 
of covering those areas with vertical photo¬ 
graphs was not warranted if the oblique photo¬ 
graphs could be used. 

The question of the most suitable method was 
considered from fundamentals with the object 
of developing a method which would lend itself 
to production in large quantities, without large 
numbers of highly trained specialists and with 
the greatest possible accuracy. The arguments 
which led to photographic rectification in the 
problem of delineation from oblique photo¬ 
graphs (see Section 3.3.5) led also to rectifica¬ 
tion here. The practical problems in using recti¬ 
fied images, for which the datum scale and 
contour interval are constant in the area of the 
image, are much easier than when the oblique 
photographs themselves or intermediate partial 
rectifications are used. The theoretical accuracy 
also is greater with rectified photographs if 
an optimum plotting procedure is used. Be¬ 
tween photographic rectification and projection 
of a rectified image, as by the multiplex, the 
former was chosen because of its better image 
quality and easier operation. Plotting directly 
in orthographic projection was chosen as lead¬ 
ing to simpler instruments. For control, the 
existing radial line method using the rectified 
photographs appeared quite satisfactory with¬ 
out fundamental change. 

Description of the Method 

No mapping method consists of a specific set 
of procedures which is applied unchanged to 


every job. The instruments and procedures 
which constitute a method are actually rather 
general techniques for performing the various 
operations necessary to mapping, and their 
specific application may be varied from time to 
time to meet changing conditions. To describe 
a method, therefore, it is necessary to describe 
it with respect to some application, but it is ob¬ 
viously valueless to select a concrete example so 
that procedures may be described explicitly, 
since the same conditions may never arise 
again. 

In the following description of the MFA 
method the application in mind is topographic 
mapping, at whatever scale may prove feasible 
from the wing pictures of trimetrogon photog¬ 
raphy, of an area containing suitably located 
elevation control. In mapping from such pho¬ 
tography the vertical photograph also would 
be used. But this method is concerned only 
with oblique photographs; the verticals would 
be treated according to any of the existing ver¬ 
tical methods. The operations are: 

1. Photography . Strips of 60-degree oblique 
photographs, taken with a 6-in. camera on 
9x9-in. film. The principal planes are parallel, 
and are perpendicular to the line of flight. 
Spacing of exposures along the line of flight is 
the same as the spacing for verticals with 55 to 
60 per cent overlap, but flight lines are spaced 
three to four times the flight altitude. 

2. Preparation. The negatives are processed 
and indexed by standard procedures. 

3. Rectification. Photographs are rectified 
according to one of the procedures described 
later in this section. Two rectified photographs 
are produced: one consisting of several prints 
of different parts of the photograph, for plot¬ 
ting; the other consisting of one large paper 
print, for control. 

4. Control. Minor control points are located 
by a radial control plot, using any one of the 
standard methods for radial control, and using 
rectified photographs. The details of procedure 
are affected by two characteristics of oblique 
photographs: 

a. The nadir point is not in the image area 
of the photograph. It must be located 
by using the accompanying vertical 
photograph, or by trigonometric com- 




TECHNICAL INVESTIGATIONS 


193 


putation if there is no vertical photo¬ 
graph. 

b. The nadir points of photographs in the 
adjacent strip appear, and should be 
used. Such cross-strip azimuth lines 
strengthen the plot. 

5. Plotting. Because of the great relief dis¬ 
placement in oblique photographs, both planim¬ 
etry and contours are plotted in succeeding 
operations in the plotter developed for the pur¬ 
pose. This and its use are described in this 
section. 

6. Preparation of the Manuscript. The plots 
from the several pairs of photographs are all 
at slightly different scales. They are brought to 
the scale of the control plot and compiled ac¬ 
cording to standard procedures. 

The rectification and plotting techniques 
which were developed to utilize high oblique 
photographs are radically novel. The other op¬ 
erations may all be performed using standard 



Figure 5. Change of tilt angle in rectification. 


techniques, and there is no need to discuss them 
further here. The techniques for rectification 
and plotting, and some of the theory, are de¬ 
scribed below. 

Rectification of High Oblique Photograph 

Principles of Rectification. The most elemen¬ 
tary principle of rectification is illustrated in 
Figure 1, by the projection of plane II through 
L onto plane III. The negative is projected 
through a perspective point onto a horizontal 
plane with all parts in the same relative posi¬ 
tions as at the time of exposure. 

This arrangement was used in the pinhole 
cameras but is never used when a lens serves 


for projection. Since plane III is not at an infi¬ 
nite distance, the focal length required of the 
lens changes with change in tilt, and this is im¬ 
practical. It was mentioned in Section 3.3.1 
that a perspection is not uniquely determined 
by a given object and image, but that one de¬ 
gree of freedom remains. This is represented 
in Figure 5. 

Plane II, L, and plane III are here in the 
same positions as in Figure 1. If plane II is ro¬ 
tated about S' to any other position, and L is 
rotated about K through the same angle to L', 
then the projection of II' through L' on III 
gives identically the same image on III as the 
projection of II through L on III. It is easily 
seen that j, H , and S' remain in the same rela¬ 
tive positions on II' as on II, and according to 
one of the properties stated in Section 3.3.1 this 
is sufficient to assure that the perspections are 
the same. 

In order that II or II' focus on III with a 



Figure 6. Illustrative two-stage rectification. 


lens at L or L', in the position drawn in Figure 
5, the axis of the lens must lie on Lj or L'j'. It 
is obvious that the focal length required 
changes with change in t, even though the per¬ 
spection is the same. 

In practice this property is used inversely, 
so that a fixed focal length may be used for dif¬ 
ferent perspections by using in the rectification 
an angle t' different from the angle t represent¬ 
ing the tilt at which the photograph was taken. 
Rectifying cameras are built with freedom to 
change the angle between the object and image 
planes, and for any particular photograph the 
camera is adjusted so that 

F esc T = f esc t. 






194 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


where F is the perpendicular distance from the 
lens to the object plane in the rectifier, T is the 
angle between object and image planes of the 
rectifier, / is the principal distance of the aerial 
photograph, and t is the tilt of the aerial photo¬ 
graph. In fact, in practical design this property 
is used still more intensively, so that the angle 
T seldom even approximates t. In order that 
good process lenses of long focal length and 
narrow field may be used, most rectifying 
cameras are designed with F and T several 
times larger than / and t. 

The negative is placed on the object plane of 
the rectifier so that its isocenter and horizon 
coincide with the isocenter and horizon of the 
rectifier, and the desired perspection is thereby 
achieved. The isocenter of the rectifier is a 
point on its object plane having with respect 
to the image plane the same angle-preserving 
property as j in Figure 1. The horizon of the 
rectifier is a line on its object plane having with 
respect to the image plane the same property 
as H in Figure 1, namely, that there is no image 
of H on plane I (or plane III). 

This presentation of the principles of rectifi¬ 
cation is different from that usually given. 
Ordinarily the subject is treated trigonometri¬ 
cally, with reference to obtaining the correct 
distance between points on the rectified image. 
Although not detailed, the treatment here has 
been with respect to the angle-preserving prop¬ 
erty of the isocenter and the lack of image (or 
zero scale) at the horizon. While this theory is 
obvious once it has been stated, the theoretical 
contributions to the theory of rectification came 
from explicit recognition of the following 
simple facts. 

1. On the object plane of a rectifier there ex¬ 
ist a point and two lines having special proper¬ 
ties with respect to the image plane, namely, 

an isocenter (conformality), 

a horizon (scale = 0), 

an isoline (scale = 1). 

2. On an oblique aerial photograph there ex¬ 
ist similar quantities having the same proper¬ 
ties with respect to the datum plane (but re¬ 
duced in size) or to the desired rectified photo¬ 
graph. A necessary and sufficient condition for 
obtaining a rectified image at a desired scale 
is that the isocenter, horizon, and isoline of the 


photograph be placed in coincidence with the 
corresponding quantities on the object plane of 
the rectifier. 

3. The process for rectification is quite im¬ 
material, and may use any means so long as it 
provides an object plane and image plane, 
makes a straight line in the object plane appear 
as a straight line in the image plane, and pro¬ 
vides on the object plane an isocenter and hori¬ 
zon. The existence of an isoline is implied by 
the other conditions. 

The practical importance of these facts lies in 
item (3). If a process can be devised having a 
sufficient amount of freedom in the steps be¬ 
tween the object and image planes, then by 
proper use of this freedom it might be possible 
to reduce distortion. Ordinary rectification has 
only one degree of freedom, and this is used to 
permit a fixed focal length. Multiple-stage rec¬ 
tification has an unlimited number of degrees 
of freedom, and it turns out that these may be 
used to reduce distortion. 

Multiple-Stage Rectification. The process 
which was devised has been called multiple- 
stage rectification, since in it an oblique photo¬ 
graph is subjected to several photographic pro¬ 
jections in succession. The image in one step is 
used as the object in the next step, and the final 
image is a rectified photograph. The object and 
image planes in the projections may be either 
parallel, as in a ratio camera, or inclined, as in 
a tilt camera. A simple example of this is 
illustrated in Figure 6 showing two successive 
tilt projections. 

An object (photograph or negative) is 
placed on 0 1 and photographed on h. This 
image is developed, placed on 0 2 (the relative 
positioning between I ± and 0 2 must be specified 
as part of the process), and photographed on 
/ 2 . A point at a on 0 1 appears eventually at c 
on 1 2 . 

Except in a certain special case, there always 
exist a point and a line on the first object plane 
having the properties of isocenter and horizon 
with respect to the last image plane. In Figure 
6, for example, by starting with a line through 
L 2 parallel to / 2 , and tracing backward, the line 
h 0 1 is found. This line obviously has no image 
on I 2 . Only if, in transferring h o2 to I lf h n were 
to fall at the point k on 7i would there be no 




TECHNICAL INVESTIGATIONS 


195 


line h 0l on Ox. This occurrence gives rise to the 
special case and in this case the transformation 
between the first object plane and the last 
image plane is an affine projection, rather than 
a general perspection. 

If a horizon exists on 0 lt then a unique iso¬ 
center also exists. This is more difficult to show 
than is the existence of a horizon, and will not 
be attempted in this summary report. Its exist¬ 
ence is shown in the literature. 4 * 5 In Figure 6, 
j 01 is the isocenter. While not a conformal point 
for either perspection separately, it is con¬ 
formal for the combination. The lack of con¬ 
formality in each of the two steps cancel. Since 
this process provides object and image planes, 
and a horizon and isocenter on the object plane, 
it is a rectification process. 

Applications. Several advantageous applica¬ 
tions of these principles were worked out. In 
these applications it would have been possible 
to permit the inclination of object and image 
planes in the several instruments to be varied, 
thereby varying the distance from isocenter to 
horizon in order to accommodate photographs 
with different tilts, as is done in single-stage 
rectification. For practical reasons, however, it 
was decided to build the tilt cameras with fixed 
angles, and to vary the isocenter-to-horizon dis¬ 
tances of the photographs themselves by en¬ 
largement or reduction before rectification. 
This is really a further application of the prin¬ 
ciples already stated, and the entire process 
may be considered a multiple-stage rectification 
process, in which the first step is a variable 
ratio printing. By this process the rectified 
photographs would all come out at different 
scales, and hence an additional variable ratio 
printing must be added as a last step. 

1. One tilt projection. The distortion charac¬ 
teristics and field of the rectifying Hypergon 
lens were so good that it could be employed in a 
rectifying camera of standard type for 6-in., 
60-degree oblique photographs. Limitations 
would be imposed on the scale ratio during rec¬ 
tification; in particular, the field, even of the 
Hypergon, was not great enough to permit a 
sizable reduction during rectification. The rec¬ 
tified image of the useful portion of a high 
oblique photograph, if not reduced in scale, 
measures about 30x40 in., which is too big to 


reduce conveniently photographically. Since 
freedom to vary the scale was essential, the 
standard type of rectification was not pursued 
in this project, but was applied in a slightly 
different fashion requiring three photographic 
steps, only one of which is a tilt projection. The 
steps are: 

a. Reduction of normally about 3X, hut 
variable to adapt the perspective index 
of the reduced print to that of the rec¬ 
tifying camera. 

b. Rectification at 1/1 scale in a single 
stage, in a fixed rectifying camera de¬ 
signed for photographs of 60-degree 
tilt and 2-in. principal distance. 

c. Enlargement or further reduction of 
the rectified photograph to desired 
scale. 

This process requires a variable ratio camera 
adaptable to double use, or two such cameras, 
and a single-stage fixed rectifier designed for 
1/1 rectification of reduced high oblique photo¬ 
graphs. In both this process and process (2) 
below the variation in tilt of the aerial photo¬ 
graphs to be processed is taken care of by vary¬ 
ing the initial reduction or enlargement rather 
than by varying the angle of the tilt camera. 
These instruments were constructed and are 
described later. 

2. Two tilt projections. This application con¬ 
templates five photographic steps, which are: 

a. Ratio printing at normal 1/1 ratio, but 
variable to adapt the perspective index 
of the print to that of the rectifying 
camera. 

b. and c. Rectification in two stages of tilt 
projection, both of them identical and 
performed by two successive projec¬ 
tions in the same fixed-angle tilt cam¬ 
era. During the rectification a reduc¬ 
tion of 3X is imposed. 

d. Contact print, for reversal. 

e. Enlargement or further reduction of 
the rectified print to desired scale. 

While more steps are required in this process, 
it is preferred to process (1) for several rea¬ 
sons. The conditions under which the reduc¬ 
tion is imposed are better for preserving image 
quality. The lens is larger, and in general tol¬ 
erances are less critical. Light enters the image 



196 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


plane much less obliquely, thereby increasing 
the illumination and reducing the effect of non¬ 
uniformity over the image surface. 

The process requires a variable ratio camera 
as in process (1), and a two-stage, fixed-angle 
rectifying camera designed for rectifying 6-in., 
60-degree oblique photographs, with 3X reduc¬ 
tion during rectification. These instruments 
were constructed and are described later. 

3. Three tilt projections. For achieving accu¬ 
rate rectification in practice the preceding 
methods have two faults. First, identification 
and measurement of points for determining 
tilt must be done on the oblique photographs. 
As remarked in Section 3.3.4, this operation is 
less accurate than identification on vertical or 
rectified photographs. Second, several steps in¬ 
tervene between the determination of tilt and 
the ultimate rectified photograph. Accidental 
errors occurring in the intervening steps will 
accumulate in the rectified photograph. 

The following procedure was therefore de¬ 
vised to permit determination of tilt on a nearly 
rectified photograph immediately preceding the 
final step. Three photographic steps are re¬ 
quired : 

a. and b. Approximate rectification of the 
high oblique photograph, using a rough 
tilt determined from the horizon. This is 
done in two stages on the two-stage rec¬ 
tifying camera of process (2). The per- 
spection is so chosen that little tilt re¬ 
mains in the approximately rectified pho¬ 
tograph, and the isocenter of this photo¬ 
graph with respect to the datum plane has 
been moved from near one edge to the 
center of the image area. This permits the 
photograph to be treated further in a rec¬ 
tifier of the type ordinarily used with 
near-vertical photographs. Accurate re¬ 
sidual tilt is determined by identification 
and measurement on this photograph. 

c. Final rectification and enlargement or re¬ 
duction to desired scale simultaneously on 
a variable-angle, small tilt rectifier of 
standard design. 

This application requires a near-vertical rec¬ 
tifier of standard design and a two-stage fixed- 
angle rectifier similar to that required in 
process (2) but designed to use aerial roll film 


in the first stage. A variable ratio printer is not 
used. These instruments were not made, but the 
method was tested experimentally at Aero 
Service, using the existing two-stage rectifier 
and a Brock rectifier and Brock enlarger. 

Instruments 

The following instruments, necessary for 
testing and demonstrating the methods of recti¬ 
fication described above, were constructed: 

Variable Ratio Printer. This camera, for use 
in the first and last steps of methods (1) and 
(2) above, was designed and constructed by 
Aero Service Corporation. It is illustrated in 
Figures 7 and 8. 

Item A is the light box, containing four 100-w high- 
pressure air-cooled mercury-vapor arc lights diffused 
by opal glass. For the first step in each method the 
object is an aerial negative. The roll of film negatives 
passes from one spool to the other, and is held, one 
frame at a time, between pressure plates in the object 
plane B. The image for method (2) is received on a 
14xl7-in. glass plate C. For method (1) the glass plate 
at C is replaced by an adapter carrying a lantern slide 
plate. This adapter was not constructed. Means are 
provided for positioning all parts accurately. 

For the last step in each method, the object is a 
portion of a 14xl7-in. glass plate carrying the rectified 
image. It is inserted in the slot D, and may be posi¬ 
tioned so that any small portion lies in the field of the 
lens. The image is received on paper held between two 
glass plates at C. 

One-Stage Fixed Rectifier. This camera, for 
use in method (1), was designed by MFA and 
constructed in the shop of the Mount Wilson 
Observatory. It is illustrated in Figures 9 and 
10 . 

Item A is the light box, containing one 250-w medium- 
pressure air-cooled mercury-vapor arc light, which is 
condensed on the lens by an elliptical reflector. The 
object is a lantern slide at B. The image is received 
on a 14xl7-in. glass plate resting on the eight pads C, 
and clamped there by opposing pads on the cover D. 
The Hypergon lens, 1.484-in. focal length, //30, is seen 
at E. 

The object is positioned automatically by three stops 
against which the edges of the object plate rest. These 
stops are duplicated on the image plane of the variable 
ratio printer, where all positioning of the image is done. ^ 

The perspective index of this camera is 2.297 
in. The preliminary reduction must be such as 
to reduce the perspective index of the aerial 
photograph to this value. 



TECHNICAL INVESTIGATIONS 


197 


Two-Stage Fixed Rectifier. This camera, for 
use in method (2) was designed by MFA and 
constructed in the shop of the Princeton Uni¬ 
versity Physics Department. It is illustrated in 
Figures 11 and 12. 

Item A is the light box, containing two 250-w medium- 
pressure air-cooled mercury-vapor arc lights diffused 


thereby carrying position from the first to the second 
stage of rectification. The latter set of stops may be seen 
at E. 

The perspective index of this camera is 7.072 
in. The preliminary enlargement or reduction 
necessary for the use of this camera in rectifica¬ 
tion method (2) must be such as to bring the 




Figure 7. Variable ratio printer—object end. 

by opal glass. The object is a 14xl7-in. glass plate at B, 
clamped against eight pads lapped to a plane. The 
image is received on another 14xl7-in. glass plate held 
similarly at C. The Hypergon lens, 4.402-in. focal 
length, may be seen at D. 

The position of the object for both the first and 


Figure 8. Variable ratio printer—side view. 

perspective index of the aerial photograph to 
this value. As an illustrative example, the scale 
changes necessary for photographs whose prin¬ 
cipal distance (f) is 6 in. are tabulated below 
for several different tilts. 



Figure 9. One-stage fixed rectifier—image end. 

second stages is determined by three stops against 
which the edges of the plate rest. These stops are 
duplicated on the image plane of the variable ratio 
printer, where initial positioning of the image on the 
glass plate takes place, and serve to carry this position 
over to the object plane of the rectifier. The stops are 
also duplicated on the image plane of the rectifier, 



Figure 10. One-stage fixed rectifier—side view. 


Perspective 


Tilt 

Index 

Enlargement 

t 

Q — f esc t 

E = 7.072/Q 

55° 

7.325 

0.96550 

57° 

7.154 

0.98850 

58°2'22" 

7.072 

1.00000 

60° 

6.928 

1.02075 

62° 

6.796 

1.04075 


























198 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


Orthographic Plotting from Rectified 
Oblique Photographs 
Principle . The principle of stereoscopic plot¬ 
ting has been described in Section 3.3.2. If the 
photographs are rectified and are both at the 
same scale, the plotting instrument becomes 



Figure 11. Two-stage fixed rectifier—image end. 

much simpler. In addition, the photographs 
need not be viewed from their perspective 
centers. They may be viewed from any points 
perpendicularly over the nadir points, the same 
distance above each photograph, and this will 
only change the vertical scale of the floating 
mark’s motion. 

For rectified high oblique photographs, exist¬ 
ing plotters giving orthographic projection 
were unsatisfactory because of the require¬ 
ment that the points of view be over the nadir 
points of the photographs. Viewing from this 
position would lose a considerable part of the 
value of rectification, since the angle subtended 
at the perspective center of points on a photo¬ 
graph is not changed by rectification. The ap¬ 
parent scale in the background would then be 
very much smaller than in the foreground, 
with resultant loss of accuracy in plotting. It is 
desired to view the photographs from points as 
nearly as possible over the points being plotted, 
or over the center of the area of the image 
being plotted, in order to obtain uniform appar¬ 


ent scale in viewing all parts of the rectified 
photographs. 

It was found by analysis that the projection 
is orthographic if: 

1. The pair of rectified photographs is ori¬ 
ented for plotting as if they were to be viewed 
from the perspective centers (see L, Figure 
13). 

2. The points of view are then moved paral¬ 
lel, while maintaining the same separation, to 
any new positions (see L', Figure 13) ver¬ 
tically over points m on the photographs. 

3. A plotting principle is used, similar to that 
for orthographic projection, but in which the 
horizontal position of the stylus is unchanged 
when the reticles move in the direction L'n y 
rather than when they move vertically. 

For an experimental model of such a plotter 
an equivalent principle was used, namely, the 
stylus was moved horizontally as the reticle 
marks were raised vertically to pass from one 
contour to another. 

Orthographic Plotter for Rectified Oblique 
Photographs. An experimental plotter employ¬ 
ing the principle described in the preceding 
paragraphs was designed by MFA and con¬ 
structed by Chicago Aerial Surveys. It is illus¬ 
trated in Figures 14 and 15. 



Figure 12. Two-stage fixed rectifier—side view. 

Item 4 is a mirror stereoscope with two “photo¬ 
graphs” in position below it. The reticle marks B may 
be moved vertically on posts C. The stylus is at D and 
may be moved radially on the track shown. The direc¬ 
tion of the track may be changed. Item E is a parallel 
motion device, operating on the usual parallelogram 
principle, to permit only parallel motion of the pair 
of reticle marks horizontally. 

Complete adjustments are provided for setting the 
reticle marks parallel to a line joining the perspective 








TECHNICAL INVESTIGATIONS 


199 


centers, for changing their separation with different 
pairs of photographs, and for accommodating differ¬ 
ences in operators’ interocular distances. 

Partial Test of the Proposed Method 

Since many of the operations outlined in the 
brief description of the proposed mapping 
method given earlier in this section are to be 
accomplished by standard procedures, it was 
not necessary, nor was it feasible with the 
limited equipment available, to demonstrate the 
complete method during the experimental 
phase of the development. The novel proce¬ 
dures, consisting of the rectification and plot¬ 
ting steps, were tested on a single pair of pho¬ 
tographs, selected from those taken for the 


The rectified prints were contoured in the ex¬ 
perimental orthographic plotter and compared 
with an existing map of the region by means 
of profiles. Substantial agreement was found 
to within the usual mapping tolerance of 0.02 
in. in horizontal position and one-half contour 
in elevation. No systematic differences ap¬ 
peared, and hence the theory of the method 
was substantiated. Quantitative measurements 
of local accuracy could not be derived, but it 
was found in contouring that differences in ele¬ 
vation could be readily distinguished to permit 
mapping with a contour interval %oo of the 
flight altitude. This is in the region of lower 
accuracy when compared with mapping from 
vertical photographs, which permit contouring 





front elevation side elevation 


Figure 13. Change of viewpoint for plotting rectified obliques. 


studies of photographic quality (see Section 
3.3.3). They were taken with a K-17 camera, 
6-in. Metrogon lens, on infrared film at 65- 
degree tilt and 5,000 ft altitude. They are re¬ 
markably clear in the background. The terrain 
is rugged, cut by many almost vertical canyons. 

The photographs were rectified by a modifi¬ 
cation of rectification process (3). Since the 
two-stage rectifier did not accommodate film 
negatives, contact prints of the negatives on 
14xl7-in. glass plates had first to be made. 
These were rectified approximately on the two- 
stage rectifier. Since an appropriate instrument 
was not available for the last step, the final 
rectification was done on the Brock equipment 
at Aero Service Corporation. The Brock rec¬ 
tifiers project only at 1/1, and hence the re¬ 
quired scale change was imposed in the Brock 
enlarging camera. Thus, five photographic 
steps were used instead of the three proposed, 
but the photographic quality was still good. 


at y 3 oo to Ys oo of the flight altitude, depending 
on the method used, but is good for the first run 
of a new method with high oblique photo¬ 
graphs. 

Since the rectification process is practically 
free of distortion, horizontal accuracy can be 
maintained, certainly at the negative scale and 
possibly at larger scales, depending on the por¬ 
tion of the negative used for mapping. 

Plans were laid for an intensive test of the 
complete method, in cooperation with the Hy¬ 
drographic Office and other government map¬ 
ping agencies. NDRC was to complete the de¬ 
velopment of the instruments, adapting the 
two-stage rectifier for roll film and producing 
an improved plotter. The Hydrographic Office 
was to provide space and additional equipment, 
and all cooperating mapping agencies would 
loan personnel. This plan was dropped at the 
end of the war and the termination of NDRC 
contracts. The Office of Research and Inven- 






200 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


tions, Navy Department, has indicated a desire 
to take over the development and continue the 
original plan. 

3,3,8 Two-Camera Method for Water 
Depth Determination 

One problem of great importance in the 
preparations for amphibious assault operations 


water, coral analysis, wave formation, and 
several photogrammetric methods. The con¬ 
tractors under Project NA-124 were requested 
to give whatever assistance they could to these 
developments. 

All of the photogrammetric methods depended 
on photographing the bottom of the water from 
two or more positions and determining the ele¬ 
vation of the bottom by the usual stereoscopic 
procedures or computational or graphical equiv- 



Figure 14. Plotter—top view. 


was the accurate determination of inshore 
water depths by means other than soundings. 
In the spring of 1944 this problem was in the 
early stages of solution. Many methods had 
been proposed, including color analysis of the 


alents. The most practical method employed 
stereoscopic depth measurements on strip pho¬ 
tographs taken with the Sonne stereostrip cam¬ 
era at altitudes of 100 to 500 ft, the surface of 
the water being used as a plane of reference. 
















TECHNICAL INVESTIGATIONS 


201 


This method was under development by the 
Photographic Interpretation Center. 

The stereostrip camera is essentially a 
double-strip camera so arranged that one cham¬ 
ber photographs the ground slightly ahead of 
the airplane and the other chamber slightly 
behind the airplane. On such photographs, 
taken from low altitude and exposed almost 
simultaneously, both the surface and the bot¬ 
tom of clear water can be seen stereoscopically, 
the wave reflections and shadows forming pat¬ 
terns excellent for stereoscopic fusion. The two 
strips viewed simultaneously give a stereo¬ 


airbase and relative orientation. A preliminary 
study was made to determine whether the cam¬ 
eras could be mounted in the wings of a F-6-F. 
A method of calibrating the installation and 
computing depth from the stereoscopic meas¬ 
urements was devised, and the errors to be ex¬ 
pected, chiefly from bending of the wings, were 
analyzed theoretically. This material was 
turned over to the Photographic Interpretation 
Center, with a recommendation that a trial in¬ 
stallation be made to determine the amount of 
wing bending to be expected and to discover 
any other source of error. 


A 



Figure 15. Plotter—front view. 


scopic model for which the airbase (line join¬ 
ing the two viewpoints for any particular 
image) is in the line of flight of the airplane. 

Difficulty in computing depth arises because 
the length of the airbase and the relative orien¬ 
tation of the two views change continually 
with changes in altitude and pitch of the air¬ 
plane. Errors in film-speed setting due to ig¬ 
norance of true altitude and ground speed also 
lead to unknown errors in the depth determina¬ 
tion. To overcome these difficulties MFA pro¬ 
posed that two single-strip cameras be mounted 
in the wings of an airplane, thereby fixing the 


The suggestion was well received, and 
preparations were made by the Photographic 
Interpretation Center for a trial of the wing 
installation. However, by this time (the end of 
August 1944) the stereostrip camera method 
had been tested and found to work satisfac¬ 
torily in trial runs. A party was soon to be sent 
out to the Pacific to apply the method, and it 
was considered desirable to await reports on 
their success before giving high priority to a 
new development. 

Preparations for the installation of wing 
cameras proceeded slowly, with low priority. 













202 


MAPPING METHODS EMPLOYING HIGH OBLIQUE PHOTOGRAPHS 


throughout the year. At the end of January 
1945, drawings and a mockup had been com¬ 
pleted, and the final installation was ready to 
be made. This installation was delayed by work 
of higher priority, and was suspended indefi¬ 
nitely at the end of World War II. 

s 4 RECOMMENDATIONS BY NDRC 

It is not easy to foresee what map informa¬ 
tion will be required by the Services in the 
future. However, it seems clear that it will be 
desirable for the Army and Navy to have avail¬ 
able as wide coverage as possible at a scale of 
1/20,000, with means ready at hand for pro¬ 
ducing maps and charts at larger scales of 
selected regions for special purposes. 


3,41 General Recommendations 

1. Both the Army and Navy should maintain 
mapping organizations with well-trained per¬ 
sonnel during peacetime. In addition, at least 
part of the mapping work should be subcon¬ 
tracted to civilian agencies (both government 
and private) to maintain the organization and 
working relations which would prove invalu¬ 
able if these agencies should be called upon to 
map for the Services in another emergency. 

2. The Navy should establish and maintain 
small, fully equipped mobile units, specifically 
for mapping, and should keep them well 
trained. 

3. Cartography should be completely sepa¬ 
rated from photographic reconnaissance. Any 
duplication in photography will be more than 
counterbalanced by the saving achieved in 
using photographs taken specifically for map¬ 
ping. Mapping pilots and photographers should 
be selected and specifically trained for that 
task, and that should be their primary occupa¬ 
tion. 

4. The Federal Government should partici¬ 
pate in the technical developments relating to 
mapping, and should provide financial support 
for private development. The situation is dif¬ 
ferent from that in other industries where 
large production permits private concerns to 


set aside adequate funds for research. The 
major part of the map production and related 
activities in the United States is carried on by 
the Federal and State Governments, with no 
provision for technical development. All de¬ 
velopment to date has arisen: 

a. In Government bureaus, incidentally, 
in the course of production, without 
special planning or provision for this 
part of the work. This is notably ineffi¬ 
cient and not very effective. 

b. In private concerns which, because of 
their small share in production, cannot 
devote sufficient funds to carry on the 
development needed. 

c. In universities, by men not well ac¬ 
quainted with or directed to practical 
production problems. 

d. By the Federal Government, during 
World War II (but not now), through 
the NDRC. 

These sources of development work are quite 
insufficient to provide the advancement in 
theory and technique that are now needed and 
possible in photogrammetry. This is particu¬ 
larly true at this time when progress has been 
made in so many other fields, such as radar, 
which should be applied to mapping, but whose 
further development for such a special use will 
undoubtedly be expensive. 

3 ’ 4 ' 2 Technical Recommendations 

1. The developments mentioned in Section 
3.2.3 should be prosecuted. These are: aerial 
triangulation, photogrammetric sounding, and 
wide-field photogrammetry. Aerial triangula¬ 
tion may prove particularly applicable to the 
problems concerned with guided missiles. 

2. Mapping photographs should be treated 
as engineering data, not merely as pictures. 
Precision equipment, materials, and processes 
should be used throughout. Attention should be 
given to precision cameras and photographic 
materials, to special purpose emulsions, and to 
processing techniques for maintaining accu¬ 
racy. 

3. Carefully planned production tests should 
be conducted on the Merrill Flood and Associ¬ 
ates method for mapping from high obliques 





RECOMMENDATIONS BY NDRC 


203 


when all necessary equipment has been made 
available. Such a test should make it possible 
to evaluate the usefulness of the process, in 
comparison with mapping from verticals, for 
various applications. 

4. The two-camera method of determining 
underwater depths should be tested by complet¬ 
ing the installation in the wings of the F-6-F 


which has already been undertaken or by mak¬ 
ing a similar installation in another airplane. 

5. Lenses of the highest possible quality 
should be used in all mapping operations. The 
many factors discussed in Chapters 1 and 2 
which affect resolution of photographs should 
be given full consideration in selecting equip¬ 
ment for mapping. 




Chapter 4 

OPTICAL TESTING METHODS 

By Roderic M. Scott a 


*.i INTRODUCTION 

A t the request of the Frankford Arsenal, 

L NDRC undertook a study of optical test¬ 
ing methods in 1943, under Project OD-138. 
The request was primarily for a survey and 
evaluation of the methods of optical inspection 
which were employed at that time. Under Con¬ 
tract OEMsr-1197, members of the Department 
of Physics of the Pennsylvania State College 1 
were to prepare a report describing inspection 
methods, assessing them, and recommending 
any changes considered to be desirable by the 
surveying group. The development of new 
equipment and methods of inspection was to be 
a second, but equally important, part of the 
undertaking. Both government agencies and 
private contractors engaged in the manufac¬ 
ture of optical components and instruments 
were included in the survey. The requested sur¬ 
vey has been made 2 and has shown that many 
of the current methods of optical inspection are 
to a large degree dependent on the subjective 
reactions of the inspector. It was therefore one 
of the primary recommendations of the survey 
group that new methods be developed which 
would be, as far as possible, impersonal and 
which would yield quantitative results upon 
which the quality of the optics under inspection 
could be assessed. As a result of this recom¬ 
mendation, several new inspection instruments 
are now employed which reduce the effect of 
personal judgment to a minimum. 

One of the most important developments de¬ 
signed to aid in optical inspection is that of a 
device which measures the angular resolution 
of a telescopic system. This device is now exten¬ 
sively employed to control the production of 
optical instruments for military purposes. It is 
known as the kinetic definition chart [KDC] 
apparatus . 3 The apparatus was an outgrowth 
of the work of Fabry. 4 A test object made up 
of a number of parallel black lines is viewed 
a The Sharpies Corporation. 


first with the unaided eye or through a tele¬ 
scope, and then with the eye or the same tele¬ 
scope plus the instrument being inspected. The 
test object is moved toward or away from the 
observer until the parallel lines are just re¬ 
solved. The distance to the object viewed with 
the test telescope should be equal to the mag¬ 
nification of the telescope times the distance 
to the test object viewed without the telescope. 
The percentage ratio of the observed distance 
to the theoretical distance is called the KDC 
efficiency. It was established that the efficiency 
was nearly independent of the auxiliary mag¬ 
nification over a wide range. The results given 
by this instrument appear to be almost com¬ 
pletely impersonal. 

It seemed possible that the resolving power 
of the eye might affect the results of an appa¬ 
ratus such as the KDC machine. In view of the 
intimate connection between the optical proper¬ 
ties of the eye and instruments which it is used 
to inspect, a study was made of the resolving 
power of the eye as a function of pupil size. 
Further carefully controlled measures should 
be made. 

The Michelson-Twyman interferometer has 
undergone considerable development 5 and study 
to determine its usefulness as an inspection in¬ 
strument. The observed field in the interfer¬ 
ometer consists of an array of dark bands 
whose number and shape are related to the 
optical performance of the lens, prism, or in¬ 
strument under observation. Methods for com¬ 
puting the form of the interference pattern for 
a perfect example of the device or component 
under test have been described in the literature. 
Although the interferometer was very little 
used before World War II, many optical manu¬ 
facturers now find that it is a great aid for in¬ 
spection purposes. 

Another device developed at Penn State is an 
apparatus for the study of distribution of light 6 
in the image formed by an optical system. Its 
usefulness in the evaluation of an optical design 


204 



SURVEY OF OPTICAL INSPECTION METHODS 


205 


is second only to direct ray tracing whether by 
computation or by the Hartmann method. The 
technique was originally developed by the East¬ 
man Kodak Company for the evaluation of 
camera lenses and has been extended to general 
optical instruments. Both a photographic 
method and a recording photoelectric method 
have been used. 

Two small but useful inspection devices de¬ 
veloped by the group are the dioptometer 7 and 
the proboscope. The dioptometer is a small 
telescope with adjustable spacing between the 
objective and an eyepiece equipped with a 
reticle. It is used to measure the degree of di¬ 
vergence or convergence of light at some point 
in an optical system. It is very useful for meas¬ 
uring spherical aberration and the parallelism 
in sighting telescopes. The proboscope consists 
of a group of supplementary lenses which, 
when slipped over the end of a telescope or 
other optical system, bring to sharp focus the 
surfaces of each interior optical component in 
turn. This device makes easy inspection of in¬ 
dividual lenses and prisms for surface defects 
and cleanliness without requiring the disassem¬ 
bly of the instrument. 

As an adjunct to the KDC equipment, an arti¬ 
ficial sky apparatus was developed to permit 
the study of the effect of scattered light upon 
the resolving power of optical instruments. 8 In 
addition, measures of scattered light may pro¬ 
vide a means for determining the conformity to 
specifications of striated glass. The combina¬ 
tion of KDC apparatus, artificial sky apparatus, 
and a holder for the pieces of glass under test 
has been called the striaescope. 


42 SURVEY OF PRE-WAR AND CURRENT 
OPTICAL INSPECTION METHODS 

The primary task of the Pennsylvania State 
College group was to examine, evaluate, and re¬ 
port upon the methods of inspection of optical 
devices and components which were used in 
1944 to 45. The survey covered methods used 
both in production plants operated by the gov¬ 
ernment and by a large number of contractors 
for the production of military optics. The study 
included the specifications controlling the pro¬ 


duction of optical glass, components and fin¬ 
ished instruments, as well as the effectiveness 
of inspection methods which are used to ascer¬ 
tain the degree of compliance with the specifi¬ 
cations. 

It is the purpose of the specifications to fix a 
level of quality that an instrument must possess 
in order to permit an observer to receive the 
benefit of all the performance inherent in the 
design. The specifications must be sufficiently 
rigid to insure this, but they must not be more 
rigid than is required. If they are placed higher 
than necessary, production is slowed and many 
useful parts are discarded. Specifically, the spec¬ 
ifications should not ordinarily include re¬ 
quirements intended merely to improve the ap¬ 
pearance of an instrument if they do not also 
improve the performance. 

In conducting the survey, each specification 
was examined in detail and each inspection 
method and inspection device was studied un¬ 
der production conditions. It was hoped that the 
survey would produce material for an inspec¬ 
tion manual covering satisfactory specifications 
and methods as well as new and improved spec¬ 
ifications and instruments. 

4,21 Optical Glass 

Index and Dispersion 

The joint Army-Navy Specification JAN-G 
174 sets forth the specifications and tolerances 
on the optical constants of glass used for mili¬ 
tary optical purposes. For such purposes glass 
is procured in three physical forms: slabs, 
molded blanks, and chunks. Very little use of 
chunk glass was encountered. Table 1 sets forth 
the indices, partial dispersions, and tolerances 
for the types of glass procured by the Govern¬ 
ment. In addition to physical constants, the 
specifications on optical glass prescribed definite 
sampling techniques. If the melt can be identi¬ 
fied, one specimen from each melt is to be ex¬ 
amined. In the case of slab glass produced by 
a continuous process, and for lots for which the 
melt is not identified, five random samples are 
to be selected by the inspector. In the case of 
slabs and chunks, if the sample is found to 
differ from the specification by 70 per cent of 
the tolerance, several additional samples must 



206 


OPTICAL TESTING METHODS 


be selected. If any of these additional samples 
fail to meet the specifications, then twice this 
additional number are to be examined. In the 
case that any of the latter fail, the entire lot 
must be rejected. For molded blanks, the sched¬ 
ule of selection is dependent upon the size of 
the lot. If a lot consists of twenty-five or less, 
all must be examined, whereas if there are over 
one hundred, a selection of thirty-five suffices. 
In this case a single failure will be cause for 
the rejection of the lot. A supplier may inspect 


spection of optical glass. The normal refrac- 
tometer is a rapid and reliable instrument, 
provided the pieces of glass are of such shape 
and surface finish as to be usable in this device. 
Some inspectors prefer the immersion method 
in which the piece of glass is immersed in a 
liquid of the correct index of refraction. Varia¬ 
tions of the index in the fifth decimal place are 
discernible. Among the liquids which are more 
commonly used are monochloronapthaline with 
n D = 1.634 and monobromonapthaline with n B 


Table 1 


Tolerances of n» Tolerances of v 



Type of glass 

Values 

(plus or minus) 

Values 

(plus or minus) 

511-635 

Borosilicate crown, BSC-1 

1.5110 

0.0010 

63.5 

0.5 

517-645 

Borosilicate crown, BSC-2 

1.5170 

0.0010 

64.5 

0.5 

513-605 

Crown, C 

1.5125 

0.0010 

60.5 

0.5 

518-596 

Crown 

1.5180 

0.0010 

59.6 

0.5 

523-586 

Crown, C-l 

1.5230 

0.0010 

58.6 

0.4 

529-516 

Crown flint, CF-1 

1.5286 

0.0010 

51.6 

0.5 

541-599 

Light barium crown, LBC-1 

1.5411 

0.0010 

59.9 

0.5 

573-574 

Barium crown, LBC-2 

1.5725 

0.0015 

57.4 

0.5 

574-577 

Barium crown 

1.5744 

0.0015 

57.7 

0.5 

611-588 

Dense barium crown, DBC-1 

1.6110 

0.0015 

58.8 

0.4 

617-550 

Dense barium crown, DBC-2 

1.6170 

0.0015 

55.0 

0.4 

611-572 

Dense barium crown, DBC-3 

1.6109 

0.0015 

57.2 

0.4 

562-510 

Light barium flint, LBF-2 

1.5616 

0.0015 

51.0 

0.4 

588-534 

Light barium flint, LBF-1 

1.5880 

0.0015 

53.4 

0.4 

584-460 

Barium flint, BF-1 

1.5838 

0.0015 

46.0 

0.3 

605-436 

Barium flint, BF-2 

1.6053 

0.0015 

43.6 

0.3 

559-452 

Extra light flint, ELF-1 

1.5585 

0.0015 

45.2 

0.3 

573-425 

Light flint, LF-1 

1.5725 

0.0015 

42.5 

0.3 

580-410 

Light flint, LF-2 

1.5795 

0.0015 

41.0 

0.3 

605-380 

Dense flint, DF-1 

1.6050 

0.0015 

38.0 

0.3 

617-366 

Dense flint, DF-2 

1.6170 

0.0015 

36.6 

0.3 

621-362 

Dense flint, DF-3 

1.6210 

0.0015 

36.2 

0.3 

649-338 

Extra dense flint, EDF-1 

1.6490 

0.0015 

33.8 

0.3 

666-324 

Extra dense flint, EDF-50 

1.6660 

0.0015 

32.4 

0.3 

673-322 

Extra dense flint, EDF-2 

1.6725 

0.0015 

32.2 

0.3 

689-309 

Extra dense flint, EDF 

1.6890 

0.0015 

30.9 

0.3 

720-293 

Extra dense flint, EDF-3 

1.7200 

0.0015 

29.3 

0.3 


Note. Each molded blank or glass plate shall be homogeneous in composition. The above tolerances are not to be construed as permitting 
any measurable variation in the refractivity between different portions of the same piece of glass. 


and resubmit any samples from a rejected lot 
which meet the specifications. 

These specifications are adequate for the 
definition and control of various types of opti¬ 
cal glass. One application was found in a wide- 
angle tank telescope, where the specification on 
the dispersion should be tightened. More de¬ 
tailed studies may reveal other cases where 
general specifications should be tightened for 
particular applications. 

Two methods are available for the rapid in- 


= 1.655. These liquids may be mixed with 
mineral oil to adjust the index to the specified 
value. 

The actual sampling technique used by vari¬ 
ous inspectors differs considerably from that 
called for by the specifications. The products of 
a new manufacturer or supplier generally re¬ 
ceive 100 per cent inspection, whereas those 
from a previously known manufacturer, whose 
record has been good, will receive only a spot 
check for record purposes. This technique is 










SURVEY OF OPTICAL INSPECTION METHODS 


207 


probably more conducive to rapid inspection 
than the one specified. It requires, however, 
considerable judgment on the part of the in¬ 
spector. 

Striae 

Optical glass is subject to many defects in¬ 
troduced during the manufacture. The most 
common defects are striae, color, and inclu¬ 
sions. Poor annealing of the glass produces ob¬ 
jectionable strain. The method of grading these 
defects during the inspection of the glass de¬ 
pends upon a comparison between the indi¬ 
vidual piece under inspection and a group of 
representative, graded samples. In the case of 
striae, the grades and their description are 
shown in Table 2. Since the requirements with 

Table 2 


1. Grade AA containing no striae, streaks, or cords, 
as defined by the contract or order under which the 
material is being procured. 

2. Grade A containing no visible striae, streaks, or 
cords, when examined by the various methods de¬ 
scribed in the specification. 

3. Grade B containing striae which are light and 
scattered when used in the direction of maximum 
visibility. Using the method described below, these 
striae are just above the limit of visibility of the 
human eye. 

4. Grade C containing striae when viewed in the 
direction of maximum visibility. It is required that 
the striae must be slight when viewed by the ac¬ 
ceptable devices described below, and must be 
sensibly parallel with the face of the plate under 
inspection. In general, it is regarded that Grade C 
glass is a form of rolled plate. 

5. Grade D containing more and heavier striae than 
Grade C, and with the striae oriented sensibly 
parallel with the face of the plate. Grade D glass 
is intended to be rolled optical glass of the poorest 
acceptable grade. 


respect to striae are more strict in some cases 
than in others, a part of the specification de¬ 
pends upon the ultimate use of the glass. Table 
3 lists the acceptable striae graded for each of 
the various optical components manufactured 
for use by the Government. In general, each 
piece of glass must be inspected for striae ex¬ 
cept in the exceptional case in which molded 
blanks have been pressed from previously in¬ 
spected glass. The specifications on striae as 
well as on other defects of this type are some¬ 
what arbitrary since no data are available to 


indicate the effect on the performance of tele¬ 
scopes and periscopes of striae in individual 
optical components. A device such as the KDC 
apparatus or the resolution striaescope should 


Table 3 


The optical part 

Acceptable striae 
grade 


A B C D 


Prisms 


Abbe 

X 



Amici 

X 



Dove 

X 



measuring wedge 

X 



Penta 

X 



Porro 

X 



Leman 

X 



rhomboid 

X 



binoculars (6x30) (7x50) 



X 

ocular 

X 



all others 


X 


Eye lens 




binocular (6x30) (7x50) 




telescopes (1-power approximately) 




all others 



X 

Reticles 



X 

Field lens 




binocular (7x50) 



X 

binocular (6x30)* 



X 

telescope, 1-power* 



X 

all others 



X 

Collective 



X 

Mirrors 




first surface 



X 

second surface 


X 


Windows 



X 

Erecting lens 




telescopes (1-power to 3-power) 



X 

telescopes (1-power approximately) 

* 


X 

Objectives 




binocular (6x30) 



X 

binocular (7x50) 



X 

telescopes (1-power to 3-power) 



X 

all others 


X 



* These may be made from Grade D when approved by the bureau 
or agency concerned. 

Note. Laps, folds, stones, or firecracks shall be limited to the 
depth of one-half of the grinding stock of the blank. In glass con¬ 
taining striae, the blanks shall be formed in such a manner that the 
path of light shall be approximately normal to the plane of the 
striae, except that for the Navy Bureau of Ordnance, no firecracks 
shall be permitted. 

be used to examine a number of telescopes made 
from components containing various degrees 
of striation to determine the relation between 
the amount of striation and reduced optical per¬ 
formance. The specification should be rewritten 
on the basis of these studies and should be based 
upon the effect on performance of the instru- 















208 


OPTICAL TESTING METHODS 


ment rather than upon the appearance of the 
glass. 

It is current practice to examine each piece 
of optical glass for striae by one of two methods. 
The direct view striaescope is a device in which 
the striations appear in the field of a collimated 

SPECIMEN UNDER 

I TEST 

o 

TOP VIEW OF STRIAESCOPE 
ILLUSTRATING THE"TURN|NG"OF THE SPECIMEN 

SPECIMEN UNDER 
, / TEST 

SIDE VIEW OF STRIAESCOPE 
ILLUSTRATING THE"TILTING" OF THE SPECIMEN 

Figure 1. The striaescope for the inspection of 
slab glass. 

light beam as observed through a small tele¬ 
scope. Figure 1 illustrates the optical arrange¬ 
ment in a direct view striaescope. An instru¬ 
ment which is somewhat easier to use is the 
projection striaescope illustrated in Figure 2. 
A representative sample of three types of stria¬ 
tions appears in Figure 3. In all cases, stria¬ 
tions are seen by virtue of the Fresnel reflec¬ 
tions produced at the boundary of the striae. 
Most inspectors prefer the projection method 


2W WESTERN UNION OBSERVER 

ARC LAMP OR Vs 

50 CP AUTO T 



in which the bands produced by the striations 
in the sample are compared with those of the 
standards. 

In practice, the grading of striations of Types 
A and B is fairly straightforward. However, 
Types C and D are not so easily distinguished. 
As an example, if in a certain lot of glass, all 
pieces contained striations of Grade C, 50 per 
cent of them would be classed as D. The tech¬ 
nique of using standards for comparison is very 


difficult if the standards are of different manu¬ 
facture than the samples. 

The method of inspection appears to be ade¬ 
quate to determine compliance with the specifi- 



C O R D 



RIBBON 



REAM 


Figure 3. Inspection of slab glass for striae at 
Frankford Arsenal. 


cations, but, as pointed out, the specifications 
themselves are open to serious question. Of one 
fact there appears to be little doubt, and that 
is that very heavy striations are optically un¬ 
desirable, although there appears to be little 
data to support even this supposition. 

In one particular case, certain modifications 
































SURVEY OF OPTICAL INSPECTION METHODS 


209 


should be made in the specification concerning 
the method of inspection of molded blanks. 
These blanks have a skin which makes it very 
difficult to inspect them for striae. The skin 
should be removed before the inspection. 

Inclusions 

The specifications and inspection methods for 
the evaluation of inclusions such as bubbles, 
seeds, and stones depend upon separating the 
ultimate uses of the glass into three classes, 
namely, reticles, field lenses, and others. In 
the case of reticles or other elements in the 
focal plane, it is required that no bubbles, seeds, 
or stones over 0.0004-in. mean diameter be pres¬ 
ent. Field lenses which are nearly in focus for 
the observer must be of higher quality than 
lenses in which such inclusions act only as light 
scatters. More than one bubble of 0.003-in. aver¬ 
age diameter per cubic inch of glass is cause for 
rejection of glass for use in field lenses. For 
other uses, defects of this type may not exceed 
0.1 per cent of the total area for each 10 cm. 
of light path. In addition, the number of bub¬ 
bles may not exceed one per cubic centimeter, 
and finally no defect may exceed 0.021-in. 
mean diameter. 

The inspector peers into the piece of glass, 
illuminated by a test lamp, and compares the 
defects with a set of standards. He must make 
mental corrections for variations of volume and 
thickness between the sample and the stand¬ 
ards. In some cases the glass is immersed in a 
fluid to make the defects more easily seen. 

Except for reticles, for which the specifica¬ 
tions must be very exacting in order that the 
field be clear of undesirable spots, the require¬ 
ments are based on the ability of the industry 
to produce glass of the indicated quality. In 
this case, as with striae, there is no well- 
established connection between the size, ap¬ 
pearance, or number of inclusions in the com¬ 
ponents and the degradation of the perform¬ 
ance of the completed instrument. What is 
needed is not only an objective test method but 
also a method related to performance rather 
than the glass industry’s abilities. 

Color and Dimensions 

Light absorption and color are two qualities 
of optical glass which are either not specified 


in amount or are required to be absent. The 
governing principle is that if absorption or 
color can be detected the piece is rejected. The 
denser flints are, however, permitted to be 
slightly yellow. 

Slab glass must be flat and of the thickness 
specified for the particular application. In gen¬ 
eral, the tolerances permit +2 and —0.0 mm for 
slabs up to 20 mm. Plates over 20 mm thick may 
be 4 mm oversize. Molded blanks must have 
grinding stock both on the surfaces and on the 
diameters. 

Strain 

Optical glass which has not been properly 
annealed contains permanent strain. This de¬ 
fect may be a source of trouble both in the 
final behavior of the optical elements produced 
from the glass, because of the presence of bire¬ 
fringence, and in the preparation of blanks for 
lenses or prisms because of the difficulty of 
cutting a badly strained piece. In the case of 
slabs of optical glass which are to be cut up 
and used in the manufacture of pressings, the 
glass must be sufficiently annealed to prevent 
breakage during handling, storage, and ship¬ 
ment, and to permit the cutting without shat¬ 
tering of blanks with straight edges. All other 
pieces of glass are required to be free from 
strain to the extent that the relative retarda¬ 
tion of sodium light shall be less than 10 mp 
per centimeter. Figures 4 and 5 show a typical 
apparatus now used for the inspection of sam¬ 
ples for strain. The inspection consists of classi¬ 
fying the samples in polarized light by a com¬ 
parison with one of a group of specimens con¬ 
taining known amounts of strain. 

The practical effect of strain upon the optical 
behavior of lenses and prisms has not been 
investigated, but this question does not seem 
to be very important, since in a group contain¬ 
ing a large number of pieces of glass examined 
during this study no pieces were found to have 
permanent strain of more than half the per¬ 
mitted amount. The introduction of a quartz 
wedge into a device for testing strain would 
permit the direct measurement of the amount 
of strain. Such a device was made up and 
utilized for the actual inspection of a sample 
number of pieces by a government inspector 
who reported that he acquired considerably 



210 


OPTICAL TESTING METHODS 


more confidence in such a direct quantitative 
measurement than he had in the older qualita¬ 
tive comparison method. The older method does 
have an advantage in that it permits a large 
number of pieces to be compared to a calibrated 
sample at once and with a single glance. 

4 - 2 * 2 The Inspection of Optical Parts 
Lenses 

Physical Dimensions. The specifications of 
the dimensions of finished but unmounted lenses 
are included on the optical drawing. In general, 



Figure 4. An apparatus for the inspection of 
strain. 


the tolerances are also shown and are usually 
negative in the amount of about 0.001 in. per 
inch of diameter. The various dimensions speci¬ 
fied are the diameter, the axial, and the edge 
thickness. Most lenses are edge chamfered and 


the normal specification calls for an angle of 
45 degrees to the edge. 

Types of go, no-go gauges are in frequent 
use for checking the dimensions of lenses. Di¬ 
rect measurement with micrometers and dial- 
type gauges enjoys considerable popularity, 
particularly among company inspectors. A lens 
cell of nominal dimensions is often used as a 
gauge. The lens is placed in the cell and the 
retaining ring run up against a shoulder. The 
accuracy of the lens dimensions is then judged 
by the “rattle.” The use of go, no-go gauges on 
the thickness offers more chance of injuring the 
surface than does the dial-type instrument. 



Figure 5. Inspection of slab glass for strain at 
Frankford Arsenal. 


Centering. Not only must a lens have the 
proper diameter, but this diameter must be 
centered on the optical axis of the lens. In the 
case of two or more cemented elements, each 
element must be centered so that its optical 

















SURVEY OF OPTICAL INSPECTION METHODS 


211 


axis coincides with the mechanical axis of the 
assembly. The centering error may be divided 
into two parts: the angle between the optical 
and the mechanical axis and the displacement 
of one with respect to the other. The latter 
error results in a variation in edge thickness 
and is covered in the previous specification. The 
standard specification calls for a departure in 
concentricity of less than 3 min of arc for all 
lenses, but oculars may be in error by as much 
as 6 min. 


established within the ability of the production 
methods but are also readily measured for in¬ 
spection. The lack of concentricity in lenses is 
known to have profound effect on the perform¬ 
ance but the exact numerical values of the tol¬ 
erances necessary for performance quality in 
line with other tolerances is not known. This 
factor is very important and a careful study of 
the matter should be made. 

Focal Length. The principal optical attribute 
of a lens is its focal length. The equivalent and 



Figure 6. A lens bench for the determination of focal length. 


For inspection, the lens may be mounted by 
its edge on a rotating collet. The motion of the 
optical axis may then be observed by viewing 
an object through the lens. Conversely, the lens 
may be mounted by cups against its surfaces. 
A microscope focused on the edge permits ob¬ 
servation of any motion resulting from lack of 
concentricity. The second method is to be pre¬ 
ferred for it facilitates accurate measurement 
of the error. 

On the whole, the specifications and inspec¬ 
tion methods for the dimensions of lenses are 
quite satisfactory. The tolerances are not only 


back focal length are both specified on the 
drawing covering the particular piece. Two per 
cent is a standard tolerance. In spite of the 
variety of devices in actual use, all the inspec¬ 
tion methods utilize a form of optical bench. 
Figure 6 illustrates a conventional bench set 
up to measure the focal lengths of lenses. Any 
such device consists of a collimator, a lens 
holder, and an eyepiece or microscope fitted 
with a scale. A pattern at the focus of the col¬ 
limator is focused in the microscope. The dis¬ 
tance from the lens to the focal plane of the 
eyepiece or microscope is the back focal length. 



212 


OPTICAL TESTING METHODS 


One manufacturer marks the back of the lens 
with a glass marking pencil and starts each 
measurement by focusing on this mark. An¬ 
other sets up a “sectional” instrument and 
adjusts the focus for a lens of the correct focal 
length. This lens may then be quickly replaced 
with those under test. The required motion of 
the microscope to restore sharp focus gives 
the error directly. A setup of this type for 
testing binocular objectives appears in Figures 
7 and 8. A refinement on this device allows the 
objective to be rotated so that the concentricity 
and the focal length may be measured at the 
same time. In all the devices now in use, the 
focal length is determined for the best focus 


situation in which the lens is to be used. As 
examples, eyepiece lenses are not to be tested 
for definition separately but in an assembled 
eyepiece; collectors are to be tested not at full 
aperture but at the aperture utilized in the 
instrument. The test of definition depends upon 
a comparison with standard units. Frequently, 
the acceptance standard is selected by the gov¬ 
ernment inspector from regular production at 
the facility concerned. 

Most often the test for definition is simply an 
examination of the sharpness of focus and the 
contrast of the image formed by the lens or 
lenses with the aid of a high-powered micro¬ 
scope. It is obvious that this test may be made 


I '/z RIGHT*ANGLE 
PRISM - 2__l 


OBSERVER 


GLASS BLOCK, WHICH WHEN 
COMBINED WITH \'/ 2 " PRISM, GIVES 
GLASS PATH EQUAL TO THAT OF 

PORRO PRISM -— 


MICROSCOPE SUBSTAGE 
LAMP WITH GROUND / 
DIFFUSING LENS X 




2^' RIGHT-ANGLE 
PRISM 


ALL OPTICS ABOVE THE OBJECTIVE UNDER TEST 
FOCUS UP 8 DOWN AS A UNIT 
A FOCUSING SCALE GIVES A MEASURE OF THE 
OISTANCE FROM THE FRONT OF THE LENS TO 
THE BACK FOCAL POINT 


Figure 7. The focalometer. 


over the entire aperture of the lens and not 
merely for the paraxial rays. 

Although the specifications of focal length 
and the methods of inspection are satisfactory, 
the basis of the tolerance is again not well 
established. Here it appears that the require¬ 
ment of precision is based more upon the ability 
of the industry to produce acceptable optics 
than upon the limits set by the overall perform¬ 
ance of the instruments in which the pieces are 
to be used. 

Definition. Each objective, collective, erector, 
and ocular must be subjected to a test for defi¬ 
nition. The specification is so worded as to 
stipulate that the lens is to be tested in a 
fashion as nearly as possible approaching the 


at the same time and in the same machine as 
the previously described tests of focal length. 

Of the various specifications so far discussed, 
the one concerning definition is probably the 
least satisfactory. It is far from quantitative, 
but without further development of testing pro¬ 
cedures it cannot be improved on. Two instru¬ 
ments described in Section 4.3 could be utilized 
to place the determination of definition on a 
quantitative basis. The more useful of the two, 
in this connection, is the modified Michelson- 
Twyman interferometer. If this instrument 
were established as a basis for inspection, the 
specification could read: “The lens when tested 
in the Michelson-Twyman interferometer shall 
conform to the interferometer pattern indi- 
























SURVEY OF OPTICAL INSPECTION METHODS 


213 


cated on the detailed drawing of the given part, 
within plus or minus one interference fringe.” 
The specified interference pattern may be the 
result of computation or the observed pattern 
produced by an acceptable lens. The tolerance 
should be based on a study of the effect of 
errors in each particular lens on the behavior 
of the complete system. 



Figure 8. Device used by the Bausch and Lomb 
Optical Co. to inspect finished lenses for focal 
length. 

The KDC apparatus, which is now used for 
the inspection of complete instruments, could 
be used for the testing of individual components 
and subassemblies. In this case, a KDC effi¬ 
ciency for each lens should be established. The 
most satisfactory test method would probably 
involve a combination of the optical element 
under test with a suitable system in which all 
the intended faults of the lens are compensated 
in exactly the same way as they would be in 
the instrument for which it is designed. 

In many cases, individual lenses need not be 
tested for conformance to specification. Tests 
for axial thickness, focal length, and even defi¬ 
nition on a quantitative basis could grade the 
lenses into groups. Proper selections of com¬ 
ponents from the groups could then be com¬ 
bined to yield assemblies whose performance 
would be acceptable. This procedure compli¬ 


cates the problem of spares however. Since the 
general problem of spares has not been ex¬ 
amined, the relative merits of component 
spares versus assembly spares cannot be evalu¬ 
ated here. 

Beauty Defects. Individual lenses are in¬ 
spected for so-called beauty defects. These de¬ 
fects are of two shape types: scratches, long 
and thin; and digs, pits, and bubbles which are 
nearly circular. The permissible number of 
maximum size scratch-type defects in an optical 
system as determined by their combined length 
and location is shown in Table 4. The classifica¬ 
tion as to least, less, and most critical lenses 
refers to their positions in the instrument. 
Most critical lenses are those lying substan¬ 
tially in a focal plane, less critical are those 
lying near a focal plane, and least critical are 
those lying far from the focal plane. The classi¬ 
fication numbers for each defect, 10 to 100, are 
determined by comparison with a set of stand¬ 
ard samples. As in the case of seeds and stones 
in optical glass, these beauty defects are speci¬ 
fied more upon their appearance than upon the 
effect which they may have upon performance. 
Since these defects result in the rejection of 
many optics which may be usable, experimental 
work is needed to determine their actual influ¬ 
ence on performance. These may be in the form 
of psychological experiments, or experiments 
showing actual deterioration in some desirable 
optical property. 

Prisms, Wedges, and Windows 

Angles. Many of the specifications and in¬ 
spection procedures are precisely the same for 
prisms, wedges, and windows as for lenses. 
There is no point here in reviewing this ma¬ 
terial. In one particular case, that of the physi¬ 
cal dimensions, a new type of specification must 
be added. For prisms and wedges the various 
angles must be specified and inspected while for 
windows the degree of parallelism must be in¬ 
spected. Figure 9 illustrates the use of the dial 
gauge for the inspection of the critical dimen¬ 
sions of prisms. This instrument seems to be 
particularly useful in this connection, especially 
when it is provided with the type of fixtures 
illustrated in Figure 9. Many methods are now 
utilized for the inspection of prism angles. The 



214 


OPTICAL TESTING METHODS 


most successful of these utilize supplementary 
optical systems in which the defect is magnified 
by the displacement of an image. Two instru¬ 
ments merit special mention. One is the pro¬ 
jection equipment illustrated in Figure 10. A 
wide variety of such devices may be utilized to 
inspect prisms of various shapes. In the par¬ 
ticular case of binocular prisms, it is obvious 
that slight errors in the angles of one prism 
may be compensated for by errors in the angles 
of a second prism. If the prisms are properly 


either of individual prisms or prism combina¬ 
tions. It may be used to measure the roof angles 
on roof prisms as well. A typical interferometer 
setup appears in Figure 11. 

In the case of windows, the most common 
procedure is simply to examine a target 
through the window with a fairly high power 
telescope. If no deviation is observed as the 
window is inserted and withdrawn from the 
optical path, the window is acceptable. For 
critical wedges such as are used in heightfind- 


Table 4 


Total length of scratch-type defects 
permitted for least critical lenses 

80 scratch X A diameter of element 
60 scratch V 2 diameter of element 
40 scratch % diameter of element 
20 scratch 1 diameter of element 
10 scratch 5 diameter of element 


Permitted size of defects in less critical lenses 




For central zone % 





Beam 

diameter of surface 

Outer zone 



diameter (mm) 

scratch 

dig 

scratch 

dig 



Over 7 

80 

70 

80 

70 



5-7 

80 

50 

80 

50 



4-5 

60 

40 

60 

40 



3.2-4 

60 

30 

60 

40 



2.5-3.2 

40 

20 

60 

40 



2.1-2.5 

40 

15 

60 

30 



1.6-2.1 

30 

10 

40 

20 



1.0-1.6 

20 

5 

40 

15 



0.6-1.0 

15 

3 

30 

10 



0.4-0.6 

10 

2 

20 

5 



0.2-0.4 

10 

1 

15 

3 



Permitted size of defects in most critical lenses 


Beam 



For central zone V 2 



diameter 

Magnifying 

Focal 

diameter of surface 

Outer zone 

(mm) 

power 

length 

scratch 

dig 

scratch 

dig 

0.2 

20-10 

12.5-25 

10 

1 

15 

3 

0.4 

10-5 

25-50 

10 

2 

20 

5 

0.6 

5-3.3 

50-75 

15 

3 

30 

10 

1.0 

3.3-2 

75-125 

20 

5 

40 

15 

1.6 

2-1 

125-250 

30 

10 

40 

20 


paired, their optical behavior as an assembly 
is in every way as satisfactory as it would be 
if both prisms were perfect. The projection 
equipment may be so constructed as to test 
prisms in pairs. By far the most useful instru¬ 
ment for the testing of prisms, but one which 
has not as yet enjoyed very great popularity, 
is the interferometer. This device permits a 
quantitative determination of the accuracy 


ers, the interferometer is the most common in¬ 
spection instrument. The method consists of 
adjusting one of the mirrors of the interfer¬ 
ometer by means of a master wedge of accept¬ 
able angle. 

Definition. There is a specification concern¬ 
ing the resolution required for prisms, wedges, 
and windows. The actual values of the defini¬ 
tion depend upon the aperture of the particular 









SURVEY OF OPTICAL INSPECTION METHODS 


215 


instrument and are generally stated on the 
optical drawing. In practically all cases, a reso¬ 
lution test target set at some appropriate dis¬ 
tance is viewed by means of a high-powered 
telescope. The amount of magnification de¬ 
pends upon the aperture of the prisms and is 
at least 50X for each inch of aperture. If the 
various optics are tested on the interferometer 


three per reticle are allowed, and none wider 
than one-half the width of the line) and the 
exact location of the markings. In general, a 
toolmaker’s microscope, such as that illustrated 
in Figure 12, is utilized for the inspection. A 
center determined by the outer edge of the 
piece is the origin of all measurements, and 
each graduation is measured with respect to 



Figure 9. Dial gauges used to inspect the physical dimensions of prisms. 


for accuracy of dimensions, the patterns ob¬ 
served are also a good measure of the definition. 

Reticles 

A rather specialized type of optic used in 
military instruments is the reticle. Besides its 
obvious optical characteristics, physical dimen¬ 
sions, parallelism, and beauty defects, which 
are specified and examined by the methods 
already described, reticles must be inspected 
for the location, magnitude, and uniformity of 
graduations. The specifications cover not only 
the uniformity of width and depth of the rul¬ 
ings, but also absence of breaks (no more than 


this origin. The operation of inspecting reticles 
is a very tedious task and must be entrusted to 
experienced inspectors. In view of the great 
growth of projection inspection methods in the 
small parts industries, it is somewhat surpris¬ 
ing to find that this method is not utilized here. 
A comparison of an enlarged projected pattern 
of the reticle could be made directly with a 
standard pattern and would require consider¬ 
ably less time and experience. 

Flats 

In some larger instruments, optically flat 
mirrors are employed as substitutes for prisms. 








216 


OPTICAL TESTING METHODS 


It is common to specify either the number of 
interference rings which may exist between 
the part under inspection and a master flat, or 
the astigmatism produced when the mirror is 
utilized at a 45-degree angle of incidence. Con¬ 
formance with the latter of the two specifica¬ 
tions is easily measured by actually examining 
a test object with a high-powered telescope and 
the optical flat set at 45 degrees before the ob¬ 
jective. The measurement consists of determin¬ 
ing the difference in best focus for the vertical 
and horizontal positions of the test object. The 
checking of an aluminized flat against a master 
flat is very difficult since any damage to the 


tank telescope, the specification of this im¬ 
portant quality is very obscure. For example, 
the specification for the image quality of 7x50 
binoculars reads in part: “The optical system 
shall be free of distortion and any residual dis¬ 
tortion be compatible with the best correction 
of the other aberrations.” The meaning of this 
type of specification is obscure and impossible 
of application except in the judgment of some 
individual inspector. In the case of binoculars, 
the situation is somewhat better since the spec¬ 
ification states also that the limit of resolution 
within one degree of the center must not be less 
than 4 sec of arc for a standard black line chart. 


LENS BENCH 



MICROSCOPE 


Figure 10. Schematic diagram of mirror assembly used in lens bench measurements on M-10 periscope 
head prism. 


aluminum coat is cause for rejection. The in¬ 
terferometer is probably the most practical 
method for the checking of optical flats. 


The Complete Instrument 

Because of the great variety of optical instru¬ 
ments manufactured for and used by the Armed 
Services, it is impossible to consider them all 
individually. Inspection of three types, the tank 
telescope, the tank periscope, and binoculars, 
was studied most completely during the survey 
and the specifications and inspection methods 
for these instruments will be described in some 
detail. 

Definition 

By far the most important characteristic of 
any optical system is the quality of the image 
which it produces. Yet, except for the M-71 


The M-10 periscope consists of two optical 
systems, one of unity power and one of 6X* 
The specification requires that the unity-power 
portion produce images which “shall be clear, 
well-defined and free from objectionable im¬ 
perfections.” The 6X section must be “prac¬ 
tically free from distortion, astigmatism, cur¬ 
vature of field, coma, chromatic and spherical 
aberration.” These specifications are desirable 
goals for the optical designer, but are not 
proper requirements upon which to control pro¬ 
duction. 

By contrast, the specification for the M-71 
telescope reads: “No telescope shall have a 
KDC efficiency less than 75 per cent on the 
axis.” Here is a quantity readily measured and 
free from personal judgment on the part of the 
inspector. Moreover, the use of this specifica¬ 
tion and inspection method led to an improve¬ 
ment of 20 per cent, on the average, in the effi¬ 
ciency of telescopes produced by those facilities 


























SURVEY OF OPTICAL INSPECTION METHODS 


217 


which formerly produced the poorest tele- that certain common types of mistakes in as- 
scopes. sembly are characterized by definite values of 

The KDC apparatus, about which consider- efficiency. For these cases, then, the inspector 
ably more will be said shortly, may be seen in will not only pass on the acceptability of the in- 
Figures 23 and 24. The efficiency is simply the strument, but, if it is rejected, will point out 
ratio of the limiting distance for resolution of where the fault lies as well, 
a standard target with the telescope under test The inspection of binoculars and tank peri- 
to the distance for resolution of the same target scopes is usually made by examining the reso- 
with a telescope of high quality having the same lution of a standard target through the instru- 
entrance pupil as the one being tested. In the ment under test with the aid of an auxiliary 
case of the M-71 telescope, the efficiency is re- magnification of 3X to 6X- Figure 13 shows an 
lated to a simple telescope of good design and inspector examining binoculars. The entrance 



Figure 11. The Frankford Arsenal interferometer. 


high quality, so that the resolution on the axis 
is limited only by the entrance pupil. The effi¬ 
ciency could equally well be related to a per¬ 
fectly made M-71 telescope. The difference in 
the two efficiencies would be the loss of defini¬ 
tion imposed by the design. The KDC apparatus 
has been demonstrated to be quite free of a per¬ 
sonal equation and thus its results are desir¬ 
ably quantitative. 

One additional advantage of a purely quanti¬ 
tative method of inspection has not as yet been 
pointed out. Careful investigation may show 


pupil of the viewing telescope is 7 mm, to match 
the pupil of the eye under conditions corre¬ 
sponding to normal use. On the basis of what he 
can see, the inspector judges whether the in¬ 
strument is acceptable. Probably few good in¬ 
struments are discarded and few poor instru¬ 
ments are passed by this method. The border¬ 
line cases, however, are difficult to judge. No 
doubt at some times instruments are rejected 
which are actually better than those accepted 
at other times by the same inspector or by other 
inspectors. 









218 


OPTICAL TESTING METHODS 


Magnification, Entrance, and Exit Pupils 
For the M-71 telescope, the magnification 
must be within 10 per cent of 5X> while for the 
7x50 binoculars, it must be 7X plus or minus 
2 per cent. In addition, the magnification of the 



Figure 12. The inspection of reticles by means 
of a toolmaker’s microscope. 

two systems of a binocular may not differ by 
more than 2 per cent. It is to be pointed out 
that the focal length of the objective, the eye¬ 
piece and erectors, the linear calibration and 
the angular specification of the reticle, and the 
magnification are all related. The tolerances 
should likewise be related. There is no point in 
specifying any one of these quantities to a 
higher accuracy than is required by this inter¬ 
relation. 

The present satisfactory method of testing 
telescopic systems for magnification, where it is 
specified, depends on a micrometer measure¬ 
ment of the exit pupil when the entrance pupil 
is limited by a stop of known size. 

For the M-71 telescope, the exit pupil is not 
specified in the same way that the entrance 
pupil and magnification are. Numerically, the 
value of the exit pupil for this instrument is 


0.276 in. and the eye distance is required to be 
1.051 in. During the measurement of the en¬ 
trance or the exit pupil, the eye distance may 
be checked by observing the distance between 
the focal plane of the observing microscope and 
the eye lens when the exit pupil is in sharp 
focus. The procedure is to focus first on the sur¬ 
face of the eye lens and then displace the micro¬ 
scope along the axis until the exit pupil is in 
sharp focus. The displacement is the eye dis¬ 
tance. 

For binoculars, the exit pupil is specified as 
not less than 50 divided by the magnification. 
In this case the specification is more explicit in 
that it requires the exit pupil in the center of 
the field to be circular, except in so far as the 
area may be reduced by a cord if the reduction 
does not exceed 2 per cent. At the edge of the 
specified field (24 degrees, 10 min) the mini¬ 
mum dimension of the exit pupil must not be 
less than 45 per cent of the diameter of the 
central pupil. The measurement is made in pre¬ 
cisely the same manner as the previously de¬ 
scribed measurements involving the exit pupil 
with the one refinement that the microscope 
must be so mounted as to rotate about an axis 
through the exit pupil of the instrument. 

In general, these specifications and methods 
of measurement which involve the entrance 
pupil, the magnification, and the exit pupil are 



Figure 13. The inspection of binoculars for 
definition. 

quite satisfactory. It must be pointed out, how¬ 
ever, that the measurement of an off-axis exit 
pupil is very difficult since the bounding sur¬ 
faces are not in the same plane and therefore 






SURVEY OF OPTICAL INSPECTION METHODS 


219 


cannot be brought into sharp focus simul¬ 
taneously. The actual measurement will de¬ 
pend to some extent upon the opinion of the 
inspector. 

The M-71 telescope is not covered by a speci¬ 
fication relating to the size of the true field, but 
nevertheless at all production centers a test of 
the field is made. The design indicates a true 
field of 13 degrees and present practice requires 
a field of 12.5 degrees. The 7x50 binocular must 


mission. The specification is adequate and the 
method now employed is straightforward and 
quantitative. 

Mechanical Features 

Classed here as mechanical features are the 
specifications covering cleanliness, collimation, 
and weatherproofing. The specification for 
cleanliness is in the form of a description of the 
treatment to be applied to a telescope, peri- 


PROCEDURE : 

1- WITH BINOCULAR OF KNOWN 

TRANSMISSION^ A) IS SET TO 
KNOWN VALUE (77.8%) AND(B) 

IS ADJUSTED UNTIL THE 
PHOTOELECTRIC RESPONSE AS 
INDICATED BY THE GALVANO¬ 
METER IS BALANCED. 

2- THE STANDARD BINOCULAR IS THEN 

REPLACED BY THE BINOCULAR 
WHOSE TRANSMISSION IS DESIRE 
AND (A) ADJUSTED UNTIL THE 
GALVANOMETER IS AGAIN BALANCI 

3- (A) READS DIRECTLY LIGHT 

TRANSMISSION IN PERCENT FOR 
BINOCULAR UNDER 
INSPECTION. 



INTEGRATING 

SPHERE 


(A) 

ADJUSTABLE APERTURE 
STOP N0.2 


\ 100 % 

DIRECT READING 
TRANSMISSION SCALE 


FIELD SUBTENDED BY FILAMENT *1.5° 500 W - T 20 - P-115 V 

Figure 14. Photoelectric photometer for the inspection of light transmission. 


have a true field of not less than 7 degrees, 10 
min. For both instruments, the inspection pro¬ 
cedure involves the viewing of a target which 
may be either in the form of a scale calibrated 
in degrees or a large white circle of the required 
diameter. It is readily apparent to the inspector 
whether the field is sufficient. 

Light Transmission 

The M-71 telescope must pass not less than 
62 per cent of incident white light. The binocu¬ 
lars are required to have a transmission of at 
least 74 per cent. There is no covering specifi¬ 
cation for the light transmission of the M-10 
periscope, but an occasional instrument is in¬ 
spected in order to insure that no long-term re¬ 
duction in the quality is occurring. A compari¬ 
son-type photoelectric photometer, such as is 
illustrated in Figure 14, is normally used for 
the determination of the percentage of trans¬ 


scope, or binocular, without traces of moisture, 
dust, or other substances appearing on the sur¬ 
faces of the various lenses. It is now routine to 
shake and tap the instrument under test with 
a hard rubber rod and then to examine the 
surfaces of the various lenses from the objec¬ 
tive and eyepiece ends. If dust is found in the 
instrument, it is very difficult for the inspector 
to localize the surface. The amount of illumi¬ 
nation used in the inspection determines the 
size of the particles which are visible and, 
although it does not appear in the specification, 
a value of 300 footcandles on a uniformly 
illuminated field is standard practice. 

The proboscope (see Sections 4.1 and 4.3.5) 
permits the examination of each internal sur¬ 
face in turn, so as to establish the location of 
any offending dirt or moisture. Such a device 
may save a number of otherwise rejected in¬ 
struments since it is possible with its use to 











































220 


OPTICAL TESTING METHODS 


determine whether the defect is on a critical 
area of the lens or prism. 

The optical axis of an M-71 telescope must 
coincide with the geometrical axis within plus 
or minus 0.25 mil. The geometrical axis is es¬ 
tablished by the spherical bearing surfaces of 
the collars on the telescope tube. Figure 15 
shows a typical setup for the determination of 
collimation. V blocks supporting the telescope 



Figure 15. Collimator used to determine true 
field, collimation, and reticle accuracy. 


are adjusted so that the optical axis of an 
acceptable instrument coincides with the image 
of the collimator reticle. Test telescopes are 
then placed in the V blocks and the displace¬ 
ment of the image of the collimator reticle with 
respect to the reticle of the telescope is ob¬ 
served. Unfortunately, it appears that there is 
a good possibility that the setting for the stand¬ 
ard telescope may be in error by an amount 
greater than the tolerance. Further, the specifi¬ 
cation relates only to the optical axis as defined 
by the markings on the reticle. The extent of 
misalignment of the various subassemblies is 
not specified or measured. A lack of collimation 
in one assembly may be balanced by a corre¬ 
sponding improper adjustment of another sec¬ 
tion, and since such a combination requires that 
the axial rays for the telescope as a whole tra¬ 
verse the subassemblies as off-axis rays, de¬ 
terioration in the image quality results. Meas¬ 
urement of the KDC efficiency will show up 
such a fault. 

Besides the normal collimation test for bin¬ 
oculars, similar to that just described, the rela¬ 
tive positions of the optical axes of the two tele¬ 
scopic systems is specified. The specification re¬ 
quires that when two parallel pencils are pro¬ 


jected into the two objectives of the binocular, 
the rays emerging from the two eyepieces shall 
be parallel within the following limits for any 
setting of the interocular distance: (1) The 
angle must be less than 14 min of arc in the 
plane perpendicular to that determined by the 
two entrant pencils. The rays must not diverge 
more than 28 min of arc nor converge by more 
than 14 min of arc in the plane parallel to the 
plane of the entrant pencils. Inspection of bin¬ 
oculars under this specification requires an ap¬ 
paratus similar to the one illustrated in Figure 
16. The optical arrangement of this instrument 
appears in Figure 17. While the nature of the 
setup is obvious from the figures, the procedure 
requires some explanation. It is common prac¬ 
tice to align one-half of the binocular so that 
the image of the collimator reticle formed by 
the binocular and observed through the auxili¬ 
ary telescope is coincident with the image of the 
geometrical center of the reticle of the observ¬ 
ing telescope. The image of the reticle of the 
collimator formed by the other half of the bin¬ 
ocular is then displaced in the observing tele¬ 
scope by an amount which is a measure of the 
collimation. It is possible to provide the second 
auxiliary telescope with a reticle containing a 
figure whose extent is the limit set by the speci¬ 
fication. The acceptability of the instrument 
under test is thus determined at a glance. An 
interesting and apparently very useful device 
for inspecting the collimation of binoculars is 
shown in Figure 18. The device was developed 
by the British and is used by them for this pur¬ 
pose. It has the advantage of requiring but one 
collimator and viewing telescope, but utilizes 
three flat mirrors. The adjustment is very 
simply made, as is shown in the figure. 

In addition to the measurement of collima¬ 
tion of binoculars, the collimation must be 
maintained under set specified conditions of 
shock. Figure 19 is a schematic diagram of the 
shock test given to British binoculars in the 
National Physical Laboratory. The American 
specification calls for a similar test, but with 
the distance increased to 6 ft. If the instrument 
passes this test, a strong cord free from stretch, 
such as a sash cord, is tied around the hinge 
pin and the instrument is dropped 6 ft in such 
a manner that the fall is arrested by the cord. 





SURVEY OF OPTICAL INSPECTION METHODS 


221 


The collimation must remain within the speci¬ 
fication after these drops. These specifications 
of collimation appear to have grown with the 
industry. They are greatly influenced by the 
opinions which existed during World War I 
and it might be well worth while to conduct an 
investigation to determine the most suitable 
tolerances. It is of interest to note that the spec¬ 
ification takes into account the fact that the 


from parallax. In addition, the vertical lines 
must be parallel to the vertical lines of the 
image. The accuracy of the graduations of the 
6X portion of the M-10 periscope is specified 
as ±1 per cent plus 0.1 mil in elevation, and 
for the unity-power system ±1 per cent plus 
0.2 mil. For the M-71 telescope, the tolerance 
is ±0.5 mil, and for binoculars, where they 
are equipped with a reticle, ±3 per cent. 



Figure 16. Apparatus for the inspection of binoculars for collimation. 


human eyes are accustomed to examine objects 
from which there are divergent beams of light 
and unaccustomed to situations involving con¬ 
vergent beams of light. 

The Reticle 

An important part of most military optical 
instruments is the reticle. Much of the useful¬ 
ness of military optical instruments, such as 
sighting telescopes, depends on its reticle. The 
graduations of the reticle must not only sub¬ 
tend the angles they are designed to represent 
but they must be in proper focus and be free 


Substantially the same technique of inspec¬ 
tion is used for each of these instruments. A 
collimated image of a standard reticle is com¬ 
pared with the reticle under test, sometimes 
with the aid of an auxiliary telescope. The 
reticle in the collimator is provided with double 
marks for each range mark. If all the marks 
on the inspected reticle fall between the corre¬ 
sponding marks of the test object, the reticle is 
acceptable. 

The specification is sufficiently definite but is 
based more upon the difficulty of construction 
than upon the actual tolerance established by 



222 


OPTICAL TESTING METHODS 


field experience. In this connection, numerous 
complaints have been received which indicate 
that the graduations do not read the range 
accurately. It is recommended that these com¬ 
plaints be investigated to determine whether 
they arise from tampering in the field, inade¬ 
quate inspection, or insufficient specification. 


dioptometer greatly facilitates the inspection. 
This device, developed at Penn State, is de¬ 
scribed in Section 4.3.4. 

The methods already described will also 
check the reticle plumb and image tilt. The spec¬ 
ification of the M-71 telescope requires that 
the axes of the reticle shall be vertical and hori- 


COLUMATOR BINOCULAR 




OBSERVER 




DETAIL C 


DETAIL A 

Figure 17. Schematic diagram of binocular collimator. 


The reticle must be so placed in the instru¬ 
ment as to remove parallax for a target be¬ 
tween 500 and 525 yd for the M-71, or 700 and 
800 yd for the M-10. In addition, with the aid 
of a 3X auxiliary telescope, the reticle must be 
in sharp focus for a diopter setting of zero, 
±0.25. For inspection, the instrument is placed 
in a collimator, sometimes the same one used in 
the previous inspection, whose reticle is op¬ 
tically placed at a distance corresponding to 
the center of the specified range. The use of a 


zontal within 0.4 mil at the 400-yd mark when 
the locking pin is in a downward vertical posi¬ 
tion. The requirement for binoculars is such 
that the markings shall be correct with respect 
to the center lines of the objectives with the 
interpupillary distance set at 63 mm. The tilt 
shall not exceed one-half degree for all settings 
from 63 to 72 mm. In the case of the M-10 peri¬ 
scope, the reference is the vertical axis of the 
instrument. In addition to the plumb of the 
reticle, the image of a vertical line of a distant 



Figure 18. Schematic diagram of apparatus used to inspect binoculars for collimation at the National 
Physical Laboratory. 





























































SURVEY OF OPTICAL INSPECTION METHODS 


223 


target must not deviate by more than 30 min 
from the reference. 

Weatherproofing 

Since most military instruments must be 
used under all weather conditions, it is of con¬ 
siderable importance that the optical perform¬ 
ance be not impaired by such conditions. The 
M-71 telescope and the M-10 periscope are sub¬ 
jected to a shower equivalent to a light rain for 
5 min. Binoculars are actually immersed to a 
depth of 12 in. in a tank of water for 5 min. The 


must not enter, for the instrument must stand 
or fall on the basis of its numerical perform¬ 
ance. 

It is a further desirable feature of inspection 
not only to determine agreement with specifica¬ 
tions within the tolerance, but by measurement 
to determine the error. Two factors are to be 
noted here. The first is the concept of tolerance. 
Tolerances must be based upon real effects upon 
the performance and not upon any other fac¬ 
tor. Many of the tolerances in present specifica¬ 
tions are based upon custom, appearance, or 



r~R— 1 —. 

crc J 3 _ qf g 


(St0 


l] I 
I I I I 
Lj lj 


2 FT 


2 FT 


2 FT 


2 FT 


r THIN CLOTH 


OH 


\ r~R i-i 


1 L —1 


_Qg) | 





SAND 









1 

2 

3 

4 

5 


PROCEDURE: 

(1) SHOCKED AT 3 INTERPUPILLIARY SETTINGS 

(2 ) COLLI MAT ION CHECKED AFTER EACH SHOCK 


Figure 19. Schematic diagram of shock test for binoculars used by the National Physical Laboratory. 


instruments are examined after the test and 
again 24 or 48 hours later. 

4,2,4 Summary Results of the Survey 

The underlying philosophy of specifications 
and inspection of optical instruments for pro¬ 
duction control should be geared to the use to 
be made of the instrument. The specifications 
should not include factors of design or desir¬ 
able results of design, but must be directed only 
toward the insurance of conformance to a fixed, 
acceptable design. Since it should be the in¬ 
spector's duty to measure conformance to a 
given design and not to judge between one de¬ 
sign or another, he must be provided not only 
with definite numerical specifications but quan¬ 
titative means for the determination of con¬ 
formance to these specifications. His judgment 


the abilities of the industry. It is a natural de¬ 
sire to include only the best producible com¬ 
ponents, and from the point of view of a long¬ 
term improvement in the industry, this practice 
has advantages. On the other hand, many 
usable optics are now being discarded for the 
sole reason that the industry can do better. Ex¬ 
amples are the questionable cases of striae, 
scratches, inclusions, and defective components 
which could be used by proper selection and 
matching. As a test, the compiler of specifica¬ 
tions might ask himself how the variation of 
focal length, or thickness, or concentricity, or 
freedom from striae, of this particular lens 
affects the resolving power of the complete in¬ 
strument. If the result of a change of ±10 per 
cent in focal length, say, of a particular lens in 
a complex system makes no measurable change 
in the KDC efficiency, then such should be the 
















































224 


OPTICAL TESTING METHODS 


specification even though this lens could be 
made to a ±2 per cent tolerance. The saving in 
production time and control may be consider¬ 
able. 

In order to aid in the solution of part of the 
problem set forth here, several new instru¬ 
ments have been developed. Their purpose is 
primarily to reduce to a quantitative basis the 
more important specifications and inspection 
methods covering the resolution of complete in¬ 
struments as well as their components. At the 
close of World War II, considerable progress 
had been made in the application of these in¬ 
struments. The entire production of the M-71 
telescope is now controlled by the KDC ap¬ 
paratus. Great numbers of other instruments 
have been checked on this machine. The Michel- 
son-Twyman interferometer has been used for 
certain individual checks by various manufac¬ 
turers and is enjoying increasing popularity. 

4 3 IMPROVEMENTS IN TESTING METHODS 

As has been pointed out, one of the most 
urgently needed improvements in testing meth¬ 
ods is the introduction of types of apparatus 
and methods so designed as to yield impersonal 
numerical results. These results, if they are to 
be of maximum usefulness, must be free from 
the judgment of the inspector and must not 
require a high degree of training or skill. 

The most important single attribute of any 
optical instrument used as an aid to the eye is 
its resolving power. It is therefore highly desir¬ 
able to have available a device which can meas¬ 
ure resolving power. In the Thomas Young Lec¬ 
ture of 1935, 4 Fabry summarized the status of 
knowledge of the resolving power of the eye. 
He described equipment which could be used for 
measuring its resolving power under various 
conditions of target contrast, pupil diameter, 
and optical aid. It was apparent that under 
some conditions the overall resolving power 
was limited by the optical aid and not by the 
eye. Here, then, was a method for measuring 
the resolving power of an instrument which 
could perhaps be made independent of the eye. 
It was from this point of view that the KDC 
apparatus was developed at Pennsylvania State 
College under NDRC. 


431 The Kinetic Definition Chart 
Apparatus 

The resolving power of the eye has been 
found to be 4.5 sec of arc per inch of aperture 
for small diameters of the pupil. The resolution 
deteriorates for larger pupil diameters because 
of the optical errors of the eye. It is highly 
significant that the value 4.5 sec is actually su¬ 
perior to the value, 5.54 sec per inch, defined 
somewhat arbitrarily by the Rayleigh criterion, 
on the basis of diffraction theory. The resolving 
power is measured by the smallest angular 
separation of a set of parallel black bands sepa¬ 
rated by white bands of the same width which 
can just be resolved. The black and white 
bands, of which there are 8 to 20, are arranged 
in a square array. Such a target is known as a 
Foucault test object. A group of these objects 
was arranged so as to have the directions of the 
lines different in neighboring groups for use in 
the KDC apparatus. Generally, the objects are 
placed around a central numeral which is used 
for focusing. 

If such a chart is observed through a tele¬ 
scope as the distance from the observer to the 
chart is varied, a certain distance will be found 
beyond which the directions of the lines on the 
chart cannot be determined even though the 
magnification is such as to give an image of suf¬ 
ficient apparent size to permit easy examina¬ 
tion. The critical distance depends on just two 
factors, the limiting aperture of the system, in¬ 
cluding the eye, and the optical quality. The 
latter is the factor which we desire to measure. 

The Concept of KDC Efficiency 

In the experiment just described, the chart 
would have to be moved to such a distance that 
the separations between the centers of the black 
lines would subtend an angle of approximately 
4.5 sec of arc if the telescope were perfect and 
had an aperture of 1 in. If the target is ob¬ 
served with the unaided eye, and if the separa¬ 
tion of the black bands were % in. between 
centers, the chart would have to be placed at 
a distance of 50 ft, assuming a pupil diameter 
of 3 mm. If a perfect telescope is used as an 
optical aid, the distance is increased by the 
magnification of the telescope, provided the 



IMPROVEMENTS IN TESTING METHODS 


225 


pupil of the eye remains the limiting aperture 
of the system. If a 3-mm stop is placed on the 
entrance pupil of the perfect telescope, then 
the critical distance at which the target can 
be resolved is again about 50 ft in spite of the 
fact that the image received by the eye is many 
times the angular size that it was when the eye 
was used without aid. If the telescope is not 
perfect, i.e., if it contains defective parts, is im¬ 
properly constructed, or is of such design as to 


ing aperture and any increase in the aperture of 
the telescope will result in a decrease in the 
critical distance, and thus the efficiency will be 
reduced. 

To overcome this effect in the testing of opti¬ 
cal instruments designed to utilize the pupil of 
the eye as the limiting aperture, auxiliary mag¬ 
nification is often employed. Such magnification 
has an additional advantage in that it increases 
the size of the image and thus reduces eye- 


WHITE SECTOR 

VARIABLE POWER COLLIMATOR \SQUARE APERTURE 

.1 FN5 CAP CONTROLLING SIZE OF \ I rTARGET ^-K D C 

OBSERVER 


INSTRUMENT 



REVERSING 

SWITCH 


(LIMITING DISTANCE OF 

RESOLUTION ) 


PROCEDURE* 

(1) USING THE COLLIMATOR ALONE WITH SUFFICIENT MAGNIFICATION TO MAKE THE 
OBSERVATION OF THE TARGET COMFORTABLE, DETERMINE LIMITING DISTANCE OF 
RESOLUTION, X c ,USING A COLLIMATOR- APERTURE EQUAL TO THE EXIT PUPIL OF THE 
INSTRUMENT TO BE INSPECTED 

(2) DETERMINE THE DISTANCE OF LIMITING RESOLUTION, X c + •,FOR THE COLLIMATOR 
AND THE INSTRUMENT UNDER TEST AS A UNIT USING THE SAME MAGNIFICATION 
FOR THE COLLIMATOR AS IN (\) 


(3) THE EFFICIENCY,E, IS DEFINED BY 

E S 100 x c-f ;— % 

M X c 

Figure 20. Schematic diagram 

have aberrations, the distance at which the 
target can just be resolved is considerably less. 
For example, it might be 40 ft. The ratio of 
these two distances is a convenient measure of 
the effects of the defects. In this case the effi¬ 
ciency would be given as 80 per cent. If the aper¬ 
ture of the telescope is increased from its previ¬ 
ous value of 3 mm, but if the optics are so 
arranged as to insure that the entrance pupil of 
the instrument is still the limiting aperture of 
the system, the distances (for the perfect and 
imperfect instruments) will be increased pro¬ 
portionately, and if no additional defects were 
introduced by the utilization of the larger por¬ 
tion of the lenses, the percentage ratio will re¬ 
main the same. At some point, however, the 
pupil of the eye will once more become the limit- 


THE FOLLOWING EQUATION: 

WHERE M IS THE MAGNIFICATION OF 
THE INSTRUMENT UNDER TEST 

of KDC inspection apparatus. 

strain. Depending upon the purpose of the test, 
in a way which will later be described, the 
auxiliary telescope may contain at its objective 
an aperture whose size is equivalent to the size 
of the pupil of the eye for an observer under the 
conditions normal to the projected use of the 
telescope. 

Model 2-B KDC Apparatus 

The Model 2-B KDC apparatus consists of a 
substantial mounting for the telescope to be 
tested, an auxiliary telescope mounted between 
the eyepiece of the test instrument and the ob¬ 
server’s eye, a long track parallel to the optical 
axis of the machine, and a target box mounted 
on a car running on this track. Figure 20 illus¬ 
trates the physical arrangement of the parts. 



































226 


OPTICAL TESTING METHODS 


The auxiliary telescope has a IT/^-in. aperture 
and a 5-in. focal length. A variety of eyepieces 
provides a wide range of magnification. As a 
result of experience, 50 X for each inch of en¬ 
trance pupil of the instrument being tested is 
satisfactory for comfortable observation. Such 
a magnification provides an apparent angle of 
separation between the centers of dark bands 
of about 4 min to the eye of the observer. For 
any practical size of instrument under test, 
this magnification insures that the final exit 
pupil is of extremely small size. The support 
for the telescope to be tested consists of a pair 
of Y brackets, each constructed so as to be 
conveniently adjustable in height and hori¬ 
zontal position. The track is of lightweight con¬ 
struction and is about 50 ft long. The car sup¬ 
porting the target box is moved along the track 
by means of a motor-driven belt and pulley 
arrangement. The motor is under the control of 
the operator so that as he peers into the eye¬ 
piece he may move the target along the track 
until he has determined the critical distance. 
The target box contains, beside the target, a 
pair of light bulbs which may be adjusted to 
illuminate the target uniformly. In addition, a 
segmented white disk may be rotated in front 
of the target to control the contrast to permit a 
study of the effect of target contrast on effi¬ 
ciency. Figure 21 is a view of a typical target 
box. 

This apparatus has been found to be of con¬ 
siderable use in testing optical instruments. It 
is simple to use, and requires neither a high 
degree of experience nor skill. A drawback is 
that it requires considerable space for the 
track, which is approximately 60 ft long. An 
undesirable feature is that it uses the telescope 
under test conditions at short object distances 
whereas most instruments designed for mili¬ 
tary purposes are intended for use with objects 
sensibly at infinity. 

The Model 4 KDC Apparatus 

Because of the space requirement of the 
Model 2 KDC apparatus, a new type, the Model 
4, has been developed. This instrument is of 
table length, adequately satisfying the space re¬ 
quirement for convenient testing. The two new 
features of the design are the optical target 


which is produced by optical reduction, and the 
effective removal of the target to a large dis¬ 
tance by collimation. The Figures 22 and 23 
show the Model 4 KDC apparatus. The dia¬ 
gram, Figure 24, illustrates the arrangement of 
the optical parts to provide the effect of motion 
within a short space. In the lower right corner 
of Figure 24 is a target, modified from a 
Bureau of Standards Foucault test chart, of 
the type used in this apparatus. 



Figure 21. The target box of Model 2 KDC 
apparatus. 


The target is illuminated by two 40-w lamps 
at a distance of about 2 in. The bulbs and target 
are surrounded on the front and sides by a 
metal screen to keep direct light from reaching 
and disturbing the observer. The collimating 
unit consists of a tube with the collimating ob¬ 
jective at one end and a microscope objective at 
the other. The microscope objective forms a 
greatly reduced image of the target inside the 
tube. Motion of the target toward or away 
from the microscope objective produces a much 
smaller motion of the image in the tube and 
at the same time a change in size of the target 
image. Looking into the collimator objective, 
the changes in apparent size of the target are 
equivalent to moving the target on the track 
of the Model 2 instrument. 

The collimating objective has a diameter of 





IMPROVEMENTS IN TESTING METHODS 


227 


3 in. and a focal length of 15.75 in. The lens 
has been hand-corrected and interferometer- 
checked for high quality. Since the target dis¬ 
tance for optimum performance is usually 
stated in the specifications for the particular 
instrument to be tested, a method must be em¬ 
ployed to adjust the collimation for this dis¬ 
tance. A telescope of high quality is focused on 


this purpose, a perfect instrument consisting 
of a very carefully figured objective and eye¬ 
piece. It is always used with an aperture stop 
equal to the entrance pupil of the instrument to 
be tested. The purpose of this telescope, in 
addition to being an aid in adjustment as de¬ 
scribed, is to act as a basis of comparison in 
that the limiting distance of resolution for a 



Figure 22. The Model 4 KDC apparatus with standard telescope in place. 


a target at the specified distance, and is then 
placed in the KDC machine and used to adjust 
the collimator so that the test target is in sharp 
focus. The focusing is accomplished by chang¬ 
ing the distance between the collimator and 
microscope rather than the distance between 
the microscope objective and the real target. 
After the adjustment for target distance, the 
real target may be moved to obtain the critical 
position for resolution. 

The standard telescope just mentioned is, for 


perfect instrument may be established with it. 
It is to be pointed out here that the spacing of 
the real target and the focal length of the 
microscope objective are so chosen as to make 
the setting of the real target for the critical dis¬ 
tance for the perfect instrument correspond 
to a virtual target distance equal to that spec¬ 
ified for the instrument to be tested. 

An auxiliary telescope is employed to produce 
enough overall magnification so as to reduce 
the exit pupil below the minimum diameter of 





228 


OPTICAL TESTING METHODS 



Figure 23. The Model 4 KDC apparatus with production telescope under test. 


COLLIMATOR OBJECTIVE 



ARRANGEMENT FOR DETERMINING THE LIMITING DISTANCE OF RESOLUTION, X c+ j, 

FOR THE TELESCOPE UNDER TEST USING THE AUXILIARY TELESCOPE 


PROCEDURE 

[1] DETERMINE THE LIMITING DISTANCE OF RESOLUTION FOR THE STANDARD TELESCOPE 

[2] DETERMINE THE LIMITING DISTANCE OF RESOLUTION FOR THE INSTRUMENT UNDER 

TEST 

KDC EFFICIENCY £ f-P-t « « | QO X 



TYPE OF TARGET 

(MODIFIED BUREAU OF STANDARDS RESOLUTION CHART) 




Figure 24. The Model 4 KDC apparatus. 






















































IMPROVEMENTS IN TESTING METHODS 


229 


the pupil of the eye. Thus, the resolution of the 
eye is removed from the measurements to a 
very great extent. The objective of the auxili¬ 
ary telescope is equipped with an entrance stop. 
This stop may correspond to the diameter of 
the pupil of the average observer’s eye, or to 
the specified exit pupil of the telescope to be 
tested. The auxiliary telescope produces enough 
magnification to enlarge the apparent target to 
a comfortable size and thus reduce eyestrain. 

The mechanism for controlling the apparent 
target distance consists of a screw and slide 
arrangement which moves the real target with 
respect to the microscope objective. The posi¬ 
tion of the target along the slide is indicated on 
a scale convenient to the observer. The machine 
is so constructed as to provide a motion of the 
target from a distance of 6 in. to 16 in. from 
the microscope objective. This range has 
proved adequate for testing a wide variety of 
optical instruments. 

The motion of the real target produces some 
motion of the image at the focus of the micro¬ 
scope objective, but this motion is small com¬ 
pared to the depth of focus of the collimator ob¬ 
jective. The principal result is the change of 
size of the image of the target. The net effect 
is to produce a target at a nearly fixed apparent 
distance but with variable line spacing. 

With the Model 4 KDC apparatus, the con¬ 
cept of efficiency is based upon the real position 
of the target. If the scale reading for a distance 
corresponding to the limit of resolution of the 
standard telescope is S, and the distance for 
the telescope under test is X , the efficiency E is 

E = ^ X 100 per cent. 

o 

This relation depends upon the fact that the 
size of the image formed by the microscope ob¬ 
jective is inversely proportional to the target 
distance over the range of distances employed. 
The zero of the scale corresponds to the case in 
which the target is coincident with the micro¬ 
scope objective. 

Procedure for Making a 
KDC Determination 

The Model 4 KDC apparatus, because of its 
convenient size and constant object distance, 
has replaced the Model 2 for production control. 


The procedure for carrying out a test on this 
machine is as follows: 

1. The alignment is checked by means of 
gauges described later in this section. 

2. The standard telescope is focused on an 
object at the required object distance for the 
instrument to be tested. 

3. The standard telescope is placed in the 
KDC apparatus. The Y supports are adjusted to 
bring the axis of the telescope into line with the 
axis of the collimator, and the collimator is 
focused to bring the target into sharp focus. It 
is generally convenient to run the target car¬ 
riage in to a distance of about 7 in. so that the 
number in the center of the array is well re¬ 
solved. (See Figure 24.) This adjustment is 
made without changing the focus of the tele¬ 
scope from that of step (2). 

4. Next, the auxiliary telescope is placed in 
the machine and focused to bring the numeral 
on the target into sharp focus. 

5. With the auxiliary and standard tele¬ 
scopes in position, the target distance is in¬ 
creased until the observer can no longer de¬ 
termine the direction of the parallel lines in 
any one of the various groups of the target. 
During the measurement, the objective of the 
standard telescope is stopped with an aperture 
equal to the entrance pupil of the telescope to be 
tested. 

6. The reading of the scale at this position 
gives the value S in the equation above. 

7. The standard telescope is replaced by the 
one to be tested. It may be necessary to readjust 
the Y supports to align the axis of this telescope 
to that of the collimator. 

8. The target is run in to about 7 in. and the 
telescope under test is focused to bring the 
number on the target into sharp focus. 

9. The target is then moved away until the 
critical distance of resolution is reached. The 
criterion is the same as in step (5). 

10. The reading of the scale gives the value 
of X, and the efficiency E is, as before, 

E = ^ X 100 per cent. 

When testing a large number of identical in¬ 
struments, steps (1) through (7) need be made 
only once when the KDC machine is first placed 
in operation. Steps (8), (9), and (10) are all 



230 


OPTICAL TESTING METHODS 


that are required for each new test telescope. 

Two gauges are provided with the instru¬ 
ment to aid in the adjustment in step (1). In 
order to adjust properly the distance between 



Figure 25. The target of the Model 4 KDC ap¬ 
paratus with space gauge in position. 


the target and the microscope objective, a space 
gauge is used. Figure 25 shows this gauge in 
position. The principal plane of the microscope 
objective is 2.30 in. inside the collimator, meas¬ 
ured from the end of the tube. It is convenient 
to set the scale reading at 7.0 in. The required 
distance from the end of the collimator tube to 
the target is then 7.00 — 2.30 = 5.70 in. The 
U-shaped gauge is thus 5.70 in. between outside 
ends. With the indicator set at 7.00 and the 
gauge in place, as shown in Figure 25, the col¬ 
limator is moved along the base until its end 
contacts the gauge. 

The second gauge is used to make the proper 
alignment of the optical parts of the KDC 
apparatus. This gauge, called the alignment 
gauge, is composed of two parts, a base which 
fits the ways on the optical bench and supports 
a circular plate, and a pointer which may be 
screwed into the center of the plate. Figure 26 
shows the gauge with the pointer in place. The 
center of the target, when coincident with the 
end of the pointer, is then on the optical axis of 
the machine. Slots under the fastening screws 
supporting the target permit horizontal and 
vertical motion. The face of the gauge with the 


pointer removed is used to check the alignment 
of the collimator, and when the circular plate 
is concentric and in contact with the cell of the 
objective, the positioning is correct. The align - 



Figure 26. The alignment gauge in position for 
centering the target. 


ment gauge and the positioning screws of the 
collimator are shown clearly in Figure 27. 

Special Fixtures and Attachments for 
KDC Apparatus 

The KDC apparatus may be used not only to 
determine the resolving power of an instrument 
on its axis but off axis as well. Certain fixtures 
and attachments have been developed for the 
purpose of making these off-axis measure¬ 
ments. Besides those designed to support tank 
sights (M-70, M-71, M-72, M-76, and others), 
a special support for binoculars makes the 
measurement of these instruments easy. 

Two different types of off-axis fixtures are 
available. In one type, Models 1 and 2, the tele¬ 
scope being tested and the auxiliary telescope 
are mounted on pivoted supports so that the 
test telescope may be rotated about its entrance 
pupil and the auxiliary telescope about the test 
telescope’s exit pupil. Figure 28 is a diagram 
showing the relation of the various angles of 
rotation. A similar fixture for use with binoc¬ 
ulars is illustrated in Figures 29 and 30. 

A different type of off-axis fixture employs 
four front surface mirrors, two fixed and two 



IMPROVEMENTS IN TESTING METHODS 


231 


rotatable. Figure 31 shows the optics of the fix¬ 
ture. A typical setup may be seen in Figure 60. 
The camera shown in the figure is replaced by 
the normal auxiliary telescope for KDC effi¬ 
ciency measurements. 


Table 5. A comparison of KDC efficiency values 
for tank telescopes determined by the Model 2 and 
the Model 4 KDC apparatuses. 


M-70 telescopes 
serial nos. 

Axial KDC efficiency 
(per cent) 

Model 2 Model 4 

57228 

63 

60 

57109 

82 

84 

57098 

65 

63 

57194 

61 

65 

57104 

94 

92 

54695 

91 

90 

54532 

63 

63 

58598 

73 

72 

57209 

80 

80 

57105 

63 

65 


In order to determine the relative accuracy 
of the Model 4 KDC apparatus with respect to 
Model 2, a large number of telescopes have been 



Figure 27. The alignment gauge in position for 
squaring the collimator. 


measured by both machines. Table 5 shows the 
results for a typical set of ten M-70 telescopes. 
It will be noted that not only are the poor tele¬ 
scopes poor on both machines and the good ones 
likewise good, but the actual values of the effi¬ 
ciencies are in all cases very close together. 
This table also shows the range of efficiencies 
observed in a single type of telescope. Five tele¬ 
scopes have efficiencies less than 70 per cent, 
whereas two have efficiencies greater than 90 


per cent. These telescopes were not selected in 
any special way, and may be considered as sam¬ 
ples of production. This telescope obviously has 
a potential efficiency of something better than 
90 per cent, yet half the standard production 
models in this group fall below 70 per cent. 
Table 6 shows a comparison between the KDC 
efficiency values and a group of values obtained 
by visual grading of tank telescopes. 


Table 6. Comparison of KDC efficiency with 
visual grading. 


Instrument 

no. 

Exit pupil 

1 

Observer 
2 3 

4 

KDC 
efficiency 
(per cent) 

4553 

7 

2 

2 

2 


72 

4468 

7 

1 

1 

1 


88 

4553 

2.5 

2 

2 

2 


90 

4468 

2.5 

1 

1 

1 


97 

4524 

7 

2 

2 

2 


68 

4820 

7 

1 

1 

1 


80 

4501 

7 

2 

2 

2 


77 

4820 

7 

1 

1 

1 


80 

4501 

2.5 

1 

1 

1 


98 

4820 

2.5 

2 

2 

2 


92 

4980 

7 

1 

1 

1 

1 

95 

2763 

7 

2 

2 

2 

2 

83 

10628 

7 

4 

3 

3 

3 

82 

2 

7 

3 

4 

4 

4 

60 


Typical Results 


Because of the great number of measure¬ 
ments which have been made on various optical 
instruments with the KDC apparatus, no at¬ 
tempt will be made here to present the labora¬ 
tory results in detail. Sufficient laboratory ob¬ 
servations have been made, however, to lead to 
the acceptance of the machine as a control of 
production for the Type M-71 tank telescope 
and there are good indications that its use will 
continue to grow for other types of instru¬ 
ments. 

One question that naturally arises concerns 
the degree of correlation between the visual 
grading of a telescope and the KDC efficiency. 
A group of telescopes were graded in pairs, each 
observer deciding independently which one of 
the pair was the better. Some of the grading 
was done on a bright clear day using a distant 
landscape, while some of the grading was done 
with an illuminated target indoors in a dark 
room. A few of the telescopes were graded 
twice, once with a 7-mm exit pupil and once 


















232 


OPTICAL TESTING METHODS 


with a 2.5-mm pupil stop. The results of the 
grading and the measured values of the KDC 
efficiency appear in Table 6. 

Besides the nearly perfect correlation evi¬ 
denced in the table between visual grading and 


order at 2.5 mm. The KDC efficiencies show the 
same result. In all cases the efficiencies im¬ 
proved with reduction of aperture, indicating 
some residual spherical or chromatic aberration 
in the design. 



BETWEEN AXIS OF TARGET AND AXIS OF 
TANK SIGHT UNDER TEST 

Figure 28. The off-axis fixture for tank sights. 


KDC measurements, several other points are of 
interest. In the case of the fourth pair, the ef¬ 
ficiencies are quite close together (77 to 80 per 
cent) yet the observers could detect a difference 
in the performance. The present specification 


Another interesting result appears in Table 
7 which summarizes the KDC efficiencies of 20 
M-70D telescopes of each of five manufacturers. 
Manufacturer No. 1 has his production under 
such control that nearly every telescope is 90 


HOLES USED TO 
DETERMINE AMOUNT 
OF ROTATION 



PLAN SHOWING RELATIVE 
ROTATION OF BINOCULAR AND 


PLAN OF BASE 



on the type of telescope represented in all but 
the last group is 75 per cent minimum. As a 
basis of comparison, 92 to 94 per cent effi¬ 
ciencies at full aperture appear possible with 
the design. The pair 4501-4820 was graded in 
one order at 7-mm exit pupil and in the reverse 


per cent or better. On the other hand, manu¬ 
facturers No. 4 and No. 5 have trouble making 
any as good as this. Five telescopes manufac¬ 
tured by No. 1 measure 100 per cent and six 
measure 99 per cent. As a matter of fact, one 
made by No. 4 measures 99 per cent and one 

















































































IMPROVEMENTS IN TESTING METHODS 


233 


each made by Nos. 4 and 5 measure 97 per cent, 
so it is quite possible by the methods used by all 
manufacturers to produce telescopes with effi¬ 
ciencies of 97 per cent or better. If the specifica¬ 
tions were set at 80 per cent or better, 37 per 
cent of the one hundred samples would be re¬ 
jected yet all of manufacturer No. 1 would be 


positions off axis. Twenty, forty, and sixty mils 
to right and left were chosen as standard val¬ 
ues of the angle of view. Figures 32 and 33 
show the results for manufacturers Nos. 1 
and 5 respectively. 9 The general shape of the 
curve relating KDC efficiency and field angle 
for a tank telescope is well illustrated in these 



Figure 30. The off-axis fixture for binoculars. 


accepted. Obviously better production methods 
are required in the case of Nos. 3, 4, and 5. 


Table 7. KDC efficiencies (per cent) of M-70D 
telescopes. 


Manu¬ 

facturer 

100-90 

89-80 

79-70 

69-60 

59-0 

Total 

1 

19 

1 

0 

0 

0 

20 

2 

11 

5 

3 

1 

0 

20 

3 

1 

12 

7 

0 

0 

20 

4 

2 

3 

8 

4 

3 

20 

5 

2 

2 

7 

7 

2 

20 

Total 

35 

23 

25 

12 

5 

100 


The telescopes discussed above were meas¬ 
ured not only on the axis, for which the results 
are shown in Table 7, but also at a number of 


figures. Off axis the efficiency drops, due to 
aberrations, particularly coma and astigma¬ 
tism. 

Table 8 shows the results from KDC meas¬ 
urements for a group of 7x50 binoculars. The 
measurements on two captured binoculars are 
also included for comparison. The Japanese 
binocular had an unusually high efficiency for 
an instrument of its power and aperture. It 
should be noted that if the acceptable KDC effi¬ 
ciency is placed at 80 per cent, while nine of the 
twenty optical systems would have passed, only 
two complete binoculars would be accepted. 
Binocular No. 1 includes the second best (left) 
and the worst (right) of the twenty optical 
systems. Eighty per cent is probably too high a 









234 


OPTICAL TESTING METHODS 


requirement for this system. Seventy per cent 
may be adequate and more practical, but 90 to 
92 per cent can obviously be attained with this 
design. 

One other laboratory result is of special in¬ 
terest. A simple telescope was deliberately de¬ 
faced by scratching the objective. A far greater 
effect was produced by scratches on the front 


The Precision of the KDC Apparatus 
As is usually the case in other kinds of phys¬ 
ical measurements, the precision of the results 
obtained with the KDC apparatus depends on 
many factors. Some of the most important of 
these factors will be considered individually in 
what follows. One of the most obvious effects 
to be expected is a possible relation between 



surface of the field lens of the eyepiece. The 
telescope had originally a KDC efficiency of 
100 per cent. When the field lens had 25 per 


Table 8. KDC efficiency of ten 7x50 binoculars 
(per cent). 


No. 

Left barrel 

Right barrel 

1 

91 

62 

2 

79 

76 

3 

78 

76 

4 

78 

73 

5 

83 

86 

6 

85 

76 

7 

81 

73 

8 

72 

81 

9 

76 

87 

10 

92 

87 

Best 

92 

87 

Average 

81 

78 

Worst 

72 

62 

German (8x40) 

92 

89 

Japanese (14.7x88) 

Broken 

96 


cent of its surface scratched, the efficiency was 
95 per cent for a target contrast of 100 per 
cent, and an efficiency of 87 per cent for a con¬ 
trast of 20 per cent. Thus scratches have very 
little effect upon the performance. 


KDC efficiency and target illumination. The il¬ 
lumination may be varied by changing the bulbs 
in the target box. Those actually used ranged 
from 10 w to 100 w, two of the same size always 
being employed. The results appear in Table 9. 
The illumination in watts is the sum of the 
two bulbs, the illumination in foot-candles is 
the actual light on the target, X is the limiting 
distance of resolution, and S is computed for 
the limiting distance for the eye times the mag¬ 
nification of the instrument. The trend of reso¬ 
lution is toward larger distances for higher 


Table 9. 

The effect of 

target 

illumination on 

KDC efficiency. Model 2 apparatus. 


Footcandles 




on target 

X 

S 

Efficiency 

40 

35.5 

36.4 

97.6 

50 

36.5 

37.3 

97.8 

115 

36.9 

37.9 

97.4 

200 

37.8 

38.5 

97.3 

400 

38.1 

39.1 

97.6 

800 

38.2 

39.6 

97.8 


light intensities, both with the instrument and 
with the standard. The efficiencies remain 
nearly the same, however. It thus appears that 


































IMPROVEMENTS IN TESTING METHODS 


235 


the result is sensibly independent of the level 
of illumination. This being the case, the choice 
of illumination may be based upon ease of meas¬ 
urement. Two 40-w lamps are normally used to 
give a level of about 200 footcandles at the 
target. 



60 40 20 0 20 40 60 


ANGLE OF VIEW IN MILS 

Figure 32. KDC efficiency vs angle of view for 

manufacturer No. 1. 

If the KDC efficiency is to be a true property 
of the telescope under test and is not to depend 
on the way in which it is measured, it must be 
demonstrated that the value is independent of 
auxiliary magnification. The effect of magnifi¬ 
cation is clearly shown in Table 10. For large 
values of contrast, the only ones used in prac¬ 
tice, the limiting distance increases rapidly 
with the magnification. At some value near 
30X overall, the resolution becomes constant 
and further magnification does not increase it. 
At the value 30 X, the limit of resolution of the 
system is that of the telescope under test and 
not that of the eye. On this basis a standard 
value of about 50 X overall is generally used. 
An auxiliary telescope is chosen to raise the 
magnification of the instrument being tested to 
this amount. 

One other point in this connection has been 
investigated, namely, the relation of target size 
and number of lines to target distance for lim¬ 
iting resolution. Figures 34 and 35 show the 
observed relations. From these results it is safe 
to assume that a telescope whose KDC efficiency 
is determined on an instrument with a target 
having one size and number of lines will be 
nearly the same when determined on an instru¬ 
ment with a different target. Further, for a 
telescope whose efficiency is 100 per cent the 
angle of minimum resolution for parallel lines 
will lie between 4.62 and 4.70 sec of arc per 


inch of aperture. The carefully built standard 
S-l is assumed to be 100 per cent efficient. 

The probable error of a single KDC efficiency 
determination depends on the auxiliary magni¬ 
fication and the value of the efficiency of the 
instrument. The effect of target contrast has 
already been discussed and since all produc¬ 
tion measurements are made with contrasts of 
100 per cent only the results for this value will 
be discussed. 


Table 10. Effect of magnification on resolution. 
(Angle of minimum resolution for M-76C telescope, 
aperture 0.863 in.) 


Total 

magnification 

100% 

50% 

30% 

10% 

7.5 

5.2 

(X 

5.5 

10-5 Rad) 
7.0 

9.0 

10 

4.0 

4.6 

5.0 

8.2 

15 

3.3 

3.9 

4.3 

7.5 

20 

3.05 

3.6 

3.9 

7.2 

30 

2.86 

3.35 

3.62 

7.2 

40 

2.85 

3.28 

3.60 

7.7 

50 

2.85 

3.25 

3.60 

8.5 

60 

2.84 

3.27 

3.64 

9.2 

70 

2.85 

3.30 

3.67 

10.0 


Many measurements have been made with 
Model 2 KDC apparatus and some with the 
Model 4, for the particular purpose of deter¬ 
mining the precision of a single efficiency meas¬ 
urement. Seven different observers were in- 



Figure 33. KDC efficiency vs angle of view for 
manufacturer No. 5. 


volved in these measurements, six on the 
Model 2 machine, two of these and a third new 
observer on the Model 4 machine. Tables 11 and 
12 exhibit the results of this set of measure¬ 
ments. It is to be noted that some observed 
values of the efficiency are means derived from 











236 


OPTICAL TESTING METHODS 


25 observations, and some from 50, but a ma¬ 
jority are means of 100 observations. All of the 
probable errors indicated in the tables were 
deduced from the agreements of the individual 
measurements on the assumption that the er¬ 
rors were distributed at random. The numbers 
which appear in the tables are the probable 


server, appears in the tables. Associated with 
this value of efficiency is a probable error of 
a single observation deduced from the residuals 
within each group. The residuals were com¬ 
puted from the mean for that group. In the 
column headed, “Means,” the efficiencies are the 
simple unweighted means for each group of six. 


Table 11. The probable error of a single KDC efficiency measurement. 

Model 2 










Most 




Observer 




probable 

Telescope 

1* 

2* 

3* 

4+ 

5f 

6* 

Means 

values 

M-71 No. 2335 

84.4 ±0.65 

84.7 ±0.68 

84.5 ±0.45 

82.0 ±0.69 

83.0 ±0.49 

84.9 ±1.07 

83.9 ±0.69 

84.3 ±0.59 

M-72 No. 12242 

59.3 ±0.90 

58.3 ±0.90 

59.5 ±0.60 

58.5 ±0.77 

56.1 ±0.49 

61.2 ±0.96 

58.8 ±0.77 

58.6 ±1.06 

M-76 No. 5492 

74.4 ±0.92 

72.9 ±1.35 

71.5 ±0.66 

76.6 ±1.13 

76.4 ±0.64 

74.4 ±1.75 

74.9 ±1.07 

73.7 ±1.36 

S-3 

97.5 ±0.24 

96.6 ±0.92 

97.7 ±0.38 

99.4 ±0.66 

97.0 ±0.52 

99.4 ±0.96 

98.0 ±0.61 

97.5 ±0.38 

PE 

±0.68 

±0.96 

±0.50 

±0.81 

±0.53 

±1.18 


... 

SE 

+0.04 

-0.38 

-0.03 

+ 0.22 

-0.79 

+ 1.64 


±0.85 


* 100 observations, 
f 50 observations. 


errors of single observations and are not to be 
confused with the normally reported probable 
errors of a mean. 

The efficiencies for one or more examples of 
three different types of telescopes appear in 
the tables. On the Model 2 apparatus, each tele¬ 


The probable errors in this column are the 
means of the probable errors for each observer. 
The column headed, “Most probable values” 
contains the means, weighted according to the 
number of observations and according to the 
probable error for each group. The probable 


Table 12. The probable error of a single KDC efficiency measurement. 


Model 4 


Telescope 

1* 

Observer 

2+ 

7$ 

Means 

Most probable 
values 

M-71 No. 9911 

79.5 ±0.45 

74.7 ±0.63 

78.5 ±0.95 

77.6 ±0.68 

76.6 ±1.05 

No. 2342 

74.7 ±0.79 

84.1 ±3.48 

82.3 ±0.95 

80.4 ±1.73 

79.7 ±2.58 

No. 2337 

84.1 ±0.79 

85.0 ±1.35 

81.3 ±1.03 

83.3 ±1.06 

83.4 ±1.08 

No. 2335 

84.1 ±0.32 

86.9 ±1.11 

86.9 ±2.22 

86.0 ±1.22 

84.7 ±0.83 

M-72 No. 2 

75.9 ±1.38 

69.7 ±0.66 

75.9 ±1.78 

73.7 ±1.27 

70.4 ±1.30 

No. 12242 

64.4 ±0.53 

61.3 ±0.72 

66.6 ±1.05 

64.2 ±0.76 

62.8 ±1.34 

No. 12305 

60.4 ±0.39 

58.9 ±0.72 

59.6 ±1.05 

59.7 ±0.72 

59.6 ±0.47 

M-76 No. 4980 

97.9 ±0.18 

95.8 ±0.18 

96.9 ±0.81 

96.8 ±0.39 

96.4 ±0.57 

PE 

±0.60 

±1.11 

±1.23 

• • • 

±1.15 

SE 

+0.84 

-0.58 

+0.50 

+0.91 




* 25 observations, 
f 100 observations, 
t 50 observations. 


scope, one of each type, was measured inde¬ 
pendently 50 or more times by six observers. 
Four of the observers made 100 measurements 
on each telescope; the other two made 50. The 
mean value for each group of 50 or 100 meas¬ 
urements, for each telescope and for each ob- 


error of the most probable value referred to 
unit weight is also given in this column. The 
latter includes the probable errors of a single 
observation deduced from each observer’s prob¬ 
able error, as well as the agreement of each 
observer’s mean with the most probable value. 















IMPROVEMENTS IN TESTING METHODS 


237 


Across the bottom of the group of observations 
on the Model 2 machine is a row labeled “PE.” 
These numbers are the mean values of the prob¬ 
able errors for each observer. The row headed 
“SE” contains the systematic errors deduced 
from the agreement of each observer’s mean 



APERTURE IN INCHES 


Figure 34. The limiting distance of resolution 
vs aperture. 

with the most probable value. The measure¬ 
ments for the Model 4 machine follow the same 
system except that more than one example of 
two of the telescope types was measured. 

A number of facts concerning the expected 
accuracy of a single observation on the KDC 
machine may be deduced from Tables 11 and 
12. First and probably most important is that 
for any telescope picked at random and for any 
observer picked at random, the probable error 
to be expected from a single observation is 
about 0.85 per cent for the Model 2 machine 
and 1.15 per cent for the Model 4 machine. The 
probable error here means that for an observa¬ 
tion made by an observer, half the time the 
value which that observer obtains will have a 
residual from the correct value equal to or less 
than the reported probable error. The remain¬ 
ing half the time the value will be equal to or 
greater than the reported probable error. By 
the correct value is meant the weighted mean 
of a very large number of observations made 
by a very large number and variety of observ¬ 
ers on the particular telescope in question and 
on the particular model of the KDC machine 
employed. A survey of the most probable values 
of efficiency and their probable errors shows 
little correlation between these two numbers. 
There is some evidence to indicate that higher 


values of efficiency may be determined with 
higher precision, but in the range from 60 per 
cent to 80 per cent the probable error does not 
change significantly. 

There is considerable variation among ob¬ 
servers. Consider for a moment a group which 
includes good and bad observers. Each indi¬ 
vidual of this group makes a large number of 
measurements on a particular telescope. The 
most probable value of the efficiency will be 
the weighted mean of the means of each indi¬ 
vidual’s observations. The weights to be ap¬ 
plied will depend upon the internal agreement 
in each individual’s observation set. From the 
table we may deduce that those observers in 
this group who have large probable errors 
may be expected to have large systematic errors 
also. For example, observers Nos. 6 and 7 have 
the largest probable errors and they also have 
the largest systematic errors. Observers Nos. 1 
and 3 on the other hand have small probable 
errors and very small systematic errors. Ob¬ 
server No. 2 is quite erratic. In one case he had 
a probable error by far the largest of any in 
the entire group, but in another case he had 
a probable error equal to the smallest of any 
in the group. The mean value places him with 
the poorer observers and his systematic errors 
are fairly large. Observers Nos. 1 and 2 meas- 



Figure 35. The limiting distance of resolution 
vs target bandwidth. 


ured telescopes on both Model 2 and Model 4 
KDC machines. Observer No. 1 was the better 
in both cases, observer No. 2 taking second 
place, both on the basis of probable errors and 
on the basis of systematic errors. 

In two cases the same telescope was observed 
on both machines. In the case of telescope M-71, 





OPTICAL TESTING METHODS 


238 


No. 2335, Model 2 machine yielded a most 
probable value 0.4 per cent smaller than the 
Model 4 machine. This difference is of the same 
order as the probable error of a single obser¬ 
vation. Since the most probable values depend 
in one case upon 450 observations and in the 
other case upon 175, it seems safe to say that 
there is actually a systematic difference of 0.4 
per cent between the two machines for this 
value of efficiency. Telescope M-72, No. 12242, 
was also measured on both machines. The 
Model 2 machine yielded a value 4.6 per cent 
smaller than the Model 4 machine. This value 
is nearly four times the probable error of a 
single observation and some forty times the 
probable errors of the means. Thus in the 
neighborhood of 60 per cent efficiency it ap¬ 
pears that the Model 4 machine gives a value 
4.5 per cent higher than the Model 2. 

All of the foregoing results depend on a very 
large number of measurements made by what 
appears to be a random sampling of observers. 
Thus the conclusions drawn from the treatment 
of the observations should be trustworthy. It 
is not known, however, whether the systematic 
differences between the two machines would be 
repeated on other machines of the same model, 
or whether they are peculiar to the particular 
instruments employed. 

The Use of the KDC Apparatus 
for Inspection 

The KDC apparatus may be used either for 
the inspection of completed telescopes to deter¬ 
mine the degree to which the instrument meets 
its specifications, or for the evaluation of a 
particular optical design. So far in this chapter 
we have only been concerned with the first of 
these, and in this section will be given some 
recommendations for such use. In the next 
major section (see Section 4.4) are discussed 
some of the applications to design evaluation. 

One of the primary requisites for an inspec¬ 
tion procedure is that it be simple and rapid. 
It was thought at first that tests should be 
made on targets of reduced contrast since most 
targets viewed with military telescopes are of 
low contrast. The applicability of this assump¬ 
tion depends on the inspection philosophy. Ex¬ 
perience has shown that if a telescope performs 


well at 100 per cent contrast it will perform 
well at all other contrasts; likewise a telescope 
which is poor at high-target contrast will be 
poor by comparison at low contrasts. In fact 
the spread between good and bad instruments 
will be larger at high contrasts than at low. 
Since the measurements are not only more pre¬ 
cise at 100 per cent contrast but are also easier 
to make, it seems best to carry out tests at 100 
per cent contrast, even though this does not 
represent the conditions of use. It is not the 
purpose of this test to determine how the tele¬ 
scope will perform in use, for that should have 
been tested before the design was adopted, but 
rather to grade individual instruments by 
means of those factors upon which its perform¬ 
ance depends. 

In a similar way the KDC efficiencies of an 
instrument for off-axis rays are very valuable 
and educational for the designer, but they 
yield no more information on the ability of the 
telescope to meet the specifications than does 
a single measurement on the axis. No tele¬ 
scopes have been found which are good on the 
axis and poor when off, or poor on and good 
off the axis, as compared to a normal telescope. 
Thug one measurement suffices. 

The question of the size of the exit pupil to 
be employed during the measurements is of the 
same type. Under normal conditions of use, the 
exit pupil is limited to 7 mm or so by the pupil 
of the eye. Naturally the telescope behaves bet¬ 
ter at this reduced aperture than if it were 
used at full aperture. But since a good telescope 
at full aperture is also good, in a comparative 
sense, if the aperture is reduced, while a tele¬ 
scope which is poor at reduced aperture is still 
worse at full aperture, the test is most sensitive 
at full aperture. It is to be expected that the 
KDC efficiencies will be smaller than those de¬ 
duced from the resolving power using the eye 
without auxiliary magnification. This is unim¬ 
portant, however, since the poorest usable tele¬ 
scope will be selected by considerations other 
than a pre-established value of the efficiency. 
This telescope may then be tested at full aper¬ 
ture and the value so obtained embodied in the 
specifications. 

It is therefore desirable that the specifica¬ 
tions indicate the minimum KDC efficiency at 





IMPROVEMENTS IN TESTING METHODS 


239 


full aperture for a target of 100 per cent con¬ 
trast on the axis. The acceptance test then 
consists of one simple measurement. If a tele¬ 
scope which has an efficiency equal to the mini¬ 
mum specification is just satisfactory in the 
service for which it was designed, then all 
telescopes with this or greater efficiency will 
be satisfactory and those with lower efficiency 
will not. Thus the purpose of the test is ful¬ 
filled. 

43 2 The Michelson-Twyman 

Interferometer 

In spite of the fact that the interferometer 
was first developed and used to measure the 
motion of the earth with respect to the ether, it 
has become a very valuable tool in many widely 
different fields. The use of this device, or the 
principle on which it is based, for the measure¬ 
ment of distances with extreme accuracy is well 
known. The underlying principle of the inter¬ 
ferometer, that of portraying light path differ¬ 
ences as dark bands across the field of the in¬ 
strument, has become very useful for the test¬ 
ing of lenses, prisms, and even complete in¬ 
struments. 

F. Twyman has investigated the application 
of the interferometer to the study of optical 
systems in general. Others have developed 
methods for computing the shape and appear¬ 
ance of the interference fringes to be expected 
from known optical designs of lenses and sim¬ 
ple combinations of lenses. This work has been 
used by the group at Pennsylvania State Col¬ 
lege with the aim of applying the interferom¬ 
eter to the inspection of optical components. 

The problem is twofold. The instrument must 
be so modified as to be easily adjusted and used, 
for the interferometer is notable for its inher¬ 
ent difficulty of operation. The fringes for a 
perfect lens or other component must be com¬ 
puted or observed and the effect of errors de¬ 
termined so that an intelligent interpretation 
of the observed fringe pattern may be made in 
terms of the quality of the optical element. 

The Principle of the Interferometer 

The interferometer employs two light beams 
starting from a common source. After travel¬ 


ing over separate paths, these are recombined 
before entering the eye of the observer. Fig¬ 
ure 36 shows the arrangement of the optical 
parts for a typical interferometer. The divid¬ 
ing plate E has a half-reflecting coat on the 
side facing the objective D. The collimated 
beam of light from D is split into two equal 
beams at this surface. One beam travels along 
the path EFEJK and to the observer’s eye be¬ 
yond K. F and H are fully reflecting mirrors. 
The other half of the beam proceeds along the 
path EGHGEJK and so to the eye. 

When the beam is split at E, it is not divided 
geometrically, say into right and left or top 
and bottom halves, but each ray (all are par¬ 
allel) is divided into two equal parts. Thus the 
cross sections of the two beams proceeding 
from E are equal and they recombine as they 
return to E. Let it be assumed as a first exam¬ 
ple that the distance which one half of each 
ray must travel is exactly equal to the distance 
which the other half of that ray travels. Then 
according to the wave theory of light, the two 
halves will arrive at E in phase and recombine 
into a ray exactly equal to the original ray 
from D, less the reflection and absorption losses 
of course. It is not necessary for each ray to 
travel a distance equal to the distance traveled 
by every other ray, provided the optical path 
lengths of the two halves of each ray are equal. 
The field of view as seen from K will then be 
of uniform intensity. 

Now suppose that the mirror F is moved a 
short distance so that the path lengths for the 
two halves of each ray are different. The ray 
halves no longer arrive at E in phase and thus 
the illumination of the field of J is reduced. 
When the path lengths are different by one- 
half of a wavelength of the light used, the field 
will be completely dark. 

Next assume that the mirror F is inclined 
so that along some line across this mirror the 
path lengths for the two ray halves are equal 
but at points on one side of the line the path 
length EFE is shorter than EGHGE , and on 
the other side EFE is larger than EGHGE . 
As the observer’s attention moves across the 
field of /, he sees a bright band corresponding 
to the line on F where the path lengths are 
equal. On either side of this line the illumina- 



240 


OPTICAL TESTING METHODS 


tion decreases until the region is reached on F 
where the path length differs one-half a wave¬ 
length. Proceeding farther, additional bright 
bands correspond to path length differences of 
one, two, or any integral number of wave¬ 
lengths. 

It follows then that the interpretation of 
the light and dark bands which appear to an 
observer with his eye at K is simply one of 
path lengths. Taking any bright spot as an 
origin, any other bright spot in the field cor¬ 
responds to a path length an integral number 


ure 36 shows an interferometer set up for the 
testing of prisms. Suppose the prism G is re¬ 
placed by a perfect flat mirror, then the instru¬ 
ment may be adjusted to give a uniformly 
bright field, a uniformly dark field, or a num¬ 
ber of straight parallel bright and dark bands. 
If now the mirror is replaced by a perfect 
prism, the mirror F must be moved away from 
E since the optical length in the other path is 
lengthened by the presence of the glass of the 
prism. Since the optical length in the perfect 
prism is the same for all rays, the original 



of wavelengths longer or shorter than the 
length at the origin. The dark bands show the 
regions which have path length differences 
equal to an integral odd number of half-wave¬ 
lengths. 

It must be remembered that the important 
factor is the optical path length and not the 
physical length. In a telescope it is desirable, 
even necessary, for good image formation that 
the optical path lengths from the object point 
to the image point be equal to within a small 
number of wavelengths. 

The diagram previously referred to in Fig- 


field as seen with the mirror alone may be 
again obtained. If the prism is imperfect in 
any way (faces not flat, or nonhomogeneous 
glass) the fringe system will so indicate. Fig¬ 
ure 37 shows a set of interferograms of several 
prisms for an M-10 periscope. A number of 
different defects are shown. Views 7, 10, and 12 
illustrate the nature of the fringe system when 
the prism contains striae. The other views are 
of prisms with imperfect surfaces. No. 8 is the 
most nearly perfect of this group, but the dark 
bands would be straight, parallel, and equally 
spaced if it were perfect. 
































IMPROVEMENTS IN TESTING METHODS 


241 


4 ‘ 3 ' 3 The Pennsylvania State College LI 
Michelson-Twyman Interferometer 

The basic parts of the 1.1 interferometer are 
shown in Figures 38, 39, and 40. The light 
source is an H-4 Westinghouse mercury vapor 
lamp. It has been found that a 40-w lamp in 
series with the primary of the supply trans¬ 
former improves the stability of operation. The 


flat is mounted on a sliding carriage which is 
controlled by a screw and hand wheel con¬ 
venient to the operator. 

The upper surface of the casting, which 
forms the bed of the apparatus, is carefully 
machined to a flat surface. The front surface 
mirror in the second light path is mounted in 
an easily adjustable support which may be 
moved around over the surface of the casting. 



EXP * I MIN 



Figure 37. Interferometer patterns for sample M-10 periscope head prism. 


light from the lamp is condensed on a pinhole 
which is at the focus of a collimating lens. This 
lens has an aperture of 2.5 in. and a focal 
length of 16.00 in. 

The plane-parallel dividing plate of this in¬ 
strument is aluminized on the face nearest the 
collimator. The aluminum coat is of such thick¬ 
ness as to reflect approximately 70 per cent of 
the incident light. The length of the adjustable 
light path is determined by the position of an 
optical flat aluminized on its first surface. This 


Between the mirror and the dividing plate is 
a rotating table upon which the optic to be 
tested is mounted. A simple double convex lens 
with an aperture of 2.4 in. and focal length 
of 7 in. is employed for viewing the field. The 
power is apportioned between the surfaces of 
this lens to minimize spherical aberration. A 
green filter localizes the wavelength of the light 
in the observed field to the green mercury line. 
For some purposes a camera with which a 
record of the interference pattern may be made 



















242 


OPTICAL TESTING METHODS 


is attached to the instrument in a manner 
shown in the photograph. 

This instrument has been so designed as to 
facilitate the making of the adjustments re¬ 
quired to bring the fringes into view. The prin¬ 
cipal adjustments are the following: 

1. The collimator is adjusted for parallel 
light by autocollimation. 

2. The dividing plate is properly positioned 
by means of its leveling screws to such a posi- 


the focus of the viewing lens and in general 
will see two bright spots. One is moved into 
coincidence with the other by means of the 
mirror adjustment. If the optical paths are of 
nearly equal length, interference fringes are 
immediately visible when the eye is moved to 
the focus of the viewing lens. 

4. The full aperture of the system is then 
obtained by a slight lateral adjustment of the 
viewing lens if such be necessary. 



Figure 38. The 1.1 interferometer set up for the testing of prisms. 


tion that the image of the pinhole reflected 
from the movable end mirror (F in Figure 36) 
coincides with the pinhole. The movable mirror 
is not provided with leveling screws, since they 
are not needed in the adjustment. 

3. The end mirror in the testing path is ad¬ 
justed by means of its azimuth and elevation 
screws until the images of the pinhole formed 
by the viewing lens and the end mirrors coin¬ 
cide. The observer places his eye well outside 


The movable end mirror may be moved in 
or out until the maximum contrast of fringe 
system is obtained and the adjustable end mir¬ 
ror may be tilted by means of its screws until 
the desired centering or decentering of the 
fringes is realized. 

For the testing of prisms, flats, or optical 
systems in which the entrance and exit beams 
are both collimated, the optical unit to be 
tested is inserted into the measuring path and 







IMPROVEMENTS IN TESTING METHODS 


243 


the end mirror is so placed as to return the 
beam through the instrument. For lenses or 
optical systems which normally produce con¬ 
vergent light, the end mirror is replaced by a 
reflecting spherical surface whose center of 
curvature is coincident with the focus of the 
lens or system. Arrangements for making 
measurements on prisms, telescopes, and lenses 
are shown in Figures 38, 39, and 40 respec¬ 
tively. 

The Interferometer as a Production 
Test Instrument 

While considerably more study of the inter¬ 
ferometer as a production tool is desired, its 


use of the interferometer as a tool for the 
designer as an aid to the study of aberrations. 
And third, the interferometer may be employed 
in inspection for conformance with design. 

In this section, only the last of these will be 
considered; the others are discussed elsewhere 
in the volume. Methods are now available for 
the computation of interference patterns to be 
expected from lenses and simple lens systems. 
For prisms and flats no computation is neces¬ 
sary since the interference fringes should be 
straight, parallel, and equidistant. The writer 
of specifications for optical parts could deter¬ 
mine by computation or by observation on a 
carefully made sample the fringe system for 



Figure 39. The 1.1 interferometer set up for the testing of telescopes. 

general usefulness is divided into three fields, a perfect part. The effect of permissible errors 
First, the use of the interferometer as a guide in the part could be then determined in the 
in the figuring of individual optics. Second, the same way and a specification indicating the re- 






244 


OPTICAL TESTING METHODS 


quired fringe system and its possible variations 
set up. The inspector could either count fringes 
or compare the field with a group of pictures 
of the fields produced by sample optics. If all 
inspectors concerned with a particular part 
were provided with prints of the same set of 
pictures, great uniformity of product could be 
assured. 

The connection between the characteristics 
under examination in the interferometer and 
the actual performance of the instrument is 
not as direct as in the case of the KDC ma¬ 
chine. The latter device is probably the more 


4 - 3,4 The Dioptometer 

During the survey reported earlier in this 
chapter, it was noted that an instrument called 
the dioptometer was becoming one of the most 
frequently used devices in the inspection of 
optical instruments. When this device is com¬ 
bined with such accessories as collimators, dia¬ 
phragms, filters, and resolution charts, it may 
be used to inspect an optical instrument for the 
following: 

1. Diopter setting of the reticle 

2. Parallax 



Figure 40. The 1.1 interferometer set up for testing of lenses. 


useful for the inspection of complete instru¬ 
ments. On the other hand, the inspection of 
parts and subassemblies could be carried out 
in the interferometer very rapidly. Moreover, 
the nature of the faults in rejected components 
may be indicated by the test. 

The use of the interferometer in optical in¬ 
spection is increasing rapidly. The instrument 
has enjoyed its greatest use in the laboratory 
where it has aided in the evaluation of new 
designs and in the production of optics which 
are difficult to figure, particularly aspherical 
surfaces. Since no large-scale testing of pro¬ 
duction instruments with interferometers has 
yet been carried out in this country, experience 
has not been accumulated on which to judge the 
convenience and precision of such a test. 


3. Accuracy of the diopter scale of the eye¬ 
piece 

4. Astigmatism 

5. Curvature of the ocular and object fields 

6. Spherical aberration 

7. Chromatic aberration 

8. Resolution 

A number of different designs of dioptom¬ 
eters have been developed at the Pennsylvania 
State College. Along with the various designs 
certain accessories and procedures have been 
evolved. 

The most useful model of the dioptometer is 
the D-2 designed and constructed for studies 
at the Pennsylvania State College and at the 
Frankford Arsenal. Figures 41 and 42 show 
the instrument in use. The principal parts are 








IMPROVEMENTS IN TESTING METHODS 


245 


the objective and the eyepiece, both of which 
employ M-l aiming circle optics. The eyepiece 
is movable and the sliding tube is calibrated in 
diopters from —4 to +4 diopters. The section 
of the scale from —1 to +1 diopter is subdi¬ 
vided on the drum so that measurements in this 
region may be made to tenths of a diopter. 

The unit of the diopter is a measure of the 



Figure 41. The Model D-2 dioptometer. 


curvature of the wave fronts in a beam of light. 
A collimated beam has zero diopters curvature. 
The unit is defined as the reciprocal of the 
radius of curvature of the wave front expressed 
in meters. If the rays are converging, the sign 
of the unit is positive, while if they are di¬ 
verging the unit is negative. Thus the light 
from a point source at a distance of 1 m has 
curvature of wave front corresponding to — 1 
diopter, at 2 m to —0.5 diopter and so forth. 
Conversely, the light from a lens converging 
on a point at a distance of 1 m has a curvature 
of wave front or power equal to +1 diopter, 
while if the focal point is at a distance of 0.5 m 
the power is +2 diopters. 

The Use of the Dioptometer 

The dioptometer measures the divergence or 
convergence of the light entering the eye from 
the eyepiece of an optical instrument. What 
should be the reception of such light by the 
eye? If the eye is directed toward an object 
at a great distance, the light from the object 
has little curvature of wave front, the power 
is nearly zero diopters, and an image is formed 
on the retina by the eye lens. As the distance 
to the object is decreased the curvature of the 


wave fronts in the light from the object in¬ 
creases and the power becomes more and more 
negative; —0.1 diopter at 10 m, —1.0 diopter 
at 1 m, and —2.0 diopters at 0.5 m. The eye lens 
changes its power for each of these cases so 
as to preserve the focus on the retina. A young 
person’s eye is able to produce a good focus for 
objects at any distance between infinity and 
about 7 cm. In other words the eye can “accom¬ 
modate” light with any degree of divergence 
between 0 and —15 diopters. The eye does not 
normally receive and cannot accommodate con¬ 
verging rays of light. 

If a telescope is adjusted to produce diverg¬ 
ing light from its eyepiece, the eye can see 
a sharp image provided the divergence falls 
within the limits just discussed. On the other 
hand, if the light is convergent no adjustment 
on the part of the eye will produce a sharp 
focus. The most comfortable viewing distance 
for most eyes when relaxed is somewhere be¬ 
tween 1 m and infinity. For this reason most 
telescopes are adjusted to produce parallel light, 
or diverging light with a curvature not ex¬ 
ceeding —1 diopter. 



Figure 42. The inspection of binoculars with 
the D-2 dioptometer. 


Cross hairs or reticles are necessary in most 
military instruments, and since these devices 
are used as reference marks against the target 
they should be in sharp focus when the eye is 
adjusted to see the target in focus. The de¬ 
sirable condition of complete coincidence of the 










246 


OPTICAL TESTING METHODS 


image of the target formed by the objective 
and the image of the reticle is rarely realized. 
For this reason the specification on this point 
generally calls for the parallax being removed 
at a particular mark on the reticle. This means 
that at this point the two images do coincide. 

In order to check an instrument for this spec¬ 
ification it is necessary to measure the relative 
locations of the objective and ocular fields and 
the reticle. Figure 43 shows several typical 


the image into focus since the rays are diver¬ 
gent, but this throws the reticle out of focus on 
the retina. Under these circumstances parallax 
exists, for the reticle appears at a different 
distance than does the target. 

It is in the measurement of these quantities 
that the dioptometer finds its greatest useful¬ 
ness. As an example of the usefulness of the 
dioptometer, consider the measurement of set¬ 
ting the reticle in the desired position. In gen- 



Figure 43. The eye as part of a telescope system. 


cases. The first (A) illustrates a case in which 
the image of the target falls in front of the 
reticle and the focal plane of the ocular. The 
eye receives converging light from this image 
which it cannot focus even when the reticle is 
in focus. Case (B) is the ideal in that image, 
reticle, and eyepiece focal plane are coincident. 
A normally relaxed eye sees all three in sharp 
focus. The situations for images falling behind 
the reticle are illustrated in (C) and (D). In 
(C) the relaxed eye sees the reticle in sharp 
focus and the image of the target out of focus. 
The eye may accommodate, however, to bring 


eral the setting should be about 0.75 diopter 
negative. If an instrument is of fixed-focus eye¬ 
piece type all that is required is to focus the 
dioptometer on the center of the reticle field as 
seen through the eyepiece, and read the setting 
of the instrument. In the case of an instrument 
of variable focus, the reading of the scale on 
the eyepiece should be the same as that on the 
dioptometer. 

If the instrument is mounted with a collima¬ 
tor adjusted to simulate a target at a distance 
prescribed by the specifications, the parallax 
may be measured for any point in the field by 















































IMPROVEMENTS IN TESTING METHODS 


247 


first focusing the dioptometer on the image of 
the target at that point and then on the reticle 
at the same point. The difference between the 
two readings is the parallax. If the difference 
is less than 0.05 diopter, the parallax may be 
considered as removed. 

Supplementary Apparatus for the 
Application of the Dioptometer 

Ordinarily, the dioptometer is used in the 
position normally occupied by the eye behind a 
telescope or periscope. For convenience, a col- 


times the magnification of the telescope. Fig¬ 
ure 28 shows the relation of the parts for this 
measurement. At each field angle the dioptom¬ 
eter is focused first on the vertical bar of the 
cross then on the horizontal. The difference in 
the settings is the astigmatism in diopters. The 
results of a typical measurement appear in 
Figure 44. 

If the objective of the telescope is covered 
by a pair of stops in turn, the first blocking 
all but the central zone and the second all but 
the margin, the spherical aberration may be 


TELESCOPE FOCUSED ON TARGET ( EYEPIECE SET AT O ) 



DIFFERENCE BETWEEN PRIMARY AND SECONDARY DIOPTOMETER READINGS 



limator is used to produce a target to be viewed 
by the telescope. The collimator, telescope, and 
dioptometer are mounted on a massive base. 
A set of lens caps for the measurement of 
spherical aberration and coma and a set of 
filters to aid in the determination of chromatic 
aberration complete the complement of acces¬ 
sories. 

If the target of the collimator is in the form 
of a cross, a rapid determination of the astig¬ 
matism in a telescope may be made with the 
dioptometer. The telescope is rotated about its 
own entrance pupil and the dioptometer is ro¬ 
tated about the exit pupil of the telescope, 
through an angle equal to the original rotation 


quickly determined. With the first stop in place, 
the dioptometer is adjusted to focus on the 
collimated target. The adjustment is repeated 
with the second stop in place. The difference 
between the two settings is then the spherical 
aberration measured in diopters. In a like man¬ 
ner the chromatic aberration may be deter¬ 
mined with the aid of a pair of color filters, 
one blue and the other red. 

If the auxiliary telescope of a KDC apparatus 
is replaced by a dioptometer all of the above 
measurements may be conveniently made when 
the efficiency is determined. 

No lengthy series of measurements have been 
carried through to determine the precision with 












248 


OPTICAL TESTING METHODS 


which the foregoing observations may be made. 
Thus it cannot be stated here whether these 
measurements are more accurate than those 
made by other methods. It seems clear, however, 
that the dioptometer is a very convenient and 
simple tool to use and is adequate for the control 
of these various factors in production testing. 


or other marks are found on the interior optical 
surfaces it is often very difficult to identify the 
offending surface. The proboscope is designed 
to permit the examination of each surface in 
turn without disassembly of the instrument. 
Figures 45 and 46 show the construction of this 
device. 



AUXILIARY LENSES FOR THE PROBOSCOPE 



Figure 45. The proboscope. 


It should therefore enjoy increasing use as the 
specifications are so modified as to set forth the 
quantitative characteristics of production in¬ 
struments. 


The Proboscope 

The last of the devices developed by the 
Pennsylvania State College group for produc¬ 
tion testing is the proboscope. In the section on 
specifications and testing (see Section 4.2), it 
was pointed out that each optical instrument is 
carefully examined for cleanliness. When dust 


The apparatus consists of four major parts: 

1. A movable slide with appropriate mounts 
ing brackets for holding the instrument under 
inspection. 

2. An illuminating sphere. 

3. Two auxiliary lenses and adapter. 

4. A special eyepiece containing a reticle 
mounted in its focal plane. 

The purpose of the movable slide is to permit 
the motion of the telescope and illuminating 
sphere and auxiliary lenses when they are re¬ 
quired to such positions as to permit the exam¬ 
ination of each lens surface through the eye¬ 
piece. The auxiliary lenses form real images of 


























































EVALUATION OF DESIGNS AND DESIGN IMPROVEMENTS 


249 


the internal surfaces at the reticle of the eye¬ 
piece. 

When the proboscope is first set up for each 
type of telescope to be tested, one telescope is 
disassembled and each surface is marked with 
a number. By trial and error, the location on 
the slide required to bring each number into 
focus is determined. Thereafter each surface 
in subsequent telescopes may be examined by 
returning the slide to the known position. 

While the proboscope is not used at present 



Figure 46. The proboscope. 


to any great extent, it is felt that its use would 
greatly aid in the inspection of surface defects 
in completed instruments. Since the location of 
the defects may be determined, the correction 
of defects either by replacement of the offend¬ 
ing lenses or by cleaning of the proper surfaces 
could be done with a minimum of effort. 


4 4 THE EVALUATION OF DESIGNS AND 
DESIGN IMPROVEMENTS 

The various instruments which have been 
described in the preceding sections were de¬ 
signed for the express purpose of aiding in the 
production testing of optical parts and instru¬ 
ments. The actual results obtained with these 
instruments, in particular with the KDC appa¬ 
ratus and the interferometer, for any particular 
optical instrument being tested, depend on the 


optical design of that instrument as well as 
the quality of manufacture. These devices may 
be used, therefore, to assess the quality of the 
design as well as to control inspection. 

The principal optical errors inherent in the 
design of lenses or multiple lens instruments, 
other than chromatic errors, are spherical aber¬ 
ration, astigmatism, and coma. The observa¬ 
tions and measurement of the first two of these 
have been mentioned in connection with the 
dioptometer, which has proved to be a con¬ 
venient instrument for this use. The question 
arises as to the effects of these aberrations on 
the results given by the KDC apparatus and the 
interferometer. To study this problem, three 
new optical instruments were designed and 
built, either by or at the request of the Penn¬ 
sylvania State College group. These devices are 
known as the spherical aberration modifier , the 
astigmatism modifier, and the coma modifier. 
Each instrument was designed to be as free 
as possible from aberrations other than the one 
desired. The desired aberration is introduced 
in such a way as to be variable and under the 
control of the operator. 

When considering the aberration of lenses or 
lens systems, the concept of the Rayleigh limit 
is very useful. If the optical system is perfect, 
all rays of light starting from a point in the 
object pass through the entrance pupil and the 
optics and reach a single point in the image 
after traveling over exactly equal optical path 
lengths. Only rays which are not skew, that is, 
only rays which lie in planes containing the 
optical axis of the system, will be considered 
here. 

In an imperfect instrument, the rays start¬ 
ing from a point in the object will not converge 
on a point in the image but any two of them 
will intersect at some point. The light-path 
length from the point in the object to the point 
of intersection will not be equal, however. The 
degree of this inequality is a measure of the 
magnitude of the errors. 

Lord Rayleigh, thinking along these lines, 
postulated that if the path lengths differed by 
one-half a wavelength, the error would be just 
detectable by the eye. This path difference of 
one-half wavelength will be called a Rayleigh 
limit. It has been found in practice that one 



250 


OPTICAL TESTING METHODS 


Rayleigh limit is actually very nearly the limit 
of detectability. 

For convenience, consider light proceeding 
from a source at infinity, composed of rays 
which are parallel when they enter the lens or 
lens system. The optical path for a ray reach¬ 
ing some point P via an arbitrary point Q in 
the lens or lens system is in general different 
from the path length to the same point P for 
a ray passing through the optical center, 0. The 
optical path difference is 


A 77r 4 A hv ® 

OPD - ^ + 2^(1 + 2 sin* 9) +^sm6 


+— r 2 


. 2A h 

H— -j- r sin < 


, 2At . 
+ —j- r cos 6, 


( 1 ) 


where S and / are the semi aperture and the 
focal length of the lens; A is the astigmatism, 
the difference between the sagittal and tangen¬ 
tial foci; AL' is the spherical aberration, the dif¬ 
ference between the axial and marginal inter- 


ometer. It should be mentioned that, to a good 
approximation, the path differences for each 
type of error add to produce a total error. For 
example, if a particular lens has 3 Rayleigh 
limits of spherical aberration, 3 of astigma¬ 
tism, and 2 of coma for a particular point in 
the image, the total error will be characterized 
by 8 Rayleigh limits, which is very poor indeed. 


4,4,1 The Yerkes Spherical Aberration 
Modifier 

The spherical aberration modifier was de¬ 
signed at the Yerkes Observatory under Con¬ 
tract OEMsr-1078. The purpose in constructing 
this instrument was to provide a means for 
evaluating the effects of pure spherical aber¬ 
ration on the performance of fire-control in¬ 
struments. Figure 47 shows the optical design 



i^BSO-2 n d =1.517 
W/// E D F- 1 n d s 1.649 

Figure 47. Optical design of the spherical aberration modifier. 


cepts for parallel light; b is the coma, the dis¬ 
tance from the paraxial image to the intersec¬ 
tion of diametrically opposite marginal rays 
for an off-axis object point; r and 0 are the 
polar coordinates of the point Q with respect 
to 0; AL, Ah, and At are the rectangular dis¬ 
placements of the point P from the ideal image 
point. 

One Rayleigh limit for spherical aberration 
is 4a/ 2 / S 2 , for astigmatism A f 2 /S 2 , and for coma 
2 a f/S. The relation between these expressions 
and the preceding equation is pointed out in 
Section 4.4.2 in connection with the interfer- 


of the instrument and Figure 48 shows the 
complete modifier. The spherical aberration is 
introduced by changing the position of the 
movable thick meniscus lens. The range is from 
0 to 9 Rayleigh limits. 

The effect of spherical aberration as pro¬ 
duced by the modifier on KDC efficiency is 
shown in Figure 49. The efficiency is 100 per 
cent for zero aberration, as it should be, and 
falls off for both positive and negative values. 
Over the range of the observations, the relation 
between efficiency and the number of Rayleigh 
limits is linear. Minus-eight Rayleigh limits 




























EVALUATION OF DESIGNS AND DESIGN IMPROVEMENTS 


251 


reduces the efficiency to about 68 per cent. 
There is some indication that on the plus side 
the efficiency drops much more rapidly. 

An eyepiece containing +3.6 limits of spher¬ 
ical aberration may be used to extend the range 
in the positive direction. With this combina¬ 
tion, measurements indicate that the KDC effi¬ 
ciency drops to 70 per cent at about +3.8 limits. 



Figure 48. The spherical aberration modifier. 


The fringe system produced by the modifier 
when it is placed in one of the light paths of 
the interferometer may be computed by means 
of equation (1). Remember that the interfer¬ 
ometer measures path length differences di¬ 
rectly. For spherical aberration 
OPD = ALV 4 /4S 2 / 2 . 

The number of dark fringes n observed in 
the pattern is n\ — AL'& 4 /4£ 2 / 2 . Thus A U — 
4n\f 2 /S 2 . The value of AL' for one Rayleigh 

110 

h-100 

z 

111 

o 

£ 90 

CL 

| 80 

UJ 

o 

U- 70 

lL 

UJ 

o 

o 60 


0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 

RAYLEIGH LIMITS OF ABERRATION 

Figure 49. KDC efficiency as function of spheri¬ 
cal aberration. 

limit of spherical aberration is 4 a f 2 /S 2 . The 
amount of spherical aberration possessed by 
the lens is the number of rings in the interfer¬ 
ometer pattern times the value of spherical 


o SPHERICAL ABERRATION 
+ ASTIGMATISM 
• COMA 


i i i 


aberration for 1 Rayleigh limit. There is one 
fringe for each Rayleigh limit of aberration, 
as is to be expected from the definition of the 
Rayleigh limit. Figure 50 shows clearly the 
agreement between the predicted fringes and 
those observed with the modifier. A count of 
the observed fringes shows that the correct 
number appears in each case. The application 
of the interferometer to the measurement of 
the spherical aberration in a production tele¬ 
scope is shown in Figure 51. 


4.4.2 The Astigmatism Modifier 

With the success of the spherical aberration 
modifier, described in the preceding section, it 
seemed likely that a similar device designed to 
introduce astigmatism into an otherwise per¬ 
fect optical system would be of considerable 
use. The device developed consists of an objec¬ 
tive and an eyepiece, both of which are M-l 
aiming circle optics mounted together in a tube 
to form a small telescope of magnification 4X« 
The variable astigmatism is introduced by ro¬ 
tating the objective about an axis through its 
second nodal point. The objective is so corrected 
as to be free from spherical aberration on the 
axis. The coma remains nearly constant and 
small, since it is zero on the axis, so that over 
a wide range of angles of rotation the device 
produces images of object points on the axis in 
which astigmatism may be varied without any 
significant amount of coma. 

Figure 52 is a diagram of the optics of the 
astigmatism modifier. The figure shows an aux¬ 
iliary telescope which is properly part of the 
KDC apparatus and does not contribute to the 
function of the modifier. The amount of astig¬ 
matism introduced at each rotation angle of 
the objective may be determined in several 
ways. The computations, while not as simple 
as in the case of spherical aberration, are 
straightforward. The technique described in 
the discussion of the dioptometer for the direct 
measurement of astigmatism is applicable here 
as well. The curve relating the astigmatism to 
the rotation is smooth and approximately para¬ 
bolic in shape. At zero angle the astigmatism 
is zero while at 10 degrees the astigmatism is 
about 160 Rayleigh limits. For the optics em- 




252 


OPTICAL TESTING METHODS 


ployed in the modifier, this corresponds to about 
0.090 in difference in focal length for sagittal 
and tangential foci. The construction of the 
modifier is illustrated in Figure 53. Because of 
the fact that small angles of rotation produce 
large amounts of astigmatism, the motion of 
the objective is controlled by a screw and lever 
arrangement. 


astigmatism is placed in an interferometer and 
the fringe system is centered, the optical path 
differences will be given by the terms in the 
equation with r 2 coefficients. Thus 


OPD = Ar 2 


(1 +2 sin 2 0) A Lr 2 


1 p 

The interferometer pattern will have a maxi¬ 
mum number N of fringes across one diameter 


MODIFIER LESS EYEPIECE 



2 3 4 5 6 7 8 


NUMBER OF RAYLEIGH LIMITS 


Figure 50. Interferometer patterns for the Yerkes spherical aberration modifier. 


The effect of astigmatism in the objective of 
an optical instrument upon the KDC efficiency 
of that instrument may be readily observed by 
measuring the efficiency of the modifier for 
various settings of the rotation angle. The 
crosses of Figure 51 show the relation between 
the efficiency and the number of Rayleigh limits 
of astigmatism. The shape of the curve is very 
nearly the same as it was for the spherical 
aberration. 

Equation (1) which permits the computa¬ 
tion of the optical path differences may be used 
to predict the interferometer pattern for astig¬ 
matic images in the same way as it was used 
in the preceding section for spherical aberra¬ 
tion. If a lens or telescope possessing only 


and a minimum number n across a diameter 
perpendicular to the first. Then 
A7 , iAS 2 + A LS 2 
NX = - f 2 -’ 

and 

. iAS 2 + ALS 2 
nX = — p —• 

L+on subtraction 

(AT Nx 

(N - n)\ = 

The term N — n is the difference in the number 
of fringes across the two diameters. 

The amount of astigmatism A is 

A = W ~ n)\p 
S 2 

The factor A P/S 2 is 1 Rayleigh limit of astig- 









EVALUATION OF DESIGNS AND DESIGN IMPROVEMENTS 


253 


matism. Thus, for a lens possessing essentially 
astigmatism, the differential number of fringes 
in the interferometer pattern is independent of 
the focus when the fringe system is centered 
and the value of the astigmatism is the differ¬ 
ential number of fringes times the value of one 
Rayleigh limit for astigmatism. Conversely, the 
number of Rayleigh limits is, under the same 


44 3 The Coma Modifier 

To complete the study of the effects of the 
first-order aberrations, a device for the intro¬ 
duction of coma into an optical system was 
desired. A special objective containing spher¬ 
ical aberration was designed at the Yerkes 
Observatory. The Perkin-Elmer Corporation 



APERTURE * 0.680 INCHES 

NO. OF RAYLEIGH 
LIMITS = 11 



0.507 INCHES 



0,304 INCHES 


0.395 INCHES 


Figure 51. Interferometer patterns for an M-l aiming circle. 


conditions, equal to one-half the differential 
number of fringes. A comparison of the theo¬ 
retical and the observed patterns for about 2.2 
Rayleigh limits appears in Figure 54. The 
agreement is very good. When the fringes ap¬ 
pear hyperbolic, at the best focus those across 
one diameter must be counted as negative and 
thus the difference becomes the sum of those 
visible. In each view the difference is a little 
less than six. 


manufactured this lens and a standard M-l eye¬ 
piece completed the telescope. As before, this 
device, known as the coma modifier, produces 
images containing nearly pure coma. The ob¬ 
jective is pivoted about a point some distance 
back from the lens itself. The lens has a diam¬ 
eter of approximately 2 in. A stop at the pivot 
point limits the effective aperture to 1.41 in. 
The optical design is shown in Figure 55. Be¬ 
cause of the restrictions on the design, the 



254 


OPTICAL TESTING METHODS 


modifier has 0.8 Rayleigh limits of spherical 
aberration and some astigmatism. The spherical 
aberration is constant and may be subtracted 
from the coma. The astigmatism, however, is 


limits at 2 degrees. A photograph of the modi¬ 
fier appears in Figure 56. 

The efficiency measurements for the coma 
modifier in the KDC machine appear as closed 



Figure 52. Optical design of the astigmatism modifier. 


zero on the axis, that is, for a zero angle of 
rotation, and increases to 2 Rayleigh limits at 
2 degrees rotation. At this angle the coma is 



circles in Figure 51. It may be seen clearly that 
for more than 4 Rayleigh limits, coma has a 
greater effect upon the KDC efficiency than 
does either the spherical aberration or the 
astigmatism. This point is of considerable im¬ 
portance to the designer for he should reduce 
this coma even at the expense of the other aber¬ 
rations if he wishes to improve the resolving 
power. 

If the modifier is placed in an interferometer, 
adjustments are made to center the fringe sys¬ 
tem, and the end mirror is set for paraxial 
focus, then by equation (1) 


OPD 


6r 3 sin 0 
2 S 2 f * 


Along one diameter, then, the number of fringes 
will be a maximum where sin 0 = 1. 


Then 

bS 3 


NX ~ 2 S 2 / 

and 

= 2 NXf 
0 s . 


Figure 53. The astigmatism modifier. 

10 Rayleigh limits. The relation between the 
number of Rayleigh limits of coma and the 
angle of rotation is linear, according to compu¬ 
tation, from zero at zero angle to 10 Rayleigh 


Now 1 Rayleigh limit for coma is 2 a f/S so, as 
in the case of the other aberrations, there is 
one fringe for each Rayleigh limit of coma. In 
other words, the value of the coma is the num¬ 
ber of fringes times the value of 1 Rayleigh 
limit for coma. Figure 57 shows the observed 



























EVALUATION OF DESIGNS AND DESIGN IMPROVEMENTS 


255 


I RAYLEIGH LIMIT * 2.2 X 10 3 FOR ASTIGMATISM = — 

SIN*© 

COMPUTED ASTIGMATISM = 6 X10~ 3 INCHES 






AL IN THOUSANDTHS OF AN INCH 


BEST 

FOCUS 


TANGENTIAL 

FOCUS 


PARAXIAL 

FOCUS 


SAGITTAL 

FOCUS 


Figure 54. Interferometer patterns for the astigmatism modifier. 



Figure 55. Optical design of the coma modifier. 
























256 


OPTICAL TESTING METHODS 


and computed interferometer patterns for sev¬ 
eral values of the angle of rotation of the ob¬ 
jective in the coma modifier and thus for vari¬ 
ous values of coma. As before, the agreement is 
very good. 


4 44 Survey of Results from Aberration 
Modifiers 

The various aberration modifiers have ful¬ 
filled two purposes. First, they have permitted 
the exact determination of the effect of various 



Figure 56. The coma modifier. 


amounts of each aberration upon the KDC effi¬ 
ciency and thus upon the resolving power of an 
instrument. Second, they have provided a 
means of checking the computations of inter¬ 
ferometer fringe patterns. These functions are, 
of course, directed toward the designer of new 
or improved optical systems, but they will also 
aid in the analysis of present systems with a 
view toward their improvement. A telescope 
now in production may have a low KDC effi¬ 
ciency even under the best conditions of work¬ 
manship. It may be that interferometer pat¬ 
terns show that one or more of the aberrations 
is greater than it should be for the desired per¬ 
formance of the instrument. Then, by trial and 
observation with the interferometer, changes 
in the optics may be made to improve the per¬ 
formance. Thus an observational approach 


may partially replace laborious computational 
methods. 


4,45 The Artificial Sky Apparatus 

The artificial sky apparatus [ASA] consists 
of an internally illuminated sphere and a cam¬ 
era. The function of the sphere is to simulate 
conditions of a bright natural background. The 
instrument being tested looks through the 
sphere and out at a target, through an aperture 
on the far side of the sphere. Figure 58 shows 
the arrangement of the parts. The sphere does 
not cut into the true field of the instrument, so 
that any light from the sphere that reaches 
the camera must be scattered at least once in¬ 
side the telescope. The actual illumination of 
the surface of the sphere depends upon the 
number of 7.5-w bulbs used. For one bulb the 
level is 37 foot-candles, while for all six a level 
of 217 foot-candles is available. 

Because of differences in various types of 
telescopes, two spheres were built, one for use 
with M-71 telescopes and one for M-72 and 
M-76. 

Two targets of the bull’s-eye type are con¬ 
venient for use with the scattered light experi¬ 
ments. One has a white center, illuminated at 
10 foot-candles, and a black surrounding zone, 
while the other has a black center and white 
outer zone. 

Nearly all the measurements were made by 
photographing the target through the tele¬ 
scope, first with the lights of the ASA off and 
again with them on. Transmission measure¬ 
ments of the negatives made with a Leeds and 
Northrup microphotometer and recorder gave 
graphical records of the results, but the nega¬ 
tives were not photometrically calibrated, and 
so the results are only qualitative. Care must 
be exercised to insure that the quality of the 
light in the sphere and on the target are the 
same, otherwise the color sensitivity of the film 
becomes important. 

The principal results of a study of six tele¬ 
scopes, two M-71, two M-72, and two M-76, 
are: 

1. A considerable amount of scattered light 
enters all of the telescopes. This cannot help 








EVALUATION OF DESIGNS AND DESIGN IMPROVEMENTS 


257 


but reduce their usefulness under bright condi¬ 
tions. 

2. A considerable loss of contrast in the tar¬ 
get results from the scattered light. 


the effect of coated optics. The coating of optics 
of telescopes is employed to reduce the loss by 
surface reflection and thus increase the trans¬ 
mission of the instrument. What effect does 




0 5 1 L5 2 

ROTATION OF OBJECTIVE IN DEGREES 

0 2.5 5 7.5 10 

NUMBER OF RALEIGH LIMITS OF COMA 


Figure 57. Interferometer patterns obtained with the coma modifier. 


ARTIFICIAL SKY 
SPHERE 



Figure 58. The artificial sky apparatus. 


3. The actual amount of scattered light 
varies from instrument to instrument, even of 
the same design. 

A result of considerable interest concerns 


such a coating have on the scattered light? Two 
good M-71 telescopes, one with coated optics 
and one without, were observed visually, each 
by two observers. Both had about the same 


































258 


OPTICAL TESTING METHODS 


KDC efficiency when measured in the normal 
way. When used with the ASA, slightly more 
light seemed to be scattered by the instrument 
with coated optics. The bull’s-eye targets with¬ 
out the ASA, however, gave preference to the 
coated telescope. From these simple experi¬ 
ments it appears that coating optics makes 
little difference in the scattered light and if 
there is any difference it is in the direction of 
more scattering when coated. On the other 
hand, the increased transmission made the 
coated telescope much more useful for targets 
of low contrast at low levels of illumination. 

The ASA has been used in another way, 
namely, in measuring the effect of striae. A 
block of glass to be observed for striae is placed 
in the telescope between the objective and its 
focal plane. The increase in scattered light 
from the ASA, and thus the change in the 
KDC efficiency of the telescope, becomes a 
measure of the effect of the striae. A second 
method has been to examine the contrast of the 
bull’s-eye target with and without the block of 
glass placed before the objective. When in place 
the block is illuminated from the side. 

It is regrettable that the termination of the 
work before a large group of sample blocks of 
glass became available prevented thorough 
testing of this method for the determination of 
the effect of striae. 


4.4.6 Photoelectric and Photographic 
Procedures for the Evaluation 
of Optical Instrument Design 

As an aid to the lens designer, one of the 
most useful kinds of information would be the 
results of experimental ray tracing. The Hart¬ 
mann test, which involves covering the aper¬ 
ture of the objective with a diaphragm con¬ 
taining a number of small holes and observing 
the direction of the rays passing through the 
various holes, gives this sort of information. 
The test requires a large amount of careful 
measurement, however, and is time consuming. 
A less difficult, and perhaps more practical, de¬ 
termination is that of observing the distribu¬ 
tion of intensity in the region of the image 
since this depends on the integrated contribu¬ 


tion of rays over the entire aperture. Since 
most requirements relating to telescope per¬ 
formance involve the resolution of parallel lines 
or the discrimination of the edges of objects, it 
seems most useful to study the energy distribu¬ 
tion in the image of a line source. 

Equipment has been developed for recording 
the energy distribution in the image of a dis¬ 
tant line source produced by a telescopic sys¬ 
tem and focused on a plane at a considerable 
distance behind the eyepiece. Both photoelec¬ 
tric and photographic methods were employed. 
Figure 59 is a diagram of the photoelectric 
scanning apparatus. The device is a modified 
KDC machine. The original target has been re¬ 
placed by a line source which may be mounted 
either in a vertical or horizontal position. The 
auxiliary telescope is not used but in its stead 
a scanning slit and phototube assembly are em¬ 
ployed. This assembly travels slowly across the 
image in a direction perpendicular to the opti¬ 
cal axis. The electrical output of the photocell 
is recorded on a recording microammeter, 
which shows directly a plot of light intensity 
across the image. 

Figure 60 illustrates the instrument modified 
for photographic recording. The image of the 
line is photographed through a wedge whose 
gradient of density is parallel to the line 
source. Figures 61 and 62 are typical results 
obtained by the two methods under the same 
conditions and with the same telescope. 

Two results are obtainable from the type of 
observations illustrated in the figures. The 
records, either photoelectric or photographic, 
indicate the manner in which the image quality 
deteriorates with a change of field angle. Re¬ 
sults of this type permit an analysis of the 
region of confusion, including the effect of 
skew rays, in a far shorter time than would 
be required by ray tracing and thus serve as 
an effective basis for evaluation of lens design. 


45 CONCLUSION 

The optics group at Pennsylvania State Col¬ 
lege has made a complete survey of the meth¬ 
ods of inspection of optical instruments. While 
this was their primary task, together with 




CONCLUSION 


259 


OFF'AXIS FIXTURE 


I 

PHOTOELECTRIC SCANNING 




INSTRUMENT 
UNDER TEST 


CAMERA 


4 MM MICROSCOPE 
OBJECTIVE 


T- 14 BULB 


FILTER 


; 


COLLIMATOR 
LENS-EFL* 15.75 


Figure 60. The photoelectric KDC machine. A camera replaces the photocell. 































260 


OPTICAL TESTING METHODS 


recommendations for changes in methods 
where such were indicated, they have in addi¬ 
tion developed numerous instruments and de¬ 
vices to aid in the testing procedures. Among 
these the kinetic definition chart apparatus has 
come to play a major role in the testing of the 
definition of certain classes of tank telescopes. 
The interferometer , while not so widely used as 
yet, has been demonstrated to be of consider¬ 
able use in the determination of the quantita¬ 
tive behavior of optical components and com¬ 
plete instruments. These two devices represent 


well as others of the same design. There is good 
indication then that, for such cases, studies of 
manufacturing methods could lead to general 
improvement in the performance without de¬ 
sign change. 

Perhaps the most significant result of the 
survey is the recommendation that the speci¬ 
fications be reexamined to determine whether 
they, in reality, control the production in such 
a way as to produce the greatest number of 
usable instruments. The necessary perform¬ 
ance should be determined by a study of the 


WITHOUT FILTER 



Figure 61. The photoelectric KDC machine records. 


a group which were designed to aid in the 
placing of the entire method of specification on 
a more quantitative basis. 

During the survey of methods of inspection 
it was found that many of them depended upon 
the subjective observations of the inspector. It 
is recommended that as many such cases as 
possible should be replaced by quantitative 
measurements. When such measurements are 
introduced it has been found that many instru¬ 
ments which pass the inspection are by no 
means as well constructed and thus behave as 


conditions of expected use. When this has been 
established, the specifications should then be 
written so as to produce optics of the necessary 
quality. Factors which do not affect this quality 
should not be specified, and certainly no quality 
should carry a specification which requires the 
degree of perfection to be greater than de¬ 
manded by the minimum acceptable perform¬ 
ance. 

The optical perfection of an overall optical 
system which has been established as barely 
acceptable should be expressed in units based 



















CONCLUSION 


261 


on the Rayleigh limit. The inspection may then 
be carried out on the interferometer by the 
simple interpretation of fringes. The variation 
in focal length, concentricity and alignment, 
spacing and mounting, and the resolution for 
each element which corresponds to one Ray¬ 
leigh limit in overall performance of the sys¬ 
tem, should be computed or measured experi¬ 
mentally. 

It is unfortunate that the work under this 


mains to be done in adapting the use of the 
KDC apparatus to the study of components and 
the effect of their errors on the performance 
of the instruments as a whole. 

The Michelson-Twyman interferometer has 
been developed to the point where it may be 
readily employed for inspection purposes pro¬ 
vided the specifications are so modified as to be 
applicable to such measurements. Future work 
on the application of this instrument should 


EXPOSURE TIME = 2 MINUTES 



0 

▼ 

► 



► 


▼ ► 

* ► 


* ► 



+.6 



+.5 



+.4 

+.3 


+.2 

_l 

10 

▼ 

► 


* 

► 


* ► 

▼ ► 


▼ ► 

1 

2 


+.6 



+.5 



+.4 

+3 


+.2 

[LEFT) 

20 

* 

► 


▼ 

► 


▼ ► 

* ► 


* ► 

Ui 

_i 

o 

2 


+.5 



+.4 



+.3 

+.2 


+.1 

< 

O 

Ui 

40 

* 

► 



► 


^ ► 

^ ► 


^ ► 

u. 


-.1 



"2 



-.3 

-.4 


-.5 


70 


► 



► 


^ ► 

- ► 


^ ► 



-1.0 



-U 



-1.2 

-1.3 


-1.4 








DIOPTER 

SETTING (DIOPTERS) 





1 - 

LINE 

SOURCE 

ORIENTED 

PERPENDICULAR TO THE PLANE 

OF DISPLACEMENT 

OF 

THE BEAM 




LINE 

SOURCE 

ORIENTED 

PARALLEL 

TO THE PLANE OF 

DISPLACEMENT OF 

THE 

BEAM 


Figure 62. Wedge photographs of line source. 


contract terminated before the Specification 
and Inspection Manual could be prepared. 
There is a need for a manual of this type and it 
is recommended that this work be vigorously 
pursued. 

The KDC apparatus has been shown to be a 
valuable tool in optical research and design. It 
has been used to study fundamental concepts 
such as the effects of aberrations on perform¬ 
ance, the efficiency at various angles of view, 
the effects of target contrast, and of component 
defects. However, a great deal of work yet re¬ 


assess its relative usefulness when compared to 
other quantitative methods. In the study of the 
application of the interferometer to the devel¬ 
opment of new designs, the possibility of using 
the instrument as a means for the investigation 
of the use and performance of nonspherical 
surfaces is attractive. There is always the pos¬ 
sibility that the approach to new design prob¬ 
lems can be made on an experimental basis. 
Such an attack would make extensive use of the 
interferometer. 

The continued development of such instru- 



262 


OPTICAL TESTING METHODS 


ments as the modifiers would be of great aid in 
establishing the minimum quality of optical in¬ 
struments acceptable for military purposes. 
The philosophy represented in these instru¬ 
ments, that of measuring the results on the 
performance of known amounts of individual 
errors, could well be extended to such factors 
as striae, scattered light, misalignment, sur¬ 
face defects, and so on. 

The general tendency toward automatic re¬ 
corders such as photoelectric KDC machines 
should be continued as new electronic tech¬ 
niques become available. All ways in which the 
judgments of the observer may be removed or 
made less important should be exploited. The 
photoelectric scanning of images is a develop¬ 
ment of this type. It may be safely predicted 
that at some time not too far in the future a 
completely automatic inspection of optical in¬ 
struments will be possible. 


46 RECOMMENDATIONS BY NDRC 

1. Present specifications should be carefully 
reviewed for all optical instruments, on the 
basis of the survey that was made under NDRC 
and on the basis of all other available informa¬ 
tion. Specifications should be based on a level 
of quality adequate to insure that instruments 
give the performance inherent in their de¬ 
signs, but should not be more rigid than is nec¬ 
essary to achieve this result. They should be 
based, as far as possible, on the use of imper¬ 
sonal methods of inspection. 

2. Present methods of inspection should be 
reviewed in detail. Every effort should be made 
to introduce objective methods so that there 


can be no differences of opinion as to whether 
an instrument meets specifications. 

3. Further development work should be con¬ 
ducted vigorously on new methods of inspec¬ 
tion, employing all available techniques. These 
should include photoelectric and automatic 
methods whenever possible. 

4. The use of the interferometer for inspect¬ 
ing individual elements and subassemblies 
should be studied further. With adequate fix¬ 
tures for making adjustments expeditiously, it 
seems likely that the interferometer offers one 
of the most promising impersonal means for 
carrying out inspection. Much remains to be 
done in the way of setting up satisfactory 
methods for interpreting fringe patterns and 
of establishing appropriate criteria for inspec¬ 
tion. 

5. Studies based on correlations with field 
tests of performance should be made to estab¬ 
lish the level of efficiency that should be re¬ 
quired in KDC tests for inspection. 

6. Further studies should be made to deter¬ 
mine the best value of auxiliary magnification 
that should be used with the KDC apparatus. 

7. The resolving power of the naked eye, 
with various apertures, should be fully investi¬ 
gated. This program should be undertaken in 
close cooperation with the Army-Navy-NRC- 
Vision Committee, to insure that all physiolog¬ 
ical considerations are taken into account in 
planning the program. 

8. The effect of coated optics on scattered 
light in telescope systems should be measured 
to determine whether present methods of coat¬ 
ing lead to any loss of contrast. 

9. A manual covering recommended methods 
of inspection should be prepared as soon as the 
studies outlined above have been completed. 




Chapter 5 

BINOCULARS AS AIDS TO VISION 

By H. K . Hartline a 


51 SUMMARY 

I nvestigations OF the optimum design fea¬ 
tures of binoculars for use at night was un¬ 
dertaken at Dartmouth College (Contract 
OEMsr-1058), Brown University (Contract 
OEMsr-1229), and the University of Pennsyl¬ 
vania (Contract OEMsr-1228). Tests in indoor 
observing ranges were made with binoculars of 
various magnifications and exit pupils. An 
evaluation was made of the physical and physi¬ 
ological factors governing the assistance to 
vision furnished by binoculars. 

The range at which small targets could be 
detected at levels of illumination corresponding 
to night conditions was increased by increasing 
the magnification of hand-held binoculars up to 
10x, using instruments with objective di¬ 
ameters of 50 mm and 70 mm. Further increase 
to 14 X yielded poorer performance. Range of 
target detection was increased significantly by 
increasing the exit pupil diameter of 5x in¬ 
struments up to 6 mm. Further increase 
yielded only slight additional gains. 

Binoculars mounted in alidades yielded 
slightly better ranges of detection than hand¬ 
held binoculars by about 8 per cent. 

The maximum gain in range of detection ob¬ 
tained with binoculars was 4 to 4% times that 
obtained with naked-eye observation, under 
comparable conditions. This gain was fur¬ 
nished by 10 X binoculars (50-mm and 70-mm 
objectives) at brightness levels comparable to 
a clear moonless sky. 

The range of detection of targets is de¬ 
creased by light losses within the binocular and 
by brightness losses resulting from failure of 
the instrument's exit pupils to utilize the full 
aperture of the pupils of the observer’s eyes. 
Serious brightness losses result from misalign¬ 
ment of the instrument with respect to the 
eyes, caused by improper holding and aggra- 

a Eldridge Reeves Johnson Research Foundation, 
University of Pennsylvania. 


vated by eye movements as the observer scans 
the field of view. In addition, angular tremor 
movements caused by unsteady holding and 
magnified by the instrument decrease the visi¬ 
bility of targets. Quantitative allowance for the 
effects of these factors on the visibility of tar¬ 
gets with the aid of binoculars can be made 
with moderate success. 


52 INTRODUCTION 

The study of binoculars as optical aids to 
vision was undertaken by NDRC in May 1943 
to provide basic information which would as¬ 
sist the Bureau of Ordnance in selecting the 
most effective optical characteristics for night 
binoculars. The aim of Project NO-210, which 
was formally established in January 1944, is 
described as follows: “The primary purpose of 
the project is to determine the optimum rela¬ 
tion between magnification, angular field of 
view, exit pupil, size and weight and such other 
factors as may appear. The project is directed 
primarily at night glasses, though not neces¬ 
sarily so limited.” Since these factors are 
closely interrelated, and since the “optimum 
relation” may be expected to be different for 
different applications and to be affected by ex¬ 
traneous but important practical considera¬ 
tions such as cost, availability, and ease of 
production, it was clear that a broad program 
of investigation was required. 

NDRC set two aims for itself in undertaking 
this investigation. The first was to obtain as 
quickly as possible empirical information con¬ 
cerning the relative effectiveness for night use 
of the various binoculars that were available 
immediately or that could be produced readily 
by slight modification of existing instruments. 
Binocular testing projects were instituted in 
which observations were made with a variety 
of binoculars under controlled laboratory con¬ 
ditions. The object of these testing programs 


263 



264 


BINOCULARS AS AIDS TO VISION 


was to reveal the design trends that appeared 
most promising, and to determine the type of 
instruments that merited field tests under serv¬ 
ice conditions. The second aim was broader. It 
was to analyze in greater detail the influence of 
various factors of binocular design on extend¬ 
ing the range of human vision. For this the pri¬ 
mary program of binocular testing was ex¬ 
panded to include a variety of experimental in¬ 
struments, so chosen that the influence of spe¬ 
cific factors could be isolated in turn by holding 
constant all (or most) others. Concurrently, 
theoretical and experimental analysis of the 
effects of various factors was attempted in 
terms of the known physical properties of the 
instruments and established principles of 
visual physiology. Ultimately it was hoped to 
acquire sufficient understanding of the prin¬ 
ciples governing the utility of binoculars to 
permit the prediction of the optimum design 
for any particular application, or at least to 
narrow the range of possible designs to be se¬ 
lected for final field tests. 

The project at Dartmouth was a testing 
program to determine, for hand-held binocu¬ 
lars, the effect of magnification in increasing 
the range of visibility at low brightness levels. 1 
A more extensive testing program was under¬ 
taken at Brown University, where the effects 
of exit pupil, magnification, and other factors 
were analyzed, and observations with hand¬ 
held and mounted instruments were com¬ 
pared. 2 ’ 3 At the University of Pennsylvania, 
the application of principles of visual physi¬ 
ology to the use of binoculars was studied by 
the analysis of data from the Dartmouth and 
Brown testing programs and by specially de¬ 
signed experiments. Particular attention was 
given to the problem of “unsteadiness of hold- 
ing” of hand-held instruments. 4 Study of the 
effects of angular field of view, planned for a 
later phase of the projects at Brown and Penn¬ 
sylvania, did not come under consideration be¬ 
fore the end of the program. All of the projects 
were laboratory studies, performed under con¬ 
trolled conditions of observation. No formal 
program of testing under field conditions was 
undertaken, as this could be done more profit¬ 
ably after the groundwork in the laboratory. 
To acquire experience in the use of binoculars 


under field conditions, however, the contractors 
and section members participated in a limited 
number of outdoor tests, particularly those 
carried out in September and October 1945 at 
New London by the Bureau of Ordnance. 

5 3 OBSERVING CONDITIONS 

Visual observation is a task requiring many 
diverse discriminations and judgments to be 
made under a wide variety of conditions. Not 
only must various features of complex visual 
scenes be discriminated, but the observer must 
interpret these features in relation to the gen¬ 
eral situation; the psychological as well as the 
physical factors are intricate. Experience, skill, 
and motivation of the observer are of para¬ 
mount importance. When binoculars are used 
as a visual aid, skill in handling them must be 
acquired. The advantages that may be gained 
by the use of binoculars must be evaluated with 
proper regard to all these considerations. A 
scientific approach to the study of these sub¬ 
jects must begin of necessity by simplifying 
the problems and standardizing the conditions 
of observation. This is all the more true when 
the real subject of the investigation is the ob¬ 
server and the use he can make of an instru¬ 
ment, rather than the instrument itself. These 
restrictions of the problems, essential to sound 
scientific method, inevitably limit the validity 
with which conclusions drawn from laboratory 
experiments can be applied in practical situa¬ 
tions. 

In the binocular testing programs, efforts 
were made to preserve as many of the essential 
conditions of practical observation as was pos¬ 
sible without too great sacrifice of scientific 
control. Where necessary, however, experi¬ 
mental instruments and methods were em¬ 
ployed without regard to their immediate prac¬ 
tical relevance if they contributed to the ulti¬ 
mate understanding of general principles 
which operate no less in practice than in the 
laboratory. The results of these laboratory ex¬ 
periments are thus to be considered as con¬ 
tributing basic understanding and informa¬ 
tion; the successful evaluation of the utility of 
binoculars in practice will depend on recogniz¬ 
ing the significant factors that distinguish the 



OBSERVING CONDITIONS 


265 


practical from the laboratory situation, and on 
analyzing correctly their effects. It is, there¬ 
fore, essential to consider in detail the experi¬ 
mental conditions that prevailed throughout 
the study, pointing out where these conditions 
agree with those of practical observation with 
binoculars and in what respect simplifications 
and limitations have been introduced. 

In the first place, no direct account has been 
taken throughout this study of the effect of the 
atmosphere on the visibility of objects, either 
with or without the aid of binoculars. This 
simplification of the problem is permissible be¬ 
cause knowledge of the optical properties of the 
atmosphere is available from other sources and 
from this information it is possible to calculate 
the effect of the atmosphere on the visual ap¬ 
pearance of objects. 5 Experiments with binocu¬ 
lars included the study of visibility of targets 
of reduced contrast. Allowance for atmospheric 
effects can, therefore, be made by calculation. 
Such calculations must always be made in the 
final assessment of the utility of binoculars in 
practice, although at night the ranges of visi¬ 
bility are usually short and the effects of the 
atmosphere are often small. 

As directed by the project request, the em¬ 
phasis in this program was on the nighttime use 
of binoculars. This was an important restric¬ 
tion of the problem, although the range of 
illumination that had to be investigated was 
still considerable. Dark clouds or sea back¬ 
grounds on a heavily overcast moonless night 
have brightnesses less than 10 m^iL (1 mpL = 
10~ 9 Lambert), which is only 10 times the abso¬ 
lute threshold of vision for the average observer 
(approximately). The range between 100 mpL 
(clear moonless sky) and 10,000 mpL (moon¬ 
light) is of maximum importance; above this 
brightness level conditions merge with those of 
twilight, which were not considered in this 
program. (Studies at twilight brightness levels 
have been reported.) 6 Except possibly at the 
very lowest brightness levels, binoculars are 
useful over this entire range. 

Below about 1,000 mqL, the properties of the 
retinal rods govern seeing (scotopic vision). 
Such vision is colorless, not very acute, and un¬ 
able to distinguish small gradations of con¬ 
trast. Maximal sensitivity is in the peripheral 


visual field (2 to 20 degrees from the direction 
of fixation, depending on the brightness level), 
and seeing requires special skill that can be 
acquired only by experience. The pupils of the 
eyes of observers are dilated almost to their 
maximum. “Night glasses” take advantage of 
this increased light-gathering power of the 
dark-adapted eye by providing larger exit 
pupils than are necessary at higher bright¬ 
nesses; this is their sole distinction. Although 
the properties of rod vision dominate at night, 
brightness levels above 1,000 m\xL are of im¬ 
portance; in this range the retinal cones begin 
to function (photopic vision), superseding the 
rods. Central vision (foveal) is possible at 
these levels, acuity and contrast discrimina¬ 
tions are higher, and color can be perceived. 
However, visual capacities are far below the 
maximum achieved in daylight. Whether domi¬ 
nated by rod or cone function, vision at night 
is characterized by a strong dependence on the 
amount of illumination available. This depend¬ 
ence made it necessary to cover the entire 
range of brightness representing night condi¬ 
tions in the experimental study of binoculars. 

In practice, the visual scene is usually very 
complex. At night, it is true, much of the com¬ 
plexity of the actual scene is not visible. Never¬ 
theless what can be seen by peripheral rod vi¬ 
sion is indistinct and seeing is uncertain at 
low brightness levels. Skill and experience on 
the part of the observer are required in dis¬ 
criminating one object from another, and in 
interpreting whether, for example, an object 
glimpsed is a “target” or only a fleeting shadow 
or reflection of no significance. In the binocu¬ 
lar testing programs, this situation was con¬ 
siderably simplified. The problem was confined 
to seeing a single small target of high or mod¬ 
erately high contrast against a background of 
uniform brightness. Seeing an airplane against 
the sky is an important practical example of 
this situation. The problem was, furthermore, 
confined to the simple detection of a target in a 
known location, or in one of several possible 
known locations with respect to an orienting 
mark. Thus the important problems of search 
and recognition were not included at this stage 
of the study, and the necessity for discriminat¬ 
ing a “real” target from some irrelevant fea- 



266 


BINOCULARS AS AIDS TO VISION 


ture of the visual scene was eliminated. It is 
quite possible that these more complicated vis¬ 
ual tasks might be differently affected by the 
use of binoculars than is simple detection, 
therefore generalizations must be made with 
caution. Nevertheless, the detection of an ob¬ 
ject or of a recognizable feature is basic to all 
these problems, and it is logical to consider it 
first. 

Very simple targets were presented for ob¬ 
servation in these studies. Small dark circular 
spots were caused to appear against a uni¬ 
formly lighted background. For the most part, 
targets of high contrast were employed, al¬ 
though enough data on targets of reduced con¬ 
trast were obtained to cover the range of prac¬ 
tical importance. In practice, targets may have 
irregular shapes and often appear lighter than 
their background. The laboratory simplifica¬ 
tions in these cases, however, are not to be con¬ 
sidered as seriously restricting the general ap¬ 
plicability of the results, for it is probable that 
they are of slight importance. 2 * 4 ' 13 ’ 14 It is 
known that, to a fair degree of approximation, 
the shape of the target does not affect its visi¬ 
bility at low brightness levels. For given 
brightnesses of target and background, only 
the angular area of the target determines 
whether it will be detected; the shape is unim¬ 
portant unless the target is greatly elongated 
in one direction. Many actual targets satisfy 
this requirement. It is also a good approximate 
rule that targets of given angular area are 
equally visible regardless of whether they ap¬ 
pear dark against a light background or light 
against a dark background, provided the abso¬ 
lute value of the brightness difference between 
target and background is the same. The prod¬ 
uct of angular area by absolute difference in 
brightness (incremental flux from the target) 
is the important quantity governing visibility, 
and for practical purposes may be treated as 
a single variable. 

Seeing at the threshold of visibility is uncer¬ 
tain. Indeed, the “threshold” is not a sharply 
defined magnitude, but rather a range of mag¬ 
nitudes within which the probability of seeing 
varies from zero to unity. 7 * 8 It has been estab¬ 
lished that at the absolute threshold of vision 
less than 10 quanta of light absorbed by the 


retina are required for seeing a flash of light. 8 
Where so few quanta are required, statistical 
fluctuations will be especially noticeable; the 
lower the average intensity of the visual stimu¬ 
lus, the less frequent will be the occasions on 
which the required number of light quanta will 
be absorbed within a time interval during 
which excitatory effects in the retina can be 
integrated. These physical fluctuations of the 
stimulus are sufficient to account for the uncer¬ 
tainty of seeing at low brightness levels. 8 

The relation between the (average) magni¬ 
tude of the stimulus and the frequency with 
which it is seen has considerable practical im¬ 
portance. Knowing it, various problems of 
threshold seeing can be treated by statistical 
methods. In some instances in practice, interest 
may center on seeing at a low level of proba¬ 
bility to yield the earliest possible warning of 
the presence of an object. In other cases, only 
“practical certainty” is important. Knowledge 
of the “frequency of seeing” curve permits cal¬ 
culations to be made for any desired level of 
probability. 

The value of a statistical definition of 
“threshold” has determined the procedure 
adopted for measuring thresholds experimen¬ 
tally in this study. Observers were allowed a 
specified period of time in which to examine a 
given field of view and to report the position of 
any “target” they saw. A number of such pres¬ 
entations at constant background brightness 
were made with the magnitude of the visual 
stimulus (size of target) varied in discrete 
steps covering the range from zero frequency 
of seeing to 100 per cent seeing. b Enough ob¬ 
servations were obtained to permit a moder¬ 
ately reliable estimate of the frequency with 
which each size of target could be seen over 
the range necessary to yield the entire “fre¬ 
quency of seeing” curve for each experimental 
condition. Typical “ogives” obtained in this 
way are reproduced in the Dartmouth final re¬ 
port. 1 The supplement to the Brown final re- 

b This method of determining threshold has been 
termed the “method of constant stimuli” in the litera¬ 
ture of psychophysics. That this is a glaring misnomer 
when applied to visual stimuli at low brightness levels 
is evident from what has been said concerning the 
origin of the uncertainty of seeing near the absolute 
threshold of vision. 





OBSERVING CONDITIONS 


267 


port 3 contains tabulations of the original data, 
from which the ogives may be reconstructed. 

For most practical purposes it is convenient 
to define the “threshold” as the target size that 
can be detected with a frequency of 50 per cent. 
At Dartmouth, the 90 per cent level of fre¬ 
quency of detection was also considered in the 
final report. At Brown, separate experiments 
were performed to establish the important fact 
that the shape and slope (on a logarithmic 
scale) of the “frequency of seeing” curve was 
practically independent of the conditions of ob¬ 
servation, within the limits of the experiments. 
A single parameter (the target size for 50 per 
cent detection) was, therefore, sufficient for 
the comparison of visual performance under 
various conditions. In the experiments at the 
University of Pennsylvania similar procedures 
were adopted. Target brightness was some¬ 
times varied instead of target size in determin¬ 
ing the threshold; these alternatives are essen¬ 
tially equivalent for practical purposes. 

The criterion of detection that was used was 
based upon the observer’s ability to report the 
position of the observed target correctly. Tar¬ 
gets were caused to appear in one of several 
possible positions with respect to a centrally 
located orientation mark that was easily vis¬ 
ible. The observers soon became familiar with 
the situation and knew where to look for pos¬ 
sible targets. With the exception of a few spe¬ 
cial experiments a target was always present 
in some one of the positions. Consequently a 
certain number of reports would be correct by 
pure chance even if the observer were only 
guessing, and actually saw no target at all. It 
was necessary, therefore, to apply a “correc¬ 
tion for guessing” to determine the true fre¬ 
quency of detection for each level of stimulus 
magnitude. The details of this statistical pro¬ 
cedure are given in the final reports. 

The restrictions and simplifications of these 
laboratory observations and the considerations 
of “frequency of seeing,” “correction for guess¬ 
ing,” etc., seem at first artificial and far re¬ 
moved from problems of practical seeing. Such 
is not entirely the case. Essentially the same 
problems are actually present in practice; they 
are merely unrecognized. In viewing any visual 
field, the observer must always answer for him¬ 


self the question: “Do I see an object?” Un¬ 
certainty may exist, and the observer must 
then perform for himself a short “frequency of 
seeing” experiment to decide whether a suspi¬ 
cious sensation of light or shadow is real or 
spurious. Only the uninitiated think that seeing 
at threshold levels is a simple matter of clear- 
cut presence or absence of a visual sensation. 
In practice, it is true, the observer cannot com¬ 
pare a restricted number of locations to deter¬ 
mine which of them contains the target. Tar¬ 
gets may be present in an infinite number of 
positions, or not at all. The skilled observer ex¬ 
amines a field systematically, often assuming 
that a target is present unless he fails to ob¬ 
tain a reasonably reliable visual confirmation. 
The use of an orientation mark in the labora¬ 
tory experiments is not entirely alien to prac¬ 
tice. Easily visible features of a visual scene 
are customarily utilized by skilled observers as 
orientation marks in searching the field syste¬ 
matically. When such features are not present, 
systematic observation is much more difficult 
and less trustworthy. Situations do arise in 
practice where no such features are visible, as 
when scanning the sky with binoculars; these 
the laboratory experiments fail to represent. 

Conditions of observation in the laboratory 
are, of course, greatly idealized as compared to 
field conditions. This is necessary in well con¬ 
trolled experiments, although the hardships 
and diverse circumstances of practice have a 
decisive bearing on the ultimate utility of bin¬ 
oculars. Effects of wind, vibration, and move¬ 
ments of the observing platform were deliber¬ 
ately avoided in the basic laboratory experi¬ 
ments ; their effects on the use of binoculars in 
practice may be very great. Toward the end of 
the program, a few observations were made at 
Brown with a rolling platform (Scoresby ma¬ 
chine), and comparison was made between 
hand-held and alidade-mounted instruments. 
Measurements of angular vibration and un¬ 
steadiness of hand-held binoculars on ship¬ 
board were made by the Pennsylvania group. 

The selection of observers to take part in the 
laboratory tests was made with care. An effort 
was made to parallel the Service requirements 
of age and physical vigor; visual requirements 
were similar. A large proportion of women was 



268 


BINOCULARS AS AIDS TO VISION 


employed as observers at Brown. There is no 
reason to believe that their visual performance 
is different from that of men; if anything, they 
tend to be more patient and conscientious in a 
tedious laboratory experiment. A careful pe¬ 
riod of training was required for each observer 
before data were collected. Unreliable perform¬ 
ance and failure to develop skill during this 
initial training period was cause for rejection, 
as was failure to show a conscientious attitude. 
Discomfort and fatigue were, of course, 
avoided as much as possible in the laboratory 
experiments; boredom was unavoidable. 

In practice, the situation in these respects 
varies widely. The laboratory conditions might 
be considered analogous to the most favorable 
conditions of practical observation, except that 
motivation is doubtless greater where the 
stakes are higher. Insufficient or incorrect 
training and failure to select reliable, observers 
are all too common in practice. In military 
service, discomfort and fatigue may be exces¬ 
sive at times. Boredom is relieved only when 
there is an element of danger. It would be 
grossly incorrect to fail to take into account 
such elements in assessing the ultimate prac¬ 
tical utility of binoculars. 

The binoculars that were used in these 
studies were high-quality instruments, either 
standard Navy equipment or carefully de¬ 
signed modifications of them. Special objec¬ 
tives or special oculars were manufactured to 
yield instruments with the desired magnifica¬ 
tion and exit pupil. The objectives were dia- 
phragmed in some experiments to provide the 
exact values of exit pupil required. All optical 
surfaces were coated with a low-reflection film 
(lithium fluoride). The transmission of the 
instruments was between 70 and 80 per cent. 
Great care was exercised in the laboratory ex¬ 
periments to have the binoculars correctly ad¬ 
justed for each individual observer. The cor¬ 
rect setting of the binocular for interpupillary 
separation was made carefully in every experi¬ 
mental session. The optimum focus was deter¬ 
mined by experiment for each observer who 
then set the instrument he used at this value 
in every test. Such precautions are at least pos¬ 
sible in practice even though they are not 
always taken. 


The discussion of these factors affecting ob¬ 
servation serves as more than a delineation be¬ 
tween practical and laboratory conditions. 
Recognition of elements unfavorable to obser¬ 
vation in practice can lead to their elimination 
in some cases. Ability to deal systematically 
with various perceptual aspects of seeing 
marks the difference between a skilled and un¬ 
skilled observer. Recognition of elements that 
contribute to skill can lead to improvement in 
training procedures. The experimental precau¬ 
tions, the systematizing, and the simplifications 
that have been found necessary to obtain re¬ 
liable results in the laboratory thus contribute 
to the understanding and solution of practical 
problems. 

54 EXPERIMENTAL PROJECTS 

3 4,1 Binocular Testing Program at 
Dartmouth College 

Equipment and Methods 

The testing program at Dartmouth College 1 
dealt with the comparative effectiveness of bin¬ 
oculars of various magnifications, all possess¬ 
ing a 5-mm exit pupil. The binoculars employed 
were 6x30, 7x35 (a standard 7x50 with ob¬ 
jective diaphragmed to 35 mm), 8x40, and 
10x50. The experiments were conducted in an 
indoor observing range, in which a white 
screen (12x12 ft) was placed 60 ft from an 
observing station and illuminated uniformly 
at the desired level. 

The brightness levels chosen were 40, 400, 
4,000, and 40,000 mqL. They were adjusted with 
the aid of a University of Rochester sky pho¬ 
tometer. The original calibration of this instru¬ 
ment, and a later verification at Pennsylvania, 
were based on a tungsten standard having a 
color temperature of 2360 K, standardized at a 
high brightness level by conventional pho¬ 
tometry, and attenuated physically to the low 
levels required. This is in accordance with the 
specification for low-brightness photometry 
proposed by the American Standards Associa¬ 
tion. The same specification was adopted at 
Brown and at Pennsylvania. 

The “targets” were circular holes in the 



EXPERIMENTAL PROJECTS 


269 


screen, and appeared black (100 per cent con¬ 
trast) against the dimly lit background. They 
could be made to appear in any one of four pos¬ 
sible positions equally spaced in a circle sub¬ 
tending an angular radius of 0.7 degrees about 
a weak red fixation spot at the center of the 
screen. Six different target sizes were used for 
each set of brightness and magnification condi¬ 
tions. 

Three full-time observers were employed, 
after a preliminary period of training. They 
were dark-adapted for 45 min before each ex¬ 
perimental session. For each observation, a 
warning signal was given, and the observers 
were allowed 6 sec to look. At the end of this 
time they signaled the position in which they 
saw the target. They were not forced to guess 
if they could not see the target. The observers 
stood to make the observations, and held the 
binoculars in their hands without resting them 
on any solid support. 

For each target presentation, the actual po¬ 
sition of the target was recorded, and also the 
position reported by each of the observers. The 
correct responses could thus be noted and the 
errors analyzed. 

In any experiment of this kind a certain 
number of errors will be made in reporting 
target position, even with observers trained to 
do their best conscientiously, and even if the 
observer is not forced to guess. Indeed, failure 
to make any errors is evidence that the ob¬ 
server is being too cautious; this will cause him 
to miss targets that are merely a trifle uncer¬ 
tain. Presence of errors means that the ob¬ 
server is “playing his hunches,” doing a certain 
amount of guessing of his own accord, and pre¬ 
sumably working at the very lowest limit pos¬ 
sible for him. Since a target was always pres¬ 
ent in some one of the four positions in these 
experiments, there was one chance in four that 
any guess made would be correct, even though 
the observer did not actually see the target. It 
is desirable to make allowance for these for¬ 
tuitously correct guesses, for in practice, tar¬ 
gets can appear in an infinite number of posi¬ 
tions, and no appreciable number of lucky 
guesses will occur. The necessary allowance for 
guessing can be made from the number of in¬ 
correct responses that were recorded, for on 


the basis of pure chance these must constitute 
three-fourths of the total number of guesses. 
One-third of the number of incorrect reports, 
therefore, equals, on the average, the number 
of reports that were correct guesses. This num¬ 
ber subtracted from the total number of cor¬ 
rect reports yields the best estimate of the 
number of observations in which the target 
was “actually” seen. The number actually seen, 
for any given target size, divided by the total 
number of presentations of targets of this size, 
gives the “probability of detection,” corrected 
for guessing, for this size of target. 

In the analysis of the Dartmouth data by 
Section 16.1 it was found that these considera¬ 
tions were complicated by the fact that the in¬ 
correct reports tended to favor those positions 
adjacent (on the circle about the fixation 
point) to the true position of the targets, as 
though the observer was “almost right” in 
some cases. The incorrect reports which gave 
the position diametrically opposite to the actual 
target position were, therefore, considered to 
give a truer estimate of guessing. The discus¬ 
sion of the final method of analysis of the Dart¬ 
mouth data is given in the appendix to the final 
report. 1 For a discussion of “frequency of see¬ 
ing” and the general problems of determining 
threshold by this method, the report CAM No. 
110 may be consulted. 7 

Results 

Figure 1 exemplifies the way in which the 
data collected at Dartmouth were presented, 
after the correction for guessing had been ap¬ 
plied. These curves (ogives) show the relation 
between probability of detection and size of 
target for the four binoculars studied and for 
naked-eye observations. This set of curves was 
obtained from observer B for black circular 
targets at a screen brightness of 40 mpL. Simi¬ 
lar sets were prepared for other screen bright¬ 
nesses and for the other observers. Figure 1 
shows that smaller targets could be detected 
with binoculars than could be seen by the 
naked eye. In general, the higher the magnifi¬ 
cation the more was the “frequency of seeing” 
curve displaced in the direction of smaller 
target sizes (in the particular case of Figure 1, 
the 7x and 8x ogives constitute an exception). 



270 


BINOCULARS AS AIDS TO VISION 


If some arbitrary level of probability of detec¬ 
tion is chosen as defining “threshold,” the target 
sizes required to meet this criterion may be 
read from these graphs. This was done for the 
various cases, and the thresholds of the three 
observers averaged to obtain a mean threshold 
for each magnification and brightness level. 
The results are given in Table 1, for a 50 per 
cent level of detection, and for a 90 per cent 
level (corrected for guessing). The data in this 
table are presented in terms of “relative 
range.” (The relative range is the reciprocal of 


computation, the smaller ranges in the IX col¬ 
umn being the expression of the decreased 
acuity of vision caused by the brightness losses 
of the instrument. If brightness loss were the 
only factor affecting observation through bin¬ 
oculars, the observed ranges obtained with any 
particular binocular should agree with the 
ranges calculated by multiplying the “lx” 
ranges by the magnification of the instrument. 
This expectation is met with a fair degree of 
approximation, as may be seen by inspection of 
Table 1. Figure 2 presents these data (for 50 



-0.2 O 0.2 0.4 0.6 0.8 1.0 1.2 

LOG e 


Figure 1 . Curves relating probability of detection (corrected for guessing) with target diameter (0, in 
milliradians), for naked-eye observations and for observations with binoculars of various magnification 
(6X, 7X, 8X and 10X, all having 5-mmexit pupils). (Dartmouth College results.) 


the angular diameter of the threshold target in 
radians.) 

The threshold ranges obtained from naked- 
eye observation (for two of the three ob¬ 
servers) are entered in Table 1 in the column 
headed “N. E.” In the next column, headed “lx” 
are hypothetical ranges computed for ob¬ 
servers using a binocular of unit magnification 
having an exit pupil the same size (5 mm) as 
that of the binoculars actually used, and a 
transmission equal to the average of the trans¬ 
missions of the actual instruments. The naked- 
eye observations furnished the basis of this 


per cent probability of detection) in graphical 
form. It shows the linear increase of range of 
detection with magnification. Solid lines con¬ 
nect the observed points, the lowest point for 
each curve (plotted at M = 1) is the value for 
naked-eye observations at that brightness level 
(column “N. E.”). The dotted lines show the 
relation predicted from the “lx” ranges mul¬ 
tiplied by magnification (column “lx”)* Fig¬ 
ure 2 shows a slight lack of agreement between 
observed and predicted ranges at the highest 
brightness level, and also at the highest mag¬ 
nification. Moreover, the values for the 7X in- 





EXPERIMENTAL PROJECTS 


271 


strument seem to be consistently better than 
predicted, and somewhat out of line with the 
rest of the observations. However, analysis of 
the results shows that these discrepancies are 
only on the borderline of statistical signifi¬ 
cance. 

The principal result of the Dartmouth 
studies, and the one that is of practical im¬ 
portance, is that the use of binoculars greatly 
increases the range of detection of small tar- 

Table 1 . Relative ranges of detection for black 
circular targets viewed against backgrounds of 
various brightnesses with binoculars of various 
magnifications (5-mm exit pupils). (Dartmouth 
College results.) 

Data are given for two levels of probability of de¬ 
tection, corrected for guessing [P c (d)]. “Relative 
range” is the distance ( d ) at which a target of 
unit diameter is detected with a probability P c (d). 
Naked-eye ranges (N.E.) are included, obtained by 
2 of the 3 observers. The column headed “IX” con¬ 
tains ranges interpolated from N.E. observations at 
different brightness for a hypothetical binocular of 
unit magnification, having a 5-mm exit pupil and a 
transmission equivalent to the average of the actual 
instruments used. 


Back¬ 

ground 

bright¬ 

ness 

m/iL 

N.E. 

IX 

6X 

7X 

8X 

10X 


40 

120 

87 

580 

705 

625 

750 


400 

190 

160 

935 

1,150 

1,340 

1,540 

Pc(d) 

4,000 

390 

294 

1,710 

2,420 

2,160 

2,880 

= 50% 

40,000 

1,100 

759 

3,580 

5,110 

4,770 

6,660 


40 

95 

74 

410 

480 

460 

565 


400 

130 

120 

705 

895 

1,010 

1,130 

Pc(d) 

4,000 

300 

220 

1,360 

1,730 

1,660 

1,860 

= 90% 

40,000 

850 

590 

2,720 

3,440 

3,020 

3,570 



gets at night, and that this advantage is 
greater, the higher the magnification, at all 
brightness levels and at all levels of probability 
of detection. This holds true for magnifications 
up to 10x, and shows no sign of falling off at 
this value. The results indicate that considera¬ 
tion should be given to the use of higher mag¬ 
nification for hand-held binoculars than was 
customary in practice. This general result was 
confirmed by subsequent studies. 

The detailed relation between magnification 
and range of detection is of considerable theo¬ 
retical, and ultimately practical, interest. In 
the Dartmouth results the observed ranges 
agreed with those predicted on the basis of 


magnification alone, after allowance for simple 
brightness losses caused by physical loss of 
light transmitted by the instrument, and by an 
exit pupil smaller in size than the natural pupil 
of the observer’s eye. This means that in the 
Dartmouth studies the observations were not 
affected by misalignment of the binoculars 
with the observer’s eyes or by tremor move¬ 
ments of the binocular image caused by un- 



Figure 2. Relative ranges at which circular 
black targets were detected with a probability of 
50 per cent with the aid of binoculars of various 
magnifications (5-mm exit pupils), at 4 levels of 
background brightness. Data from Table 1. 
(Dartmouth College results.) 

steady holding. It is difficult to believe that 
these factors are without effect in practice, and 
it is somewhat surprising to find them without 
effect even in laboratory experiments. The 
other experimental studies at Brown and at 
Pennsylvania failed to confirm this particular 
conclusion drawn from the Dartmouth results. 


Binocular Testing Program at 
Brown University 

Equipment and Methods 

The testing program at Brown 2 was essen¬ 
tially similar to that at Dartmouth, but was 









272 


BINOCULARS AS AIDS TO VISION 


larger in its scope. More observers were em¬ 
ployed, and a wider variety of experiments was 
undertaken. 

An observing range was constructed, capable 
of being darkened completely. A white matte 
screen was placed 70 ft from the observing 
station at which observation booths were con¬ 
structed to accommodate six observers at one 
time. A Leitz VIII-S projection lantern was 
used to project dark circular targets from 2-in. 



Figure 3. Relation between relative range and 
exit pupil diameter for 5X binoculars, at three 
levels of background brightness. Arrows indicate 
the average diameter of the natural pupil at the 
respective brightness levels. The relative ranges 
for naked-eye observations were 280, 320, and 
630 for 40 m/uL, 400 m/*L, and 4000 m/xL respec¬ 
tively. (Brown University results.) 

square glass slides onto the screen. Screen 
brightness was varied, and controlled photo¬ 
metrically, over a range from 4 m\iL to 12,600 
mpL. The low brightness photometer used was 
constructed at the University of Pennsylvania 
(described in final report of Contract 
OEMcmr-209) 4 and standardized in accordance 
with the American Standards Association for 
low brightness photometry. The photometry at 
Dartmouth, Brown, and Pennsylvania was thus 
in agreement. 

The contrast of the targets (in absolute val¬ 


ues) could be varied from the maximum ob¬ 
tainable with the projector (close to 100 per 
cent) to any lower value. Data thus obtained 
could be used in computing the effects of dif¬ 
ferent atmospheric conditions. 

The targets were caused to appear in any 
one of six possible positions equally spaced on 
a circle subtending an angular radius of 3 de¬ 
grees about a central orientation mark. For 
any given experimental session (particular 
binocular, particular background brightness), 
target sizes were selected to cover the entire 
range of frequency of detection, and presented 
in random order in various positions, also 
taken in random order. In each observing booth 
was a push-button panel used by the observer 
to indicate the position in which the target was 
seen. The recorded responses were transferred 
to punchcards for convenient computation. 

As in the Dartmouth experiments, a “correc¬ 
tion for guessing” must be applied to restore 
the situation to a practical status where guess¬ 
ing has a negligibly small chance of success. 
In these experiments there were six possible 
target positions, but since target positions were 
never repeated in any one “run,” one-fourth 
rather than one-fifth the number of incorrect 
responses was taken as the estimate of the 
number of guesses that were correct by chance. 
No analysis of the errors was made, as was 
done at Dartmouth. Presumably the greater 
angular separation of the targets (3 degrees, 
instead of 1 degree as at Dartmouth) would de¬ 
crease the possibility of error from this source. 

The observers were recruited from the un¬ 
dergraduate students of Brown University and 
Pembroke College. They were carefully se¬ 
lected and trained; altogether approximately 
sixty were employed. 

It is difficult in a brief summary to convey 
an adequate impression of the amount of 
thought and effort that went into these binocu¬ 
lar testing programs, and to the significance 
of the details of method and procedure. The 
very fact that reliable measurements could be 
made of observer performance with binoculars 
is one of the results of this general program of 
study. The precautions and controls that were 
necessary to accomplish this furnish insight 
into the factors affecting the use of binoculars. 







EXPERIMENTAL PROJECTS 


273 


Many of the same factors requiring control in 
these laboratory studies are important in prac¬ 
tical observation. The original contractors’ re¬ 
ports may profitably be read with practical 
problems kept in mind. 

Results 

Exit Pupil Series. The first set of experi¬ 
ments undertaken at Brown was the compari¬ 
son of instruments with various exit pupil 
sizes, all of the same magnification (5x)- 
The experimental instruments were identical 
(5x50), except that the objectives were dia- 
phragmed, giving exit pupil diameters of 2, 
4, 6, 8, and 10 mm. Observations were made at 
three levels of background brightness; the re¬ 
sults are shown in Figure 3. This figure shows 
the unmistakable advantage of increasing the 
aperture of the instrument so that its exit pupil 
makes fuller use of the observer’s expanded 
pupil at low brightness levels. In detail, how¬ 
ever, the results are somewhat disappointing, 
for they show negligible improvement in range 
when the exit pupil is increased beyond 6 mm. 
The natural pupils of observers adapted to 
these low brightness levels are on the average 
considerably greater in diameter than 6 mm. 
Direct measurements by infrared photography 
of the pupil diameters of the Brown observers 
were made by the group at Pennsylvania. 
These showed that the average pupil diameter 
for the range of brightness of these experi¬ 
ments was approximately 7 mm. It is not easy 
to understand why the improvement in range 
noted in Figure 3 below 6 mm does not con¬ 
tinue up to at least 7 mm. 

Magnification Series. A second extensive se¬ 
ries of experiments concerned the effect of in¬ 
creasing magnification of binoculars having the 
same objective diameter. In such instruments 
the increased advantage from greater magnifi¬ 
cation would be expected to be offset partially by 
the effects of decreased exit pupil diameter. The 
set of instruments was prepared by altering 
the eyepieces of standard 7x50 binoculars so 
that the shape and weight and balance of the 
various instruments were the same. In planning 
this experiment it was felt that, if the results 
indicated a definite optimum magnification 
other than the standard 7x, comparatively 


slight changes in the design of the standard 
instrument, involving only the eyepiece, could 
be made to yield a profitable improvement in 
practice. 

The results of this series of experiments are 
given in Table 2, and in Figure 4. In the table, 
the results are given in terms of “binocular 
gain,” which is the factor by which the naked- 
eye range of detection is to be multiplied to 
yield the range of detection afforded by the 
binocular. Under ideal conditions, this would 
equal the magnifying power of the instrument. 



Figure 4. Relation between relative range and 
magnification at various levels of background 
brightness, for binoculars with 50-mm objectives. 
Naked-eye observations are plotted at M = 1. 
(Small dots at the breaks of the two uppermost 
curves give values interpolated from Figure 20, 
OSRD Report 6128.) 2 (Brown University re¬ 
sults.) 

Because of loss of brightness of the image re¬ 
sulting from physical loss of light, combined 
with failure of the instrument’s exit pupil to 
utilize the full aperture of the observer’s eye, 
the “ideal” gain cannot be expected. Allowance 
can be made for the loss in range from these 
simple causes. The resulting “theoretical gain” 




274 


BINOCULARS AS AIDS TO VISION 


is given in the bottom row of Table 2. The 
most striking feature of these results is the 
very serious failure of the observed binocular 
gains to measure up to the values that may be 
expected. Evidently there are additional factors 
that affect the results besides the simple bright¬ 
ness losses that have been considered. These 
results of the Brown studies are thus at marked 
variance with findings at Dartmouth, and no 
certain explanation has been found for this 
discrepancy. 

The results show a maximum in the relation 


Table 2. Binocular gains at various levels of 
background brightness, for binoculars of various 
magnifications (50-mm objective diameter). 
(Brown University results.) 

“Binocular gain” is the range at which a target can 
be detected with the aid of a binocular divided by 
the range at which it can be detected with the 
naked eye, all other conditions remaining the same. 


Background 

brightness 

m/xL 

5x50 

7x50 

10x50 

14x50 

4 

2.9 

2.7 

2.6 


40 

3.1 

4.1 

4.5 

2.3 

400 

3.1 

3.7 

4.1 

2.7 

4,000 

2.7 

2.8 

3.2 

2.5 

“Theoretical gain” 
(see text) 

4.1 

5.8 

7.2 

7.8 


between binocular gain and magnification in 
this series where the objective aperture was 
held constant. This maximum is at 10 X at all 
brightness levels except the lowest (4 mpL). 
The 7 X binoculars gave gains that were slightly, 
but significantly, lower. The gains obtained with 
the 14 X instrument were definitely lower. This 
may have been partly due to the reduced eye 
relief which is inevitable with higher power, 
and partly to the reduced field of view which 
made it more difficult to examine the six target 
locations efficiently in a specified time interval. 
Eyepieces with somewhat increased eye relief 
were supplied by the Perkin-Elmer Corpora¬ 
tion. It seems unlikely, however, that further 
improvements would make it possible for 14x50 
binoculars to equal the performance of well- 
designed 10x50’s. 

A similar series of experiments was per¬ 
formed with instruments of 70-mm objective 


aperture and various magnifications. The re¬ 
sults obtained were of the same general char¬ 
acter as for the 50-mm series. After making 
allowance for observer differences, it was con¬ 
cluded that the ranges with the 70-mm instru¬ 
ments were approximately 5 per cent greater 
than with the analogous 50-mm instruments. 
This disappointing meager return is less than 
is to be expected from elementary theoretical 
considerations, but is in keeping with the find¬ 
ings of the exit pupil study where increases in 
pupil diameter above 5 mm gave disappoint¬ 
ingly small increases in the range of visibility. 

The foregoing results were all obtained with 
targets of high contrast (approximately 100 
per cent) ; in practice, target silhouettes are 
often of low contrast. In addition, an impor¬ 
tant effect of the atmosphere is to reduce the 
apparent contrast of objects. For these reasons, 
experiments were conducted with targets of 
reduced contrast. According to simplified theory 
the threshold of visibility of a target, as has 
already been noted, is determined at any given 
level of background brightness only by the 
product of angular area of the target by the 
absolute difference in brightness between tar¬ 
get and background. The latter is proportional 
to the target contrast (at any fixed background 
level) ; the former varies inversely with the 
square of the distance to the target. Hence the 
“threshold range” should vary in proportion to 
the square root of the contrast. This was borne 
out by the experimental results, for contrasts 
greater than 50 per cent. For targets of lower 
contrast the observed ranges were materially 
less than predicted by this simple relation, and 
presumably must be treated by more exact 
theoretical analysis. There was some evidence 
that the loss in range for low-contrast targets 
was less for binoculars than for naked-eye ob¬ 
servations. For the details of these findings, 
the Brown final report 2 must be consulted. 

When the relative range is plotted against 
background brightness for various instruments 
of the 50-mm series, the graphs are quite accu¬ 
rately linear in most cases. These relations com¬ 
bined with the simple law governing the effect 
of contrast on target visibility make it possible 
to express the Brown results (for the 50-mm 
series) tiy empirical equations which form a 










EXPERIMENTAL PROJECTS 


275 


convenient condensation of the observational 
data: 

Naked Eye log R = 0.275 ( 6.49 + log B) + V 2 log C 

5x50 log R = 0.247 ( 9.4 + log B) + % log C 

7x50 log R = 0.205 (12.25 + log B) + % log C 

10x50 'log R = 0.195 (13.19 + log B) + % log C 

14x50 log R = 0.280 ( 7.75 + log B) + % log C 

Additional Experiments. The project at 
Brown University succeeded in its primary 
purpose of providing a large number of relia¬ 
ble data on the effects of fundamental features 
of binocular design on the detection of targets 
at low brightness levels. In addition, numerous 
other minor problems were attacked, of interest 
both for their significance in practical observa¬ 
tion and for their bearing on the question of 
why binoculars fail to come up to expectations. 
The Brown final report 2 summarizes the results 
of these parts of the projects: 

The effects of unsteadiness in holding the binocular 
were investigated by a series of observations in which 
the binoculars were mounted in standard Navy alidades. 
The results showed an average gain in range from 
alidade mounting of about 8%. These experiments 
cannot be considered, however, as completely eliminat¬ 
ing the tremor effects, since the observer was still re¬ 
quired to move the binocular about in scanning the field. 
Furthermore, little benefit could be expected from the 
alidades in diminishing the tremors of head and eve. 
Specially designed head-rests [“eye-guards”] for hold¬ 
ing the eyes in a fixed position with respect to the 
binocular, thus reducing the relative motion of the 
eye and the instrument, were also tested. The results 
of this study failed, however, to show any gain in 
range under laboratory conditions (absence of wind 
and ship vibration). 

It was particularly desirable to determine the effect 
on detection of angular motions such as those experi¬ 
enced on shipboard. This problem was studied by seat¬ 
ing an observer in a moving Scoresby machine during 
target observations. The results indicate a rather re¬ 
markable postural adaptation on the part of the 
observer, since no definite loss in range was detected 
for periods of oscillation greater than 12 seconds even 
when accompanied by an angular motion of 14°. For a 
9-12 second period having the same amplitude the 
range was decreased on the average by 15%. 

Since certain other night vision studies had been 
carried through with target presentations much shorter 
than the 30-second period which was adhered to in the 
present program, it seemed desirable to make a series 
of comparative studies between the ranges obtained 
with 30-sec. and 6-sec. exposures. . . . Both naked-eye 
and binocular ranges were less, as might be anticipated, 
for the 6-%ec. period, but in general the deficit in range 


with binoculars was somewhat greater than with the 
naked eye, thus leading to slightly lower binocular 
gain. Earlier studies had shown that little, if anything, 
was to be gained by exposures longer than 30 sec. 

Still another conceivable cause for low binocular gain 
lay in a possible lack of perfect coordination between 
the two eyes when using binoculars. In order to explore 
this possibility, a series of comparisons was made be¬ 
tween the ranges attained with a single eye and the 
two eyes acting together. The results show a relative 
advantage for the two eyes over a single eye with 
naked-eye observations of between 19% and 26%, 
whereas the corresponding advantage when binoculars 
are used is less than 15%. The difference may possibly 
be due to a greater difficulty with clipping when the 
two exit pupils of the instrument are to be simultane¬ 
ously aligned with the pupils of the two eyes. 

Commenting on the results of the entire 
project, the final report makes this interesting 
statement: 

Perhaps the most striking feature of the results 
obtained is the minor extent to which variations in 
binocular design affect performance. Thus, increasing 
the exit pupil from 5 mm. to 10 mm. leads to much 
lower gains than might be anticipated. Similarly, in¬ 
creases in binocular power do not yield the full returns 
which might be hoped for, particularly at the highest 
and lowest intensities of illumination. Furthermore, 
the benefit which accrues from alidade mounting would 
appear to be much less than might be anticipated. . . . 

The maximum ranges that could be obtained 
at low levels of illumination were given by the 
10x50 and 10x70 binoculars mounted on ali¬ 
dades. These ranges exceeded those given by 
hand-held standard 7x50 binoculars by only 
about 15 per cent. 

This result should not minimize the impor¬ 
tance of improvements in the design of binoc¬ 
ulars. That these advantages do not come up 
to expectations must not be allowed to obscure 
the fact that all good design features of binoc¬ 
ulars contribute to good performance. A large 
exit pupil is better than a small one at night; 
alidade mounting aids observation with binoc¬ 
ulars; a binocular instrument is better than a 
monocular. Particularly noteworthy are the ef¬ 
fects of magnification: Under laboratory condi¬ 
tions, the range of visibility at night is in¬ 
creased by increasing the magnification of a 
constant-aperture binocular, up to 10 X- The 
fact that each of the various features of binoc¬ 
ular design contributes disappointingly small 
returns is all the more reason for giving close 




276 


BINOCULARS AS AIDS TO VISION 


attention to all the details, since when taken 
together these add up to yield important advan¬ 
tages in observation. 

5,4,3 Investigations at the University 
of Pennsylvania 

The project at the University of Pennsyl¬ 
vania 4 was set up apart from the specific pro¬ 
grams of binocular testing to permit an experi¬ 
mental analysis of the factors affecting visi¬ 
bility through binoculars. It was hoped by such 
a study to acquire an understanding that would 
ultimately make it possible to predict the effects 
of design features not specifically considered 
in the testing programs, and that would make 
future exhaustive testing projects unnecessary 
for every new type of binocular developed or 
for every new set of Service demands. This is 
an ambitious hope for a problem that is es¬ 
sentially concerned with the human observer. 
Nevertheless such an aim is essential if the 
entire project is to have any significance be¬ 
yond the specific results obtained with partic¬ 
ular instruments and particular observers. 

This aim, of course, was by no means lacking 
from the testing projects at Dartmouth and 
Brown; but these were primarily concerned 
with providing results that could be interpreted 
directly in practice, and were often unable to 
go into the analysis of factors that were diffi¬ 
cult to understand. The final report of the 
Brown project 2 goes into many questions of 
interpretation; the discussion of these will be 
included after the description of the results 
from Pennsylvania. 

At the University of Pennsylvania an effort 
was made to correlate known facts of visual 
physiology with the findings of the binocular 
testing programs at Dartmouth and Brown 
and to conduct experimental studies as needed 
to clarify understanding of these findings. 

Studies of Pupil Size 

It was evident from the beginning that ac¬ 
curate information was required concerning the 
size of the natural pupil of the human eye, both 
as it is affected by the brightness of the ex¬ 
ternal surroundings to which the observer is 


adapted, and as it varies from individual to 
individual and from moment to moment in each 
individual. It was also necessary to have accu¬ 
rate knowledge of the contributions of all areas 
of the pupil to the total apparent brightness of 
the retinal image. The first project undertaken 
at Pennsylvania was, therefore, to survey the 
literature on the pupil, and to add experimental 
knowledge where necessary. 9 

It is known that at high brightness levels 
light passing through the margins of the pupil 
is not so effective in producing a sensation of 
brightness as light passing through its center 
(Stiles-Crawford effect). This is perhaps the 
result of a directional property of the retinal 
cones which may admit light only within a 
limited entrance angle. The original papers on 
this subject report that this effect is not pres¬ 
ent for the rods, i.e., for rod vision; all areas 
of the pupil are equally effective. A survey of 
the literature 4 on this subject indicated that 
this conclusion was amply verified by compe¬ 
tent workers. Therefore, for most brightness 
levels at night, where rod vision only is con¬ 
cerned, one may consider the effective bright¬ 
ness of the retinal image to be proportional to 
the area of the pupil that is utilized by the 
binoculars. Only at the highest levels of bright¬ 
ness that were investigated in these projects, 
where cone vision begins to be of importance, 
is it perhaps necessary to take into account the 
Stiles-Crawford effect. 

A number of careful studies have been made 
of the size of the pupil as a function of the 
brightness level to which the eye is adapted. 
However, all of these taken together include 
only a few subjects; it was felt necessary that 
a short survey be undertaken to gain some ac¬ 
curate knowledge of the variations to be ex¬ 
pected in large numbers of people. The experi¬ 
mental study 9 of the pupil that was undertaken 
at the University of Pennsylvania employed 
the usual method of infrared photography. The 
subjects first measured were ten observers 
from the Brown University project. Fortu¬ 
nately, it was not necessary to devote as much 
effort to this survey as had been anticipated, 
for a British report 10 became available in which 
was presented the exact information that was 
desired. The measurements performed on the 




EXPERIMENTAL PROJECTS 


277 


ten observers at Brown provided useful con¬ 
firmation of the British data, in addition to 
furnishing pupil data for specific subjects con¬ 
cerned in the binocular testing project. A fur¬ 
ther result of this study was the finding that 
fluctuations in pupil size, from moment to mo¬ 
ment and from day to day in a given individual, 
while present, are not large enough to have 
any bearing on the present problems. It was 
also concluded that while accommodation 
affects pupil size, the slight amount that most 
observers exert when using binoculars at night 
will have no significant effect on pupil size. 

An analysis of the British data is included 
in the report on the pupil survey. It was found 
that expressing the pupil size either in terms 
of diameter or area resulted in a distribution 
that was fitted satisfactorily by the, normal 
error function. The distribution of areas, how¬ 
ever, resulted in standard deviations that were 
nearly the same for all brightness levels, and 
hence this is a convenient presentation for pur¬ 
poses of computation. These valuable data from 
the British report 10 are reproduced in Table 3, 

Table 3. Average area of the pupil of the human 
eye, at different low levels of brightness adaptation. 
(From OSRD report No. 6098.) 

Mean areas for a sample of the population (52 
subjects), and standard deviations of the dis¬ 
tributions; measurements from British report 
A.R.L./N.2/0.502. (1942).io 


Adaptation 
brightness 
(candles/sq ft*) 

Mean pupil 
area 
(mm 2 ) 

a 

(mm 2 ) 

0 

43.86 

9.33 

1 X 10-6 

43.61 

9.23 

1 X 10-5 

41.81 

9.13 

1 X IO- 4 

39.50 

9.45 

1 X 10-3 

38.27 

9.04 

1 X 10-2 

34.34 

9.77 


* 1 candle/sq ft = 3.36 millilamberts. 


which gives the values of the mean pupil area 
at each of five brightness levels, with the cor¬ 
responding values of standard deviation. For 
convenience, Table 4 presents the pupil diam¬ 
eters for various brightness levels in mjiL, cal¬ 
culated from the mean areas interpolated from 
the data of Table 3. 

These data on the size and variability of the 
natural pupil of the eye have been used in 


calculations concerning the choice of exit pupil 
size for night binoculars. 11 Only those observ¬ 
ers whose pupils exceed a specified diameter 
will profit by an increase in the size of the exit 
pupils of binoculars above this value. There¬ 
fore, increases in exit pupil size above the size 
of the smallest human pupil (about 5 mm) yield 
diminishing returns. Quantitative calculations 
have been made, based on the observed distri- 


Table 4. Diameters of pupils of average area, for 
various levels of brightness adaptation. (Inter¬ 
polated from British data.) 


Adaptation 

brightness 

(m^L) 

Diameter 

(mm) 

Dark 

7.46 

10 

7.40 

100 

7.20 

1,000 

7.04 

10,000 

6.48 


bution in size of the human pupil, which yield 
the visual information on which choice of exit 
pupil size can be based. It is to be noted that 
the actual measurements made at Brown on the 
effect of exit pupil (see Figure 3) show an even 
smaller advantage from large exit pupil size 
than these calculations would allow one to 
expect. 

The size and variability of the human pupil 
likewise has a bearing on the effects of errors 
in the interpupillary setting of binoculars. 12 
Such errors cause one or the other, or both, of, 
the telescopes to be misaligned slightly, and 
this leads to brightness losses which somewhat 
reduce the advantage that a binocular instru¬ 
ment possesses over a monocular. This is true 
only for cases in which the exit pupil of the 
instrument is exactly the same size as the ob¬ 
server’s pupil, otherwise a certain amount of 
error in interpupillary setting can be tolerated 
without brightness loss to either eye. Calcula¬ 
tions of the net effect of this error, and its 
bearing on the design of instruments with fixed 
interpupillary settings, are presented in the 
article cited. 

Analysis of Dartmouth Results 

It was a primary purpose of the project at 
Pennsylvania to determine to what extent ob- 











278 


BINOCULARS AS AIDS TO VISION 


servations with binoculars could be explained 
in terms of known physical and physiological 
principles. It was expected that the effects of 
unsteadiness of holding would appear as dis¬ 
crepancies between the ranges of target visi¬ 
bility actually observed with various instru¬ 
ments and those calculated from naked-eye 
observations, making allowance for simple 
brightness losses. These effects would then be 
subjected to whatever experimental analysis 
seemed appropriate. 

When the results of the Dartmouth tests be¬ 
came available, they were analyzed in terms of 
known principles of visual physiology. 13 Since 
naked-eye data obtained at Dartmouth were 
very sparse, a report from the University of 
Rochester 14 was utilized to supplement the 
Dartmouth data. This report furnished data on 
the naked-eye visibility of targets, relating 
target size, target brightness, and background 
brightness at low levels of illumination. Analy¬ 
sis showed that the visibility of a target was 
determined by the product of its angular area 
and the brightness difference between it and its 
background. For a given level of background 
brightness, this total flux added (or subtracted) 
by the target was approximately constant for 
all values of target size below 10~ 4 steradians. 
It was the same for a dark target viewed 
against a bright background as for a target 
that appeared brighter than its background. 
The threshold flux increment contributed by 
the target varied in a regular way with the 
’background brightness, increasing slowly from 
the basic value set by the threshold flux from 
a point source seen against a completely dark 
background to somewhat more than 10 times 
this value at approximately 0.5 j.iL. Over this 
part of the brightness range, the Rochester 
data could be described by Hecht’s treatment 
of brightness discrimination. 15 At 0.5 |iL the 
data showed a break, an effect explained by the 
transition from rod to cone vision, and the in¬ 
crease of threshold flux increment with back¬ 
ground brightness was less rapid than at lower 
levels. The Rochester data thus furnished a 
basis for computing the threshold size of a 
target of any contrast seen against a back¬ 
ground of any given brightness within the 
range covering night conditions. The Dart¬ 


mouth naked-eye observations were found to 
agree well with these Rochester data. 

To apply this treatment to the observations 
with binoculars it is necessary to consider the 
limiting pupillary aperture. It is convenient to 
convert all of the values for background bright¬ 
ness, for both naked-eye and binocular observa¬ 
tions, into terms of relative “retinal illumina¬ 
tion.” This was done by multiplying all bright¬ 
nesses by the area of the limiting aperture— 
natural pupil or exit pupil, whichever was 
smaller. Treated in this manner, and after 
allowing for the magnification and transmis¬ 
sion of the binoculars, it was found that the 
Dartmouth binocular data were in good agree¬ 
ment with the naked-eye data. The points for 
all the instruments, 6x, 7X> 8X> and 10X, at 
the four.brightness levels used clustered about 
the same curve relating threshold flux incre¬ 
ment to background brightness that described 
both the Rochester and Dartmouth naked-eye 
data. In other words, the simple brightness 
losses caused by the instruments were sufficient 
to explain the visibility of the magnified target 
images seen through the binoculars. No residual 
effects of unsteady holding remained to be ex¬ 
plained. 

The treatment just outlined is given in detail 
in the interim report 13 cited, and also in the 
Dartmouth final report. 1 It is the basis on which 
the straight lines of Figure 2 were drawn, 
showing that the observations with binoculars 
agreed with the “expected” values. This result 
was rather surprising, since it was believed 
generally that unsteady holding caused large 
losses in the efficiency of hand-held instru¬ 
ments, particularly at high magnifications. The 
skepticism concerning this conclusion, ex¬ 
pressed in the interim report cited, was justi¬ 
fied, for the results presented in a British re¬ 
port on binocular studies 16 and the results of 
the Brown studies 2 - 3 when they became avail¬ 
able showed unmistakably that the simple con¬ 
clusion drawn from the analysis of the Dart¬ 
mouth data was open to question. The reason 
for the discrepancy is not clear; it may be 
noted, however, that the visibility ranges ob¬ 
served with binoculars at Dartmouth were in 
reasonably good agreement with those obtained 
at Brown with comparable instruments at 




EXPERIMENTAL PROJECTS 


279 


equivalent background brightnesses, while the 
naked-eye data did not agree well. The Dart¬ 
mouth naked-eye ranges were considerably 
smaller than those observed at Brown, and it 
is possible that the two observers who provided 
these observations at Dartmouth had not de¬ 
veloped the same skill in this slightly different 
observing situation that they had in their more 
extensive experience of observing with binoc¬ 
ulars. At Brown, on the other hand, naked-eye 
observations received the same attention as the 
binocular observations and were obtained in 
parallel observing runs. Underestimation of the 
naked-eye ranges of course leads to overestima¬ 
tion of the “binocular gain,” and this seems to 
be the best interpretation that can be found 
for the higher binocular gains recorded at 
Dartmouth. Whatever the explanation, the 
weight of experimental evidence favors the 
lower gains reported in the Brown studies. 

Experimental Analysis of 
Unsteady Holding 

Experiments were designed specifically to 
test the relative value of the threshold for 
naked-eye observation and for observation 
through binoculars. They showed a marked 
discrepancy between the two, in keeping with 
the findings at Brown. 2 - 3 Calculated at the 
retina, nearly 4 times as much light (total flux 
increment) was required at threshold when 
the target was viewed through a 10x70 instru¬ 
ment, hand-held, as was necessary with naked- 
eye observation. It was shown, moreover, that 
this discrepancy was the result of errors in 
holding the instrument in alignment with the 
eyes, combined with the effects of angular 
tremor that are unavoidable when the instru¬ 
ment is held in the hands. The experimental 
proof of this consisted in mounting the binoc¬ 
ulars rigidly, and fixing the observer’s head by 
means of a mouth-bite, so that his eyes were 
in correct alignment. Even with these precau¬ 
tions the movements of the eyes in scanning 
the binocular field caused misalignments that 
resulted in failure to utilize at all times the 
full brightness available. By requiring the ob¬ 
server to use a fixation mark and by providing 
artificial pupils in front of his eyes these errors 
were eliminated. Observations through the bin¬ 


oculars, compared with observations made with 
binoculars removed but with the observer and 
artificial pupils undisturbed, then yielded the 
same value for threshold (making proper al¬ 
lowance for the magnification and for the 
transmission of the instrument). This experi¬ 
ment proved that no factors had been over¬ 
looked that might affect seriously the detection 
of targets with the aid of binoculars. 

Experiments next were performed to deter¬ 
mine the relative contributions to lost efficiency 
by tremor and by alignment errors. A small 
spot of light was projected on a screen by 
means of a specially constructed, very light 
projector that could be clamped on the binoc¬ 
ular itself without adding appreciably to its 
weight and balance. When thus mounted, the 
effects of tremor in holding the binocular were 
eliminated, and the image of the spot as seen 
by the observer appeared perfectly steady in 
the instrument’s field. Thresholds obtained in 
this way were appreciably lower than when 
the same projector was mounted rigidly along¬ 
side the observer, so that the projected spot 
was stationary on the screen and, when viewed 
through the binocular, was subject to the usual 
unsteadiness. In both cases, of course, the usual 
alignment errors were effective, and the thresh¬ 
olds obtained with tremor eliminated were still 
not as low as was to be expected from naked- 
eye observation. Quantitatively, the results in¬ 
dicated that the losses were approximately 
equally divided between effects of tremor and 
effects of errors of alignment. 

Both misalignment and angular tremor of 
hand-held binoculars were measured directly 
in special experiments, suggested by one of the 
members of Section 16.1. The alignment of the 
observer’s eyes with the exit pupils of the bin¬ 
ocular was recorded by infrared photography. 
Since the exit pupil and entrance pupil of an 
optical instrument are conjugate, a camera can 
be focused on the objective of the binocular to 
photograph the image of parts of the observer’s 
eyes that are within the exit pupil. If the nat¬ 
ural pupil is smaller than the instrument’s exit 
pupil, or is not lined up with it, part of the 
observer’s iris is photographed, imaged in the 
circle of the objective lens. The area included 
between the margin of the natural pupil and 



280 


BINOCULARS AS AIDS TO VISION 


that of the exit pupil can be measured and the 
consequent brightness loss resulting from such 
“clipping” can be computed. Figure 5 is an 
example of the pictures thus obtained. The 
preliminary results indicate that unsteadiness 
of holding the instrument was not so much a 
source of clipping as were eye movements exe¬ 
cuted in scanning the binocular field. Special 



B 


Figure 5. Photographs by infrared illumination 
of the observer’s eyes imaged in the objective 
apertures of a 9x63 binocular. The rims of the 
objective lenses are faintly outlined in white. 

Upper photograph (A) : binocular well aligned, observer 
looking at the center of the instrument’s field; exit pupils 
fall almost entirely within the observer’s pupils, only a 
narrow crescent of the edge of the iris (darker than the 
pupil) being visible inside the objective aperture. Lower 
photograph (B): binocular incorrectly aligned, the result 
of looking off to the side (observer’s right) of the field; 
the exit pupils are “clipped” by the edges of the observer’s 
irises, which are visible as crescents covering more than 
half of the instrument’s aperture (darker than the pupil 
on the right side of the picture, brightly banded on the 
left). The right-hand objective (in the picture) was held 
slightly high in both photographs. (University of Pennsyl¬ 
vania studies.) 

training and experience might make it possible 
for an observer to reduce losses from this 
source; a sufficiently large exit pupil would, of 
course, eliminate it. 

Direct measurement of angular unsteadiness 
was made from records of the actual tremor 
of hand-held binoculars. This was done in the 
laboratory and on a moving vessel (Navy bin¬ 
ocular tests from a DE operating in Gardiner’s 
Bay, off New London). For this a small, light 


camera (/ = 95 mm) was attached to the bin¬ 
ocular being tested, its film exposed for a sec¬ 
ond or more to a distant light that was being 
observed through the binocular. The irregular 
path traced by the image of the light on the 
film thus recorded the angular movements of 
the instrument during the exposure. The pre¬ 
liminary results obtained indicated that, in the 
laboratory, random tremor movements take 
place which in 1 sec cover an area approxi¬ 
mately equal to a circle 1 mil (3V2 min) in 
radius. On shipboard, even under the compara¬ 
tively quiet conditions of the test, the tremor 
movements were two to three times as great 
as this. They were not greatly different with 
the largest instrument (10x70) than with the 
smallest (6x30). Wind (estimated 20 knots) 
increased them greatly; resting the elbows on 
a solid part of the vessel reduced them mark¬ 
edly, even in wind. Figure 6 gives examples of 
the traces photographed. 

While these measurements of alignment er¬ 
rors and angular unsteadiness are interesting 
and valuable, the solution of the problem by 
such a direct approach promises to be long and 
difficult. The effects of brightness loss calcu¬ 
lated from measured amounts of “clipping” can 
be translated fairly precisely into loss in range 
of visibility, for any given instant of time- 
However, as the eyes scan the binocular field 
and the “clipping” thus introduced varies from 
moment to moment, the net effect is not easy 
to assess. The effects of angular tremor are 
even more difficult to estimate. Discussions of 
these points are to be found at numerous places 
in the Brown final report, 2 as well as in the 
final report from Pennsylvania. 4 

Analysis of Brown Results 

Because of the difficulties in making a direct 
analysis of the effects of unsteadiness of hold¬ 
ing, the project at Pennsylvania devoted con¬ 
siderable attention to a less direct approach, 
based on the analysis of the data accumulated 
in the binocular tests at Brown. It was assumed 
that the failure to explain the binocular obser¬ 
vations in terms of naked-eye data, after al¬ 
lowance for simple brightness losses, could be 
taken as a measure of the combined effects of 
misalignment and angular tremor, since these 





EXPERIMENTAL PROJECTS 


281 


were shown at Pennsylvania to be the only re¬ 
maining factors. It was expected that the analy¬ 
sis would reveal certain consistencies that 
might permit useful generalizations to be 
drawn concerning the nature of these factors. 

Thus it was expected that the series of ex¬ 
periments with binoculars of fixed magnifica¬ 
tion but varying exit pupil would reflect the 
fact that alignment is less critical if the exit 
pupil is either considerably larger or consider- 



Figure 6. Records of the angular tremor of 
hand-held binoculars. Photographs of a distant 
light source taken with a camera mounted on a 
10x50 binocular under various conditions of ob¬ 
serving from the deck of a moving vessel. 

Upper row (A-C), 1-sec exposures; lower row (D-F), 
3-sec exposures. A, calm, elbows not rested. B, windy (esti¬ 
mated 20 knots), elbows not rested. C, windy, elbows rested 
on steady support. D, windy, elbows not rested. E, windy, 
elbows rested on vibrating support. F, windy, elbows rested 
on steady support. Scale of angular mils at bottom. (Uni¬ 
versity of Pennsylvania studies.) 


ably smaller than the natural pupil. This it 
failed to do, and the reason for this failure is 
not understood. 

The analysis of the effects of tremor was 
more promising; moreover, the explanation of 
these effects required no new visual data. It 
has been pointed out that the visibility of a 
small target is determined by the product of 
its angular area and the brightness difference 
between it and its background. If the target is 
very small (less than 10^ 5 steradians), this 
threshold flux increment is approximately con¬ 
stant, irrespective of the size of the target. 


However, if the incremental flux from the tar¬ 
get is spread over a large area of the retina, 
perfect spatial summation of the excitatory 
effects it produces no longer takes place, and 
a greater flux increment is required to render 
it visible. Naked-eye observations of large tar¬ 
gets having low contrast provide the quantita¬ 
tive relation between target size and threshold 
flux increment at each level of background 
brightness. These data furnish a quantitative 
explanation for the diminished efficiency of 
binoculars of high magnification. The funda¬ 
mental angular tremor of holding, magnified 
by the optical power of the instrument, in effect 
spreads the light in the image of the target 
over an area of the retina that is too large for 
complete spatial integration. 

The application of these considerations to 
the analysis of the Brown data was made in 
the following way. The naked-eye data on tar¬ 
gets of low contrast that were obtained in the 
course of the binocular tests provided the re¬ 
lation between target size and threshold flux 
increment at the four levels of background 
brightness that were used. Since the instru¬ 
ments introduced various amounts of bright¬ 
ness loss, it was necessary to devise a method 
for interpolating between the four observa¬ 
tional brightness levels so as to construct 
curves relating target size to flux increment 
for any required brightness level, as computed 
at the retina. This interpolation was guided 
by an analysis of preliminary naked-eye data 
from the Tiffany Foundation project (Section 
16.8, NDRC), although the data actually used 
were solely from the Brown observers. For any 
given binocular, therefore, it was possible to 
construct, for each brightness level used, the 
curve relating target size to flux increment that 
would be expected to describe the observations 
with that binocular if there were no effects of 
misalignment or tremor. The fact that the 
threshold value of flux increment of the target 
actually observed with the particular instru¬ 
ment did not fall on this curve at the actual 
value of target size (magnified image) was 
taken as evidence of the effects of unsteady 
holding. If it be assumed that angular tremor, 
magnified by the power of the instrument, in 
effect spreads the flux over an area larger than 





282 


BINOCULARS AS AIDS TO VISION 


the actual target size, the size of this “effective 
area” may be computed from the curve. The 
mean angular radius of the area covered by 
the center of the target image in its erratic 
movements over the retina may thus be esti¬ 
mated for instruments of different magnifica¬ 
tion. The results are shown in Figure 7. The 



MAGNIFICATION 

Figure 7. Calculated values of the amount of 
image tremor that must be assumed for hand¬ 
held binoculars of various magnifications to ac¬ 
count for the difference between the naked-eye 
visibility of targets and the visibility of targets 
observed with binoculars, as determined at Brown 
University. Different symbols distinguish the 
values obtained for different levels of background 
brightness and target contrast: circles, 40 m/tL 
background brightness; triangles, 400 m/xL; 
squares, 4000 m,uL; solid circles and triangles, 
contrast = 1.0; solid squares, contrast == 0.6; 
hollow figures, contrast = 0.4. (University of 
Pennsylvania studies.) 

individual points show considerable scatter, in¬ 
dicating that the analysis is still not entirely 
satisfactory. Nevertheless, there is a distinct 
trend, showing increasing “effective area” with 
increasing magnification, in accordance with 
expectation. 


The ordinates in Figure 7 give the values of 
the angular radius, in radians, of the circular 
area that is assumed in effect to be covered by 
the center of the target image in its random 
movements resulting from angular tremor 
movements of the binocular, magnified in the 
field of the instrument. The line drawn exhibits 
a proportionality between mean “tremor ra¬ 
dius” and M-l (angular movements of an in¬ 
strument of unit power produce no apparent 
image displacements). The slope of this line 
indicates a “basic tremor radius” of 1 mil, 
which is in agreement with the measured esti¬ 
mate of the area covered in 1 sec by tremors 
of hand-held instruments indoors. Of course, 
1 sec is too long an interval for temporal sum¬ 
mation in the retina, for the retinal “action 
time” is of the order of y 5 to V 2 sec at these 
brightness levels. However, the analysis as 
presented so far takes no account of the effects 
of misalignment. If allowance is made for 
brightness loss from this source, smaller values 
for the “basic tremor” are obtained in better 
agreement with known properties of the retina. 

The observations with the 14 X binoculars 
were thought (Brown University report) 2 to be 
unfavorable to this magnification because of 
poor eye relief and small field of the instru¬ 
ments. This would tend to make the calculated 
tremor radius too high at this magnification. 

Thus while the attempts of the Pennsylvania 
project to analyze the data provided by the 
binocular testing projects were only partially 
successful, they offer considerable promise. 
Further experiments are needed, and more 
knowledge must be obtained of the fundamental 
physiological processes of spatial and temporal 
summation in the retina and other parts of the 
visual nervous mechanism. It is reasonable to 
hope that it will be possible ultimately to pre¬ 
dict the aid to vision that may be expected from 
various optical instruments under various con¬ 
ditions of observation. 

Additional Projects 

Eye Guards. Two projects of immediate prac¬ 
tical interest were undertaken at Pennsylvania. 
The construction of “eye guards” for attach¬ 
ment to standard binoculars was undertaken, 
in the hope of improving the ease and comfort 










DISCUSSION 


283 


of use, and perhaps of reducing alignment 
errors. Tested briefly at Brown University, no 
significant improvement in target visibility re¬ 
sulted, in the quiet conditions of the laboratory. 
However, binoculars equipped with these eye 
guards are considerably more comfortable to 
use, and these or similar devices would prob¬ 
ably have important practical advantages. An 
experimental prototype of the eye guard devel¬ 
oped at Pennsylvania was sent to the Naval 
Gun Factory for further engineering prior to 
Navy tests. 

Folded Binoculars. The design and construc¬ 
tion of a “folded” binocular (10x70) of uncon¬ 
ventional type was undertaken at Pennsylvania. 
This was done as an experiment to determine 
whether improvement in weight distribution 
would decrease angular unsteadiness, especially 
during long periods of fatiguing use. In this 
instrument the four reflections of the Porro 
erecting system were redistributed along the 
optical axis. The objectives were brought to 
either side of the head, and the center of grav¬ 
ity was back of the eye position. The altered 
distribution of the bulk in this design might 
make it useful in cramped quarters, as in 
airplane enclosures. It could conceivably be 
mounted on the observer’s head, to be swung 
down into place when needed and steadied by 
one hand. Tested at Brown University under 
quiet conditions indoors, this binocular yielded 
slightly poorer results than the conventional 
10x70. The possibility of lessened fatigue over 
long periods of use has not been explored. This 
line of development has greatest interest in 
making available for hand-held use, or for use 
in cramped quarters, instruments that are much 
larger than could otherwise be considered. 


55 DISCUSSION 

The laboratory studies that have been de¬ 
scribed have demonstrated clearly that binoc¬ 
ulars aid considerably in the visual detection 
of objects under conditions of low illumination 
similar to those found out of doors at night. 
This is in agreement with much practical ex¬ 
perience and with expectations based on ele¬ 
mentary considerations. This conclusion refers 


to the use of hand-held binoculars under the 
favorable conditions of the laboratory, in the 
absence of wind, vibrations, atmospheric ef¬ 
fects, etc. The conditions of the experiments 
were confined to comparatively simple problems 
of observation, where the discrimination of 
significant objects from irrelevant details of 
the visual scene were not required, and where 
different kinds of objects did not need to be 
recognized and distinguished. Problems of 
search were not included, observations being 
limited to the detection, with a specified prob¬ 
ability, of objects in one of a few possible 
known locations. The study of the utility of 
binoculars must ultimately be extended to in¬ 
clude these and other conditions that are of 
importance in practical seeing. The results of 
the present study, nevertheless, will prove use¬ 
ful in guiding decisions concerning the design 
of night binoculars and their use in practice. 

The assistance to vision that may be expected 
from the use of any hand-held binoculars at 
night, according to the fairest appraisal of the 
laboratory studies that seems possible, is at 
most a 4V2-times increase in the range at which 
objects can be detected as compared with naked- 
eye observation. This is the maximum binocular 
gain reported in the Brown University study 2 at 
the higher background brightness levels corre¬ 
sponding to a starlit sky or brighter. At lower 
brightness levels the gain was less (3 times 
naked-eye range). Binoculars that are of as¬ 
sistance on the brighter nights may be less 
useful under very dark conditions. 

The Brown University project demonstrated 
clearly that there was an optimum magnifica¬ 
tion for hand-held night binoculars of given 
objective diameter. For both the 50-mm and 
70-mm sizes, this was at 10X at all brightness 
levels studied except the lowest (corresponding 
to very dark conditions). Under the favorable 
observing conditions of the laboratory, it paid 
to increase the magnification up to 10X, but 
further increase resulted in a distinct decrease 
in performance. This falling off was so marked 
that it seems unlikely that the optimum magnifi¬ 
cation for any hand-held instrument is above 
10X. 

All conditions of practical seeing such as 
wind, vibration and motion of the observing 



284 


BINOCULARS AS AIDS TO VISION 


platform, and necessity for scanning, will tend 
to favor the use of lower rather than higher 
magnifications for hand-held instruments of 
general utility, so that a magnification lower 
than 10 X may prove to be a more acceptable 
practical compromise. 

The series of experiments on the effects of 
exit pupil size indicate that night binoculars 
should have an aperture large enough to take 
advantage of the light gathering power of the 
dark-adapted eye, with its expanded pupil. 
Notable increase in the range of detection with 
increasing exit pupil has been demonstrated 
up to a diameter of 6 mm; above this the re¬ 
turns are meager. It is not clear why this should 
be, since the average pupil diameter for ob¬ 
servers adapted to night sky brightness levels 
is approximately 7 mm, and it might also be 
expected that still larger exit pupils would 
allow a margin for minor misalignments and 
so reduce the chance of brightness loss from 
this cause. Until these experimental results 
have been verified, or a plausible explanation 
for them has been found, sacrifice of exit pupil 
size below 7 mm should be made reluctantly. 

Sacrifice of exit pupil may be justified in 
some cases. An example is the 10x50 (wide 
field) binocular, which may be compared with 
the standard 7x50 of similar size and weight. 
In this case the exchange of exit pupil size for 
extra magnification was profitable, at least for 
the laboratory conditions of observation. The 
10x50 yielded ranges that were approximately 
7 per cent higher than could be obtained with 
the 7x50 under comparable conditions, at the 
higher levels of background brightness. A fur¬ 
ther increase, of about the same amount, was 
obtained by increasing the objective diameter 
of the 10X instrument to 70 mm, thus regain¬ 
ing the 7-mm exit pupil, but since this entails 
considerable increase in bulk and weight, it is 
doubtful whether it would be warranted for a 
hand-held instrument of conventional design. 

The increase in the range at which objects 
can be seen at night with the aid of hand-held 
binoculars of the most favorable design is great 
enough to be of considerable advantage in prac¬ 
tice. Nevertheless, it is disappointingly small 
compared to what might reasonably be ex¬ 
pected. This verdict is based largely upon the 


results of the Brown University studies. 2 - 3 Ex¬ 
periments at the University of Pennsylvania 
confirm it, and it is in agreement with the 
results of a similar study by British workers. 
The Dartmouth results contradict this verdict, 
but the best appraisal that can be made at 
present favors the interpretation that the Dart¬ 
mouth naked-eye ranges must have been in 
error. They appear to be too low, and the bin¬ 
ocular gains calculated from them consequently 
appear to be too high. Whatever the explana¬ 
tion of the Dartmouth results, the conclusions 
drawn from them concerning binocular gain 
cannot be accepted. It seems quite certain that 
hand-held binoculars do not provide the full 
increase in range of vision that would be ex¬ 
pected from their magnification, after allow¬ 
ance for simple brightness losses alone. 

The attempts to come to a complete under¬ 
standing of the factors affecting visual per¬ 
formance with binoculars have been only par¬ 
tially successful. The experiments at Pennsyl¬ 
vania showed that strict experimental control 
of alignment and steadiness resulted in com¬ 
plete agreement between naked-eye thresholds 
and thresholds obtained with the aid of binoc¬ 
ulars, after allowance for magnification and 
simple brightness losses. There appear to be no 
unexplained factors remaining. Although the 
experiments were performed with bright tar¬ 
gets against a completely dark background, it 
seems likely that similar results would be ob¬ 
tained under conditions more closely resembling 
the practical situation at night. The fields of 
the binoculars in question are so large that the 
slight restriction they place on the normal 
naked-eye field can hardly be an important 
factor where search is not required. 

The experiments at the University of Penn¬ 
sylvania demonstrated that the reduced effi¬ 
ciency of hand-held binoculars, after allowance 
for simple brightness losses, is caused by fail¬ 
ure to keep the instrument in alignment with 
the eyes, especially when scanning its field, and 
by angular tremor movements from unsteady 
holding which cause the image to “dance” 
about in an irregular manner. The losses ap¬ 
pear to be about equally divided between the 
effects of these two factors, each accounting for 
the loss of roughly 0.15 log units of range. 



DISCUSSION 


285 


The direct measurements of misalignment 
and of angular tremor are valuable chiefly in 
confirming the presence of these two factors 
and in giving an approximate indication of 
their magnitude. The application of such meas¬ 
urements to a detailed analysis of visibility 
losses promises to be difficult and presupposes 
a greater knowledge of visual physiology than 
exists at present. Less rigorous application, 
however, is very instructive. Thus the fact that 
misalignment is caused largely by movements 
of the eyes themselves, rather than by unsteadi¬ 
ness of holding the binocular, indicates that 
mounting the instruments or providing them 
with eye guards to locate them accurately in 
front of the eyes will not eliminate this source 
of trouble entirely, valuable as such devices 
may be. Training the observer, on the other 
hand, may be quite effective in reducing losses 
from misalignment. The observer should learn 
to move the binoculars rather than his eyes, 
and since peripheral vision must be employed, 
he must learn to give his attention systemat¬ 
ically to various portions of the field without 
changing his direction of fixation down the axis 
of the instrument. 

The measurements of angular tremor are 
also useful in a qualitative way. From them it 
was learned that differences in size and weight 
of the instruments tested had little to do with 
the steadiness with which they could be held, 
but that external conditions of vibration and 
wind were very important. Arrangements for 
sheltering the observer from wind and provi¬ 
sion for vibration-free rests for his elbows are 
practical aids that would yield large returns in 
visual performance. 

The attempts to analyze the results of the 
binocular studies on the basis of naked-eye 
thresholds have been promising but not entirely 
successful as yet. The Dartmouth results as 
originally analyzed proved to be misleading in 
the light of later surveys; the conclusion that 
magnification alone, after correction for simple 
brightness losses, is sufficient to account for 
binocular performance is almost certainly 
wrong. The results from Brown University, 
where naked-eye observations were routinely 
obtained along with the observations with bin¬ 
oculars, are more reliable in estimating the 


binocular gains that may be expected and in 
permitting analysis of the effects of tremor and 
misalignment. Even here there are puzzling 
questions. If misalignment contributes notably 
to binocular losses, why does not the range of 
visibility increase steadily with increasing exit 
pupil, even above the value for the average 
diameter of the observer’s natural pupil? With 
an oversize exit pupil a certain amount of mis¬ 
alignment could presumably be tolerated with¬ 
out causing brightness losses from clipping. 
Furthermore, exit pupils that are smaller than 
the natural pupil should also permit some de¬ 
gree of misalignment before the effects of clip¬ 
ping become evident. Since all apparent bright¬ 
nesses are calculated from the smallest limiting 
aperture, whatever additional losses arise from 
misalignment should be least noticeable if the 
exit pupil is either much larger or much smaller 
than the natural pupil. Failure of the observed 
range of visibility to agree with the “expected” 
range should be most noticeable when the two 
apertures are exactly the same size. Then the 
slightest misalignment will result in “unex¬ 
pected” brightness loss. Analysis of the “exit 
pupil series” of experiments at Brown showed 
no evidence of this effect of misalignment. The 
reason for this is not clear, unless it be that the 
effect was masked by the considerable variation 
in pupil size in the normal population of ob¬ 
servers. 

The analysis of the effects of angular tremor 
of the hand-held instruments was more suc¬ 
cessful. Based on the assumption that rapid, 
erratic tremor movements cause the target 
image in effect to be “smeared out” over an 
angular area greater than its actual size, the 
binocular losses could be explained fairly satis¬ 
factorily by reference to naked-eye data, giving 
the increase in threshold attendant upon de¬ 
creasing the contrast in proportion to the 
increased size of the spread-out image. The 
higher the magnification, the greater is the area 
over which the target image must be assumed 
to be spread in order to account for its dimin¬ 
ished visibility, in keeping with the idea of 
a “basic angular tremor,” the effects of which 
are magnified by the optical power of the in¬ 
strument. The Brown data support this inter¬ 
pretation with moderate consistency, except 



286 


BINOCULARS AS AIDS TO VISION 


for the case of the 14 X instrument, for which 
the losses are exaggerated. There is a good 
excuse for this, since the 14X instruments were 
not carefully designed instruments and had 
limited field. The observers found it necessary 
to modify their observing technique with these 
instruments and this could easily account in 
part for their poorer performance. 

The value of the “basic angular tremor” that 
must be assumed in order to explain the Brown 
data agrees satisfactorily with that obtained 
by direct measurements under laboratory con¬ 
ditions, making proper allowance for known 
properties of the retina. In regard to the effects 
of angular unsteadiness, therefore, a consistent 
interpretation has been found. While this inter¬ 
pretation cannot be said to have been proved 
rigorously, it is the best available at present 
and it offers promise of considerable utility. 
If it can be extended on a sound basis, it may 
be ultimately possible to predict with moderate 
accuracy the aid to vision that binoculars of 
various designs will furnish under various con¬ 
ditions. Not only is it useful to know what may 
be expected in practice, but such prediction 
would be valuable in determining the optimum 
design features of binoculars for various pur¬ 
poses. 

The experimental binoculars of unconven¬ 
tional design (folded optical path) that were 
designed and constructed at the University of 
Pennsylvania proved disappointing in labora¬ 
tory tests. In the light of the subsequent find¬ 
ing that weight and balance of an instrument 
have little effect on angular tremor, this can 
be understood. It is still possible that such an 
instrument may be useful in special applica¬ 
tions, or that it may be less fatiguing to use 
over long periods of time, but it seems clear 
that no very marked improvement in perform¬ 
ance can be expected in this direction. It does 
offer the possibility of using considerably 
larger instruments than would be possible with 
conventional designs. 

The eye guards designed at Pennsylvania 
likewise yielded no very tangible improvement 
in binocular performance when tested at 
Brown, although the observers commented fa¬ 
vorably upon them. It is almost certain, how¬ 
ever, that in this case the increased comfort 


and lessened fatigue that this device is certain 
to provide warrant the adoption of this acces¬ 
sory or its equivalent. 

It was stated in the final report from Brown 
University 2 that no single improvement of bin¬ 
ocular design taken alone contributed very 
notably to increased range of detection of tar¬ 
gets. Large exit pupils gave disappointingly 
small improvements. Even the higher magni¬ 
fications were not as valuable as might reason¬ 
ably be expected, and if too high were detri¬ 
mental to performance. Binoculars mounted on 
alidades, which presumably eliminated most of 
the angular tremor, failed to give the full ad¬ 
vantage that was expected. The improvement 
in comfort obtained by the use of eye guards 
failed to give tangible returns to better per¬ 
formance. To this may be added the finding of 
the British report 16 that even the practice of 
coating the optics with low-reflection films was 
found to yield only a slight improvement in 
visibility through the instruments. It might 
well be questioned, in each instance, whether 
the slight gains obtained were worth the trou¬ 
ble and expense. Such a conclusion, however, 
would be a serious error. The very fact that 
there are many minor details all making slight 
contributions to the net efficiency makes it all 
the more necessary to pay strict attention to 
every feature of design and manner of use that 
might possibly contribute to improved visual 
performance with binoculars. In addition to 
the features that have been discussed, this in¬ 
cludes the practice of careful focusing of the 
instruments, careful adjustment of the inter¬ 
pupillary distance, and attention to other rules 
of use that are easily overlooked. Above all, it 
calls for careful training of the observers in 
the use of their eyes at night and in the special 
skills connected with the use of binoculars to 
aid their vision. 


56 RECOMMENDATIONS BY NDRC 

1. A program of laboratory studies should 
be continued, aimed at a complete understand¬ 
ing of the various factors which are involved 
when optical aids are used to increase the range 




RECOMMENDATIONS BY NDRC 


287 


of detection for targets at night. These studies 
should include: 

a. Experiments planned to explain the un¬ 
expectedly small increase in range 
which results from increasing the exit 
pupil diameter above 6 mm and the 
failure to find a reduction in the “ex¬ 
pected range” for binoculars when the 
exit pupil nearly matches the pupil of 
the eye. Explanation of this phenom¬ 
enon will undoubtedly throw important 
light on the whole situation relating to 
the night use of binoculars. 

b. Studies of the effect of increasing the 
angular field of view, both with 
“dummy” unity-power binoculars and 
with ordinary 7x50 binoculars, on abil¬ 
ity to detect targets in unknown loca¬ 
tions. This study should be made for 
targets whose location is unknown both 
in one and in two coordinates (corre¬ 
sponding to horizon and sky scanning). 
Special laboratory equipment will be 
required, perhaps based on the use of 
large mirrors mounted on axes of ro¬ 
tation. Tests on ships should also be 
made. 

c. Measurements of clipping and of an¬ 
gular tremor should be made on a num¬ 
ber of different observers and on sev¬ 
eral standard binoculars. 

d. The effect of adding extra weight on 
arms attached to binoculars, in order to 
increase moment of inertia, with pro¬ 
vision for relieving the observer from 
carrying this weight, should be investi¬ 
gated. The effect on angular tremor 
and on range of detection should be 
studied. 

2. A program of tests on shipboard should be 
carried out, with carefully planned targets, to 
establish the scoring of relative range with dif¬ 
ferent instruments on a quantitative basis. Con¬ 
sideration should be given to using a series of 
targets of different sizes, all at the same dis¬ 
tance, to eliminate the differential effect of 
haze. It would be extremely desirable to conduct 
these tests in a region where the air is highly 
transparent at night, perhaps in the tropics. 
Such a location would greatly increase the con¬ 


sistency of the results and would make it pos¬ 
sible to establish data on a reliable basis in a 
much shorter time than if the work were done 
under variable conditions of visibility. The 
program should include the following: 

a. Studies of the effect of increasing the 
angular field of view, both with 
“dummy” unity-power binoculars and 
with standard 7x50 binoculars, on abil¬ 
ity to detect targets in unknown loca¬ 
tions, both along the horizon and in the 
sky. This program should be closely re¬ 
lated to the corresponding program in 
the laboratory. The field can be con¬ 
trolled by adding diaphragms to wide- 
held binoculars. 

b. The effect of eye guards on binoculars 
should be determined under various 
conditions, including wind, cold, vibra¬ 
tion, roll, and pitch. It is likely that 
eye guards will prove to be more advan¬ 
tageous on shipboard than in the lab¬ 
oratory. 

c. The optimum combination of magnifi¬ 
cation and exit pupil (when the aper¬ 
ture is fixed) should be determined 
under various conditions, including 
wind, vibration, roll, and pitch. It is 
likely that the optimum combination 
will be appreciably different on ship¬ 
board from the result (10x50) estab¬ 
lished in the laboratory. 

d. The effect of fatigue on the best com¬ 
promise between magnification and exit 
pupil should be investigated by com¬ 
paring limiting range at various inter¬ 
vals after the lookout has started to 
observe. This test will also give impor¬ 
tant information for use in establishing 
lookout schedules. 

e. The effect of adding weighted arms to 
binoculars to increase the moment of 
inertia should be tested on shipboard, 
with provision for relieving the ob¬ 
server of the added weight by means of 
a coiled spring above or below the bin¬ 
ocular. 

f. An overall test should be made to deter¬ 
mine the increase in efficiency of a look¬ 
out that results when every provision, 



288 


BINOCULARS AS AIDS TO VISION 


which individual tests have shown to 
be effective, has been made for increas¬ 
ing his performance. The test should 
probably include the use of 10x50 bin¬ 
oculars, eye guards, weighted arms at¬ 
tached to the binoculars, vibration¬ 
absorbing elbow rests, and a shelter 
from the wind closed on three sides, 
with an opening in the front just large 
enough to cover the assigned sector 
with a reasonable margin. 

3. Lookouts should be trained in the best use 


of binoculars, on the basis of the findings which 
result from the laboratory and shipboard tests. 
Particular attention should be given to direct¬ 
ing the eye along the axis of the instrument, 
while the attention is directed peripherally, if 
this is found to be the best procedure. 

4. The extent to which emphasis should be 
put on the further development of wide-held 
binoculars should be determined as a result of 
tests made in the laboratory and on shipboard 
with instruments having fields of various 
diameters. 



Chapter 6 

HARMONIZATION OF B-29 GUNS AND SIGHTS 

By Theodore Dunham, Jr. a 


I N THE B-29 aircraft the guns are located at 
considerable distances from the sights and 
are operated by servo control. When the com¬ 
puter is not operating, the aim in directions of 
guns and sights must be held accurately paral¬ 
lel to one another at all settings of azimuth and 
elevation. This requires that the azimuth axes 
of guns and sights be parallel and also that the 
elevation axes be perpendicular to their respec¬ 
tive azimuth axes. 

At the modification centers, harmonization 
was carried out by using plane mirrors set up 
parallel to one another opposite each station, 
so that the guns and sights could be lined up 
optically without difficulty and with little ex¬ 
penditure of time. Aircraft ordinarily left the 
United States with the guns and sights har¬ 
monized, but the adjustment was often seri¬ 
ously disturbed later as a result of combat ac¬ 
tivities and rough landings. Moreover, it was 
frequently necessary to replace turrets and 
sights which required harmonization before 
they could be used. Accordingly, there was ur¬ 
gent need for a method of harmonization which 
could be used in the field. 

The middle distance yard method is usually 
employed for field harmonization. This method 
is quite satisfactory from the point of view of 
speed and accuracy, but requires an open area 
extending at least 500 ft from the bomber in 
four directions. In the Marianas, where the 
need for harmonizing aircraft was acute, it was 
not possible to provide sufficient space for this 
method. The aircraft were located on narrow 
hardstands close to one another for servicing 
between raids and there were often sharp de¬ 
clivities on both sides. Under these conditions 
middle distance yards could not be employed. 
Several such yards were established in the 
Marianas at special locations, but it was im¬ 
practical to tow the aircraft from their hard¬ 
stands to these locations solely for the purpose 
of harmonization, since to do so would seriously 
a Chief, Section 16 . 1 , NDRC. 


interrupt servicing activities and would in¬ 
crease unduly the inactive period between 
raids. 

In view of this situation, the Army Air 
Forces requested NDRC to develop a field 
method of harmonization which could be em¬ 
ployed in a limited space with portable equip¬ 
ment. The project was assigned to the Applied 
Mathematics Panel [AMP] late in 1944, as 
part of Project AC-92, which covered the gen¬ 
eral B-29 fire-control problem. Section 16.1 co¬ 
operated with AMP in devising plans and pro¬ 
viding facilities for this program. However, it 
soon became clear that the problem involved 
primarily the development and testing of spe¬ 
cial optical equipment. Accordingly, Project 
AC-127 was established and assigned to Sec¬ 
tion 16.1 in March 1945. 

The following were agreed upon by all con¬ 
cerned as satisfactory requirements for a field 
method of harmonization: 

1. Overall accuracy should be at least 2 mils, 
preferably 1 mil. 

2. The equipment should be simple enough to 
be operated by regular armorers with limited 
additional training. 

3. The device should make it possible to har¬ 
monize an airplane standing at dispersal points 
without necessarily placing it on jacks. 

4. Space requirements around the fuselage 
should not exceed what is available at stations 
in the Marianas. This is extremely limited, 
since the terrain frequently pitches off steeply 
at the very edge of the hardstands. At the most, 
not over 100 ft from the aircraft should be re¬ 
quired for operation. 

5. The time required for harmonization 
should not be unduly long, preferably not more 
than 3 or 4 hours. 

6. The equipment should be suitable for use 
by day or by night. 

7. The equipment should not be unduly 
heavy, bulky, or expensive, and should not be 
unreasonably difficult to produce. 


289 



290 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


The importance of the problem appeared to 
justify parallel efforts by more than one group, 
followed by comparative field tests of all prom¬ 
ising prototype equipment at the earliest pos¬ 
sible date. The University of Rochester 1 and 
the Massachusetts Institute of Technology 
[MIT] 2 undertook the development of the 
prism method and the wire method, respec¬ 
tively. Merrill Flood and Associates [MFA] 
developed the mirror boresight method 3 for 
harmonizing certain combinations of guns and 
sights and developed an overall procedure (the 
MFA method) 4 for harmonization which em¬ 
ploys both the wire method and the mirror 
boresight method, each for those combinations 
for which it is best adapted. MFA also cooper¬ 
ated in the final testing and evaluation of the 
various methods. Harvard University devel¬ 
oped the mirror frame method . 5 

61 THE PRISM METHOD 

6.1.1 Principle of the Method 

The prism method provides means for es¬ 
tablishing two portable targets, much closer 
than the middle distance yard targets, so lo¬ 
cated that when the gun and sight are aimed 
at these targets they will be parallel. The tar¬ 
gets are set up separately for each of the ten 
gun and sight combinations which must be har¬ 
monized. The settings of selsyns and the elec¬ 
trical adjustments are made according to the 
directions given in the Technical Order [TO] 6 
for middle distance yard harmonization. 

The method is based on the principle of lo¬ 
cating optically two lines, in a plane containing 
both the gun and the sight, which are parallel 
to one another, by making equal the two angles 
formed by the intersection of these lines with 
a third line. Two identical 6-degree double¬ 
image prisms are used to set these two angles 
equal. Figure 1 shows the arrangement of the 
various elements for harmonizing a gun and 
sight. 

6.1.2 Equipment 

The field prism is a double-image prism con¬ 
sisting of two pieces of glass cemented together 


with a partially reflecting, partially transmit¬ 
ting interface. The prism transmits one beam 
undeviated and offsets the other by 6 degrees. 
This angle is accurately established by the con¬ 
trol of the fine grinding and polishing of the 



Figure 1. Optical layout of prism method. 


parts of the prism, and is later verified after 
cementing. The prism is mounted in a swivel 
head on an M-5 Army tripod (see Figure 2) 
with provision for rotating it around the un¬ 
deviated beam. A cross-sight slit and wire are 



Figure 2. Field prisms. 


mounted on the front face of the prism unit, 
aiming across the line of sight in the plane de¬ 
fined by the deviated and undeviated beams. A 
cross-sight arm is attached at one side to aid 
the operator at the sight prism in estimating 




THE PRISM METHOD 


291 




ures 5 and 6). The sight prism has no cross 
sight. 

The field target is a white ball IV 2 in. in di¬ 
ameter, partially surrounded by a black shield, 


Figure 4. Sight prism and adapter installed on 
pedestal sight. 

and mounted on a brass tube which slides 
through the top of an M-5 tripod to permit ad¬ 
justment in height. Four 24-in. sections of tub¬ 
ing are provided, so that this target can be 
located up to 8 ft above the tripod head. 


Figure 6. Sight prism and adapter installed on 
ringsight. 

A gun target and boresight telescope are 
attached to the gun which is to be harmonized. 
The gun target is a 6-in. white disk which slips 
over the end of the gun barrel. A standard 
boresighting telescope is set in the muzzle and 
is used to sight the gun at the field prism. 


Figure 3. Sight prism and adapter for pedestal 
sight. 

The sight prism and its mount are identical 
with the field prism. An adapter (see Figure 8) 
is provided to fit the front of the pedestal sight. 


Figure 5. Sight prism and adapter for ring- 
sight. 

Figure 4 shows the sight prism on the pedestal 
sight with the Galilean telescope in place. A 
different adapter is used for the ringsight (Fig- 


the plane defined by the two beams. A 2.5X 
Galilean telescope may be attached to the prism 
unit when desired to facilitate settings and to 
increase accuracy. 




292 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


61,3 Procedure 

A crew of four men should be available, if 
possible, so that all stations can be manned 
simultaneously. Three men can do the work, 
however, since the target can be adjusted by 
the field prism operator if necessary. 

The double-image field prism is located on its 
tripod at such a distance from and at such 
orientation to the aircraft that the gun and 
sight are seen superposed. This means that the 
gun and sight subtend an angle of 6 degrees 



OF SIGHT PRISM 

Figure 7. Setting rotation of sight prism so 
that images of cross-sight arm on field prism are 
in line. 

as seen from the field prism. The gun is aimed 
at the field prism. The sight prism is placed on 
the sight, and the latter is aimed, using the 
deviated beam of the sight prism, at the field 
prism. The sight prism is rotated until the 
plane defined by its two beams agrees as closely 
as possible with the plane defined by the two 
beams of the field prism. The correct orienta¬ 
tion is judged by making the deviated image of 
the cross-sight arm on the field prism line up 
with the direct image of the cross-sight arm, 
as in Figure 7. The target is used to make this 
setting more accurately. It is located at a dis¬ 
tance from the field prism approximately equal 
to the distance of the gun from the sight, on the 
side of the sight, and is placed as nearly as pos¬ 
sible along the line of the cross-sight arm. The 
field prism operator then directs the target 
operator to place the target more exactly in the 
cross-sight plane by sighting through the cross 


sight. The sight prism operator verifies 
whether the undeviated image of the target lies 
somewhere on the cross-sight arm. If it does 
not, the sight prism operator rotates his prism 
until the cross-sight arm on the field prism ap¬ 
pears to aim directly at the target. He then 
aims the deviated reticle dot directly at the 
center aperture of the field prism, using the 
Galilean telescope as a check. If the target does 
not lie somewhere on the cross-sight arm, he 
directs the target to move until it is perpen¬ 
dicular to some part of the arm, after which the 
field prism operator directs the target to move 
back into the cross-sight plane (see Figure 8). 
This should place the direct image of the target 
somewhere within the length of the deviated 
image of the cross-sight arm, as seen by the 
sight prism operator. If so, he makes a final 
setting of the rotation of the sight prism to 
bring the target to the mid-line of the cross¬ 
sight arm. When this is done, and the reticle 
dot is still aimed at the central opening of the 
field prism, the gun and sight are parallel, just 
as they would be if they were sighted on middle 
distance yard targets. Selsyn signals can be 



Figure 8. Setting undeviated image of target 
on deviated image of cross-sight arm. 

read and electrical adjustments made in ac¬ 
cordance with directions in the TO. 6 

The above description is an abbreviated ver¬ 
sion of the full directions for setting up the two 
double-image prisms, the target, and the gun. 
Full instructions for carrying out the various 





THE PRISM METHOD 


293 


steps in a logical sequence are given in the Uni¬ 
versity of Rochester report. la The procedure is 
actually much simpler than it appears to be 
from the written instructions. With flashlights 
it can be carried out almost as easily at night 
as by day. 


ways directions. A distance of 250 ft, without 
much drop below level, is required 30 degrees 
on one side of the tail, but only 30 ft is required 
directly behind the tail. If a 12-degree prism 
were used instead of a 6-degree prism, the dis¬ 
tances would be reduced by one-half, but diffi- 


Table 1. Recommended positioning of field prism for each sight to gun combination. 


Turret and sight 

Height of field 
prism above ground 
(feet) 

Angle at sighting 
station from field 
prism to nose of 
ship (degrees) 

Distance 
from sight¬ 
ing station 
to field prism 
(feet) 

Height of target 
above ground 
(feet) 

LAT—RS* (Figure 9) 

4 

120 

170 

7 

RS—LFT* (Figure 10) 

3 

45 

300 


LFT—NS (Figure 11) 

2 

45 to left of nose 

100 

8 

ALTERNATE 

2 

100 to left of nose 

160 

8 

NS—UFTf (Figure 12) 

10 

45 to left of nose 

100 

7 

ALTERNATE 

6 

100 to left of nose 

160 

3 

UFT—RING SJ (Figure 13) 

6 

150 to left or right 
of nose 

250 

5 (or about 1 ft 
below field prism) 

RING S—UAT§ (Figure 14) 

LS—LAT Same as LAT—RS 

About 4 % ft above 
cabin of plane 

0 

10 

No target used in 
this position 

LS—TM (Figure 15) 

5 

160 to left of nose 

125 

3 

RS—TM 

5 

160 to right of nose 

125 

3 

TM—TS (Figure 16) 

4% 

180 from nose 

(straight back 
from tail) 

30 

7% 


* A 12° prism would greatly reduce distance from field prism to sighting station, 
t Field prism should be as high as possible. 

t In this position the field prism should be located as high as possible in order to view through good Plexiglas in right sight. Positions 
more toward the tail of the plane may be used, but they require a stand for the guns to be above the follower. 

§ This position is reached by standing on top of the cabin in front of ringsight. 


The recommended positions for the field 
prism for each gun to sight combination is 
shown in Table 1. The approximate locations of 
the field prism and target for each of these 
combinations are shown in the sketches in Fig¬ 
ures 9 to 16. 

The carrying case containing all of the 
equipment, except the two tripods, required for 
the prism method is shown in Figure 17. The 
total weight of the equipment, including tri¬ 
pods, is about 30 lb. 

The tripods can be placed on the ground for 
all gun to sight combinations except the ring- 
sight and upper aft turret, for which the field 
prism is mounted on top of the aircraft. It is 
necessary that the ground should not fall off 
much below the level of the aircraft for a dis¬ 
tance of about 200 ft in the forward and side- 



Figure 9. Lower aft turret to right blister sight. 

culties would be encountered in fitting a prism 
with the wider angle to the sight. 
















294 


HARMONIZATION OF B-29 GUNS AND SIGHTS 



Figure 10. Right blister sight to lower forward 
turret. 




Figure 12. Nosesight to upper forward turret. 



Figure 13. Upper forward turret to ringsight. 



Figure 14. Ringsight to upper aft turret. 



Figure 15. Left blister sight to tail mount. 























THE WIRE METHOD, MARKS I AND II 


295 


Tests of the Prism Method 

Tests were carried out at Bedford, Mass, in 
August 1945 on B-29 aircraft. Enlisted men, 
most of whom were instructors in gunnery at 
Laredo, were instructed in this method as well 



Figure 16. Tail mount to tailsight. 

as in the wire method. After two weeks of ex¬ 
perience with both methods, they were able to 
harmonize all stations, using the prism method, 
in slightly more than 3 hours, including the 
setting of selsyns. The time required for set- 



Figure 17. Case with equipment for prism 
method. 


ting up the targets at one pair of stations aver¬ 
aged about 5 min. When successive target set¬ 
ups were made, the indicated errors in har¬ 
monization showed a mean deviation of about 1 


mil. Individual settings of the gun and sight on 
the targets in any one setup showed mean de¬ 
viations of less than half a mil. 

6,1,5 Discussion 

The prism method is entirely practical. It 
provides the necessary accuracy and the time 
required is entirely satisfactory. The equip¬ 
ment more than meets requirements on weight 
and compactness. The only obvious drawback 
to the method is the extent of relatively level 
ground which is required around the aircraft. 

Three sets of equipment for the prism 
method were made up in July 1945, and were 
shipped by air to the Marianas in August with 
the twelve men who had been trained in its use. 


6 2 THE WIRE METHOD, MARKS I AND II 

6,2,1 Principle of the Method 

The wire method is based on establishing 
two lines parallel to each other by relating 
them in azimuth to different parts of a wire 
under tension and in elevation to a level bubble 
(see Figure 18). By using this principle it is 
possible to set two telescopes parallel, one in 
front of a sight and one in front of a gun. The 
first allows the observer to see the central dot 
of the reticle reflected in a triple mirror, and 
to signal to an assistant who can move the 
sight until the dot falls on the cross wires of a 
reticle in the telescope. The second instrument 
looks into a mirror mounted perpendicular to a 
bore pin in the muzzle of the gun, and allows 
the observer to direct an assistant to set the 
gun so that the image of a central bead on the 
objective falls on the reticle of the telescope. 
Under these conditions the gun and sight are 
parallel, and electrical adjustments can be car¬ 
ried out in accordance with the procedure out¬ 
lined in the TO for middle distance yard har¬ 
monization. 

6 - 2,2 Equipment 

Preliminary experimental equipment which 
was used for tests at Bedford, Mass, in April 














296 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


1945 is referred to in reports as the Mark I 
equipment. Improved equipment was tested at 
Bedford in August 1945 and is referred to as 
Mark II equipment. 2 The improvements con¬ 
sisted principally in increasing the stiffness of 
supporting members and increasing the ease of 
operation, but did not involve any change in 
general principles of design. Accordingly, the 
following brief description is based on the 
Mark II equipment. 

A piano wire, 0.020 in. in diameter, and up 
to 60 ft in length, is used to define the azimuth 
of the gun and sight. It is stretched just above 
the level of the ground by means of a cylindri¬ 
cal spring which maintains a tension of about 
40 lb. The ends are attached to aluminum plat¬ 
forms with three pointed legs, weighted with 
sandbags so that they will hold on a hardstand 
(see Figure 19). 

One-half of a standard 6x30 (Ordnance 
M13A1) binocular is used for the sighting tele¬ 
scope, while the other half is used to make the 
setting perpendicular to the wire (see Figure 
20). This azimuth-level instrument is equipped 
with a level bubble. A double-image prism (see 
Figure 21) in front of the right-hand objec¬ 
tive brings into the field of view two sections 
of the wire, approximately 7 degrees to the 
right and left of vertical. A 14-degree prism is 
used, with a 28-degree prism cemented to its 
middle third. Auxiliary lenses are moved into 
position in front of the objective to make it 
possible to focus on the wire. Cross wires are 
installed in the focal plane of the left-hand tele¬ 
scope, and a bead is mounted at the center of its 
objective. A battery and miniature lamp is pro¬ 
vided for illuminating the bead at night. The 
instrument is mounted on a standard survey¬ 
or's leveling head provided with an azimuth 
clamp and slow motion tangent screw. A pair 
of identical instruments is adjusted so that the 
lines of sight of each have the same azimuth 
and elevation when set on the same wire and 
leveled. 

It is necessary to provide means for locating 
one instrument in front of the gun and one in 
front of the sight, with an accuracy of about 
half an inch in the case of the sight, so that 
aberrations will be reduced by using the central 
part of the aperture of the sight. This requires 


a convenient mechanism for adjusting the posi¬ 
tion of the instrument, both horizontally and 
vertically. 

The instrument is mounted on a base which 
allows vertical adjustment with a coarse-pitch 
screw and nut, and which slides on a pair of 
polished stainless steel tubes for horizontal 
adjustment (see Figure 22). The whole assem¬ 
bly is mounted on a tubular mast carried on a 
fork-lift truck or “stacker,” as shown in Figure 
23. Two fork lifts would be desirable for work¬ 
ing on the top stations. If only one is available, 
the sight must be set and left undisturbed 
while the gun is adjusted. A low mast with a 
tripod base is used for the lower turrets, where 
two fork lifts would interfere with one an¬ 
other. 

A triple mirror is mounted behind the sight 
to reflect, through an angle of exactly 180 de¬ 
grees, the collimated image of the sight reticle, 
so that it can be observed with the telescope. 
The tip of the glass pyramid is ground off and 
polished so that an observer can aim the sight 
while it is in place. 

A plane mirror is mounted perpendicular to 
a boresight mandrel which is set in a collet in 
the muzzle of the gun. Rays normal to the mir¬ 
ror are then parallel to the bore. 

62 3 Procedure 

Four men constitute the most effective crew, 
but the work can be done by three men. The 
wire is stretched on the ground near the air¬ 
craft, extending somewhat beyond each of the 
two stations which are to be harmonized. An 
instrument is located in front of each station, 
and is adjusted over the wire. Each instrument 
is leveled and its azimuth is adjusted until a 
single image of the wire is seen in the right- 
hand telescope. The two instruments are now 
parallel. The triple mirror is placed behind the 
sight and the operator aims the sight at the ob¬ 
jective of the left-hand telescope. The observer 
can then see the dot of the sight reticle in the 
field of view. He directs the sight operator to 
turn the sight until the dot is centered on the 
reticle of the harmonizing instrument. At the 
same time, the boresight mirror is placed in the 
muzzle of the gun, and the gun is directed to- 



THE WIRE METHOD, MARKS I AND II 


297 


ward its instrument by gentle tapping until the 
operator of the instrument sees the bead on the 
objective centered on the recticle (see Figure 
24). If the two instruments have been previ¬ 
ously adjusted to agree in azimuth and in ele¬ 
vation, the gun and sight will now be parallel, 
and the usual electrical adjustments described 


ter tests, the same group of gunnery instruc¬ 
tors which worked on the prism method 
learned the wire method and made it possible 
to evaluate, at least in a preliminary way, the 
effectiveness of the method. 

The overall time required by three men to 
harmonize the airplane completely was usually 



in the TO can be carried out to complete the 
harmonization procedure. The MIT report 2a de¬ 
scribes the details of the various steps that 
should be taken in order to arrive at the final 
settings of the two instruments, the sight and 
the gun, in the minimum of time. 


6 * 2 * 4 Tests of the Wire Method 

Tests at Bedford were conducted in April 
1945 with Mark I equipment and in August 
1945 with Mark II equipment. During the lat¬ 


a little more than 4 hrs. The best overall time 
was 231 min, of which 131 min were spent on 
optical steps, 75 min on electrical adjustments, 
and 25 min on miscellaneous activities (wait¬ 
ing for stations to be free, etc.) not immedi¬ 
ately related to harmonization. 

The accuracy of the method was measured 
by making a number of separate settings of the 
instruments, after each of which five settings 
of the gun and sight were made on the instru¬ 
ments. The results showed that the mean devi¬ 
ations of indicated errors of harmonization 
were about 0.6 mil for successive settings of 













298 


HARMONIZATION OF B-29 GUNS AND SIGHTS 



Figure 19. Weight and spring at end of wire. 


PRISM HOUSING AZIMUTH ADJUSTMENT 



BE AO 


(FOR DAY) 


LAMP 


(FOR NIGHT) 


POLARIZED 
PLUG IN 
RECEPTACLE 
OF CELL 
HOUSING 

SCREW 
FOR HOLDING 
DRY-CELL 
HOUSING 


LENS FOR NEAR-VIEW AZIMUTH TANGENT SCREW 
OF WIRE 


Figure 20. Azimuth-level instrument. 



Figure 22. Support for azimuth-level instru¬ 
ment. 



Figure 23. Azimuth-level instrument on fork¬ 
lift truck. 





























THE WIRE METHOD, MARK III 


299 


the two instruments. Individual settings of the 
gun and sight on the instruments showed mean 
deviations of only about 0.2 mil. 


Discussion 


The wire method appears to meet all require¬ 
ments of a satisfactory method for field har¬ 
monization, as to accuracy, speed, and space re- 



Figure 24. Adjusting gun with boresight mirror. 


quired. The weight of the equipment is greater 
than that used with the prism method, but it is 
not excessive. Fork lifts are standard equip¬ 
ment and can undoubtedly be made available at 
any base in a reasonably short time, if they are 
not already available. 

Six sets of equipment were produced in July 
and sent to the Marianas in August 1945, with 
twelve men trained in its use. 

Many improvements in the equipment can 
still be made. The support for the instrument 
in the Mark II model is much better than in 
Mark I, but it should be even more rigid. A 
simple but stable tower which would support 
the instrument at any height up to 15 ft above 
the ground would be preferable to the fork lift. 


6 3 THE WIRE METHOD, MARK III 

6,81 Purpose 

The Mark III equipment was developed by 
Merrill Flood and Associates 4 while MIT was 


engaged in completing six sets of the Mark II 
equipment for overseas shipment at the earliest 
possible moment. The purpose was to make 
such changes as seemed likely to lead to greater 
speed and simplification in operation, to in¬ 
crease the accuracy if possible, and to eliminate 
the need for the use of a fork lift. 


6 ' 3 ' 2 Equipment 

The instrument was based on standard 6x30 
binoculars (Ordnance M13A1). Two changes 
were made in comparison with Mark II. The 
beam-splitting prism was replaced by a 50-de- 
gree prism (see Figure 25). The isosceles faces 
of this prism bring together two beams sepa¬ 
rated by approximately 83 degrees. It was ex¬ 
pected that this would increase the accuracy 
of setting in azimuth on the wire. The telescope 



on the left side was converted into a collimator 
with cross wires at the focus for infinity (see 
Figure 26). These were illuminated in the day¬ 
time by reflected sky light, and at night by elec¬ 
tric illumination. The triple mirror was elimi¬ 
nated, and the operator at the sight made the 





300 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


setting directly on the collimated cross wires. 

A translation device was developed which 
provided smooth motion of the instrument over 
a range of several inches laterally on rollers 
with a quick acting clamp that caused no sig¬ 
nificant rotation of the instrument (see Figure 
27). 

A high stand, in the form of a tower made 
of aluminum sections bolted together and 
mounted on wheels, was designed and con¬ 
structed to carry the translator and instrument 
(see Figure 28). The translator runs on rollers 
between tubular guides up to 15 ft above the 
ground. The front of the tower is entirely open 
so that the view is not obstructed at any level. 
Bracing at the sides, nevertheless, makes the 



Figure 26. Harmonization collimator. 

tower extremely rigid. Two translators, each 
carrying an instrument, can be mounted on the 
guides at the same time. This is convenient 
when harmonizing two stations, one of which 
is almost directly above the other. The tower 
is hinged on its base, so that it can be lowered 
to pass under the wing of the B-29. There are 
four wheels, arranged to run sideways along 
the wire. A yellow stripe on the base shows 
approximately the proper location with respect 
to the wire. Four screw jacks take the weight 
off the wheels and level the stand when it is in 
the proper position. Level bubbles are mounted 


on the base for this purpose. The observer 
climbs to the required height with one foot 
on each of two diagonal ladders, resting his 
back against a flat vertical board placed at a 
convenient distance behind the ladders. This 
position of the observer has been found to be 
entirely comfortable for observing. It seems 
likely that a tower of this general type offers a 
promising solution to the problem of locating 
the observer and his instrument securely at 
any height up to that of the highest stations on 
the B-29, without special machinery such as is 
involved when a fork lift is used. 

A low stand, allowing the translator to as¬ 
sume any position up to about 7 ft above the 
ground, was also developed (see Figure 29) for 
use with the lower turrets. 

Four wires are used, varying from 50 to 90 
ft in length. A reel case (see Figure 30) has 
been developed to carry six wires (three 90 ft 
long and three 50 ft long) in such a way that 
they can be conveniently pulled out and re¬ 
wound without becoming snarled. Errors due 
to wind are not serious if the wind velocity 
does not exceed 10 mph on the ground. 

The equipment for the wire method Mark III 
was delivered to the Armament Laboratory at 
Wright Field in November 1945. 


633 Procedure 

Five men carry out the harmonization pro¬ 
cedure. They are divided into two crews of two 
men each and a crew chief. Two high stands 
are used. One of these is provided with two 
translators and two collimators; the other with 
one of each. One low stand with a translator 
and collimator is required. Two low-power 
Galilean telescopes are provided to facilitate 
sighting. Two standard J-2 boresights and the 
usual harmonizing tools are required. 

The wire method Mark III is not intended 
to be used in harmonizing the upper two tur¬ 
rets to the ringsight, since tests have shown 
that this can be done very effectively with the 
mirror boresight method (see Section 6.4). 

A detailed procedure for carrying out the 
harmonization of a B-29 airplane in the mini¬ 
mum of time has been worked out and is de- 




THE WIRE METHOD, MARK III 


301 


scribed in the MFA report. 4 *" 1 The locations of 
the wires and stands for the successive steps is 
shown in a diagram in this report. 41 ' These de¬ 
tails will not be reproduced here since they are 


of the wire method, Mark II, a number of 
points need to be changed. 

The overall accuracy of optical harmoniza¬ 
tion between two stations was about 1.0 mil in 



Figure 27. Harmonization collimator on translator. 


not necessary in order to understand the gen¬ 
eral features of the method. 

6 . 3.4 ^egts t j ie Wire Method, Mark III 

The equipment was tested at Bedford in 
October 1945 on a B-29 aircraft with a crew of 
five enlisted men who had had previous ex¬ 
perience in gunnery, but not in harmonization. 
Time was extremely limited, so that it was not 
possible to carry out the full program that had 
been planned. It was demonstrated, however, 
that the equipment was generally satisfactory. 
While in many respects it was superior to that 


azimuth and 0.5 mil in elevation. The accuracy 
of harmonizing two stations on the middle dis¬ 
tance yards appears to be about the same. The 
mean deviations of individual settings in azi¬ 
muth on the wire with the 50-degree prism 
were less than 0.2 mil. 


6 * 3 ' 5 Discussion 

The wire method, Mark III, represents a 
definite advance in the development of har¬ 
monization equipment as compared with 
earlier equipment for the wire method. Tests 







302 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


have shown that if the use of a fork lift is not 
regarded as feasible and satisfactory, a tower 
can be designed to locate the observer and his 
instrument at the required places. It seems 
likely that the final solution lies in this gen¬ 
eral direction whenever irregularity of terrain 
makes the use of the prism method impractical. 
Much more comprehensive tests are needed to 
provide a basis for making further changes in 
the Mark III equipment. Those changes that 



Figure 28. High stand. 


seem indicated on the basis of present experi¬ 
ence are outlined in the MFA report. 40 


6 4 THE MIRROR BORESIGHT METHOD 
Purpose 

The mirror boresight method was developed 
to simplify harmonization of the two upper 
turrets of the B-29 to the ringsight. These sta¬ 
tions require high stands when the wire 
method is used, and if the prism method is used 
the field prism must be located at a distance 
from the aircraft which may be impractical 
under some conditions of terrain. The mirror 
boresight method also saves much time, since 
the necessary equipment can be installed very 
quickly. Moreover, it avoids harmonizing 
through the bad part of the Plexiglas in the 
upper dome, which is inevitable when the wire 
or prism method is used. 


64,2 Equipment 

The mirror boresight is a unit consisting of 
a small flat mirror mounted accurately at right 
angles to a mandrel which fits the usual collet 
in the bore of the gun (see Figure 31). The 
mirror faces backward, toward the breech. The 
center of the mirror is offset about 8 in. from 
the bore so that when it is inserted in a gun of 
the upper forward turret it can be seen from 
the ringsight, over the top of the turret dome. 
A white target is clamped over the front aper¬ 
ture of the ringsight. This target has a hole in 
its center for sighting. Two small electric 
lamps are mounted on either side of the hole to 
aid in sighting when reflections on the outside 
of the Plexiglas dome make visibility of objects 
inside difficult. 


6 ' 4,3 Procedure 

The mirror boresight is inserted in the 
muzzle of the right-hand gun of whichever of 
the two top turrets is to be harmonized (Fig¬ 
ures 32 and 33). Control of the turret is estab¬ 
lished for the ringsight and the power is 
turned on. The gunner moves the gun until he 
sees the reticle disk reflected in the mirror. If 
the gun is elevated as much as possible, while 
the reflection of the reticle disk on the upper 
part of the mirror is still held, Plexiglas errors 
will be reduced. If there is no error of harmoni¬ 
zation, the reticle dot will be seen midway be¬ 
tween the two lamps on the target when the 
observer looks through the hole in the target. 
If this is not the case, harmonization may be 
carried out under power, or else the gun may 
be moved to the correct position with the power 
off and then the selsyns may be adjusted to 
give zero signal in this position. 

If the harmonization is badly out of adjust¬ 
ment, the reticle dot will not be seen in the mir¬ 
ror when the power is first turned on, particu¬ 
larly in the case of the upper forward turret 
which is separated from the sight by a consid¬ 
erable distance. The reflection, however, can 
usually be located easily by moving the sight 
so that the dot moves around the mirror in a 
widening spiral, with the power on. 







THE MFA METHOD 


303 


Tests of the Mirror Boresight 
Method 

Tests were carried out at Bedford in October 
1945, but it was not possible to make accurate 
studies of time and accuracy. There was every 
indication, however, that the method is ex¬ 


it the instruments for the wire method were 
used on high stands for these high stations, and 
it reduces the space which would be required 
if the prism method were used. Moreover, since 
a single setting is made by the sight directly on 
the gun without the intervention of two instru¬ 
ments, each of which must be separately set in 



Figure 29. Low stand and high stand. 


tremely fast, easy to learn, and accurate. The 
precision of a single setting appears to be con¬ 
siderably better than 0.5 mil. The only diffi¬ 
culty encountered was that the sighting lamps 
were not bright enough when full sunlight was 
falling on the dome. 


the case of the wire and prism methods (four 
settings in all), it is obvious that the overall 
accuracy should be greater. The use of a better 
part of the Plexiglas dome, as compared with 
other methods, is another significant advantage 
of the mirror boresight method. 


645 Discussion 

There was general agreement that the mir¬ 
ror boresight method is the method of choice 
for harmonizing the upper turrets. It saves 
much time and effort which would be necessary 


65 THE MFA METHOD 

Following a study of the prism method and 
the wire method, Mark I, Merrill Flood and 
Associates developed the wire method, Mark 

















304 



HARMONIZATION OF B-29 GUNS AND SIGHTS 


III, and the mirror boresight method. The most 
effective overall method was then studied, 
using the method that seemed most satisfac- 


Figure 30. Reel case. 

tory for each gun to sight combination. This 
overall method has been referred to as the 
“MFA Method.” It employs the wire method, 


Figure 32. Inserting mirror boresight in gun. 

tion with these two methods. The details of 
procedure are outlined in the MFA report. 4a 


6 6 MIRROR FRAME FOR HARMONIZATION 
6,6,1 Purpose 

The mirror frame method for harmonization 
is based on the use of a group of plane mirrors, 
mounted on a rigid frame and adjusted parallel 
to one another, each mirror being located on 
the frame so that, when the frame is oriented 
in a specified way, it will stand opposite one of 
the stations. In view of the fact that such a 
mirror frame eliminates the need for all tar¬ 
gets and other equipment, it seemed worth 


Figure 31. Mirror boresight and target. 

Mark III, for eight of the ten combinations, 
and employs the mirror boresight method for 
harmonizing the ringsight to the upper for¬ 


ward turret and for harmonizing the upper aft 
turret to the ringsight. 

The equipment and the individual steps are 
the same as those already described in connec- 






MIRROR FRAME FOR HARMONIZATION 


305 


while to investigate its practicability. This was 
undertaken at Harvard. 5 The principal prob¬ 
lems were the size of the frame required and 
the question whether the adjustment of the 
mirrors could be maintained in the presence of 
temperature gradients in the frame. 


662 Equipment 

Careful consideration was given to selecting 
the most favorable horizontal and vertical 
angles for aiming the guns and sights so that 
the overall dimensions of the mirror frame 
would be as small as possible. The arrangement 
finally adopted is shown in Figure 34. 

In position No. 1 the guns and sights are 
aimed 6 degrees to the right of the axis of the 
aircraft and 2 degrees above the axis. Five 
stations can be harmonized with the mirror 
frame in this position, with an overall width 
of 122 in. and an overall height of 125 in. be- 



Figure 33. Use of mirror boresight for harmon¬ 
izing upper aft turret to ringsight. 


tween extremes of the various lines of sight. A 
periscope with a 24-in. offset must be used to 
raise the line of sight from the right blister 
sight over the right wing. 

In position No. 2, behind the tail, six sta¬ 
tions aim 6 degrees to the left and 2 degrees 
above the axis, and their lines of sight are in¬ 
cluded within a range of 102 in. in width and 
103 in. in height. The cover of the upper aft 
turret must be removed to clear the line of 
sight. 


Figure 37 shows the arrangements of the 
mirrors on the frame. The station covered by 
each mirror is indicated. The mirrors are 12 
in. in diameter, except for two mirrors which 
are 20 in. in diameter. One of these covers two 



stations and the other covers three stations 
whose lines of sight are close together. The 
mirrors are selected plate glass, mounted on a 
welded steel frame with provision for adjust¬ 
ing the plane of each mirror. The frame is 
divided into two parts which are hinged on one 
another for passing under the wing of the B-29 
and other obstructions. The frame is mounted 
at three points on a standard lVa-ton Army 
truck with stake body. 

663 Procedure 

The mirror frame is first located in position 
No. 1 in front of the aircraft (see Figure 38) 
at an angle of 6 degrees to the axis of the fuse¬ 
lage, with the top tilted 2 degrees toward the 
airplane, so that normals to the mirrors (lines 
of sight) were inclined 2 degrees above the 
axis, when looking forward. It was not par¬ 
ticularly difficult to make the necessary set¬ 
ting. The truck driver was guided over the in- 





























306 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


tercom by an observer at the nose and by an¬ 
other at the upper sight. These men observed 
their own reflections in the appropriate mir¬ 
rors and gave instructions for adjusting the 
azimuth of the truck until the parallax between 



Figure 35. Periscope for right blister sight. 

the two stations was equal to the spacing of the 
mirrors, so that they could both see their reflec¬ 
tions at the same time. It then remained to 
adjust the tilt of the mirror toward or away 
from the aircraft. This was done with a jack 
working against the spring on the far side of 
the truck. The periscope for the right blister 
sight was attached to the side of the fuselage 
as shown in Figure 36. After harmonizing the 
five forward stations, the same mirror frame 
was taken to the rear of the aircraft and was 
located in position No. 2, as shown in Figure 39. 

The mirrors were adjusted parallel to one 
another by using a pattern marked on the side 
of a building about 200 ft distant which repre¬ 
sented in chalk the location of each mirror on a 
1/1 scale. The observer placed his eye over each 
mark successively. If the corresponding mirror 
was correctly located, he would see his eye 
centered in the mirror. If not, an assistant 
would adjust the mirror in accordance with his 
instructions. 


6 6,4 Tests of the Mirror Frame Method 

The general procedure for lining up the mir¬ 
ror frame on a truck was tried at Bedford in 
October 1945. Time did not permit any actual 
tests of harmonization. It was shown, however, 
that no serious difficulties were involved in set¬ 
ting the frame in the proper position, and that 
when this was done it would be a simple mat¬ 
ter to carry out actual harmonization with 
targets at the sights and guns, in the same way 
as at the modification centers where mirrors 
are now mounted on the walls of the hangars. 

Preliminary tests were made to determine 
the permanence of adjustment of the mirrors 
under outdoor conditions, over a period of sev- 



Figure 36. Method of attaching periscope to 
fuselage. 


eral days, using the marks on the wall which 
were first used to adjust the mirrors. Indica¬ 
tions were that variations were less than a mil. 


6,60 Discussion 

The mirror frame method appears to be 
practical. It would be well worth while to study 
it further and to carry out experiments to de¬ 
termine variations in setting under various 
weather conditions. 



$ 



















PRESENT STATUS OF THE HARMONIZATION PROBLEM 


307 


A convenient way to set the mirrors and to 
observe variations in setting would be to ob¬ 
serve, through a hole in a wall, a 2/1 scale pat¬ 
tern of the mirrors, reflected in the mirrors 
when they are located at any distance in excess 
of about 200 ft. Every mark should be centered 
in the appropriate mirror. A low-power tele¬ 
scope would be entirely adequate for making 
readings to about 0.1 mil if a scale were pro¬ 
vided at each mark. 

The azimuth of the truck carrying the mir¬ 
ror frame could be adjusted much more quickly 
if a reflex sight were mounted where the driver 
could watch, by reference to a selected mark on 
the landscape, the effect of turning the wheels 
and running slowly forward and backward as 
in parking. This would prevent him from over¬ 
shooting the desired setting, which is all too 
easy to do with a truck which responds quickly 
to the wheel. A still better plan would be to pro¬ 
vide for rotating the frame through several de- 


UFT-1 US-2 US-1 



Figure 37. Mirror frame for harmonization. 

grees on the bed of the truck, and also to pro¬ 
vide for adjusting the level of the frame on the 
truck so that the jack could be dispensed with. 

It should be noted that the grouping of mir¬ 
rors suggested in Figure 34 and provided on 


the frame shown in Figure 37 ties the forward 
stations to the after stations only through the 
ringsight. This means, for example, that the 
right blister sight and the lower aft turret are 
tied together indirectly through the ringsight. 



Figure 38. Mirror frame in forward position. 


However, since each can be compared directly 
with the ringsight (not through the usual long 
chain of stations), this may be an entirely 
satisfactory arrangement. It would, of course, 
be possible to add two additional mirrors and 
to locate the mirror frame in a third position 
on the right side at the rear of the aircraft, so 
as to tie the right blister sight and lower aft 
turret together directly. It would probably not 
be possible to include the upper aft turret in 
the forward group of mirrors, or the upper for¬ 
ward turret in the after group, since the con¬ 
tour followers on the turrets would probably 
raise the guns top high at the required orienta¬ 
tions. In any case, it seems unlikely that it will 
be necessary to provide further connections be¬ 
tween the forward and after groups. 

6.7 PRESENT STATUS OF THE 

HARMONIZATION PROBLEM 

The work carried out under Project AC-127 
shows clearly that it is practical to provide op¬ 
tical equipment capable of harmonizing B-29 
aircraft with the necessary accuracy in the 










308 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


field, in space considerably smaller than that 
required by the middle distance yard method. 

The prism method appears to be the simplest 
method to learn, and it unquestionably involves 
lighter and simpler equipment than the wire 
method. It meets all requirements except when 
approximately level terrain to a distance of 
about 200 ft from the aircraft is not available, 
as is the case at many of the hardstands in the 
Marianas. The use of a 12-degree prism would 
improve matters, but would not solve this dif¬ 
ficulty if there are declivities within less than 
100 ft of the fuselage. 

The wire method meets all requirements, in¬ 
cluding that of limited available space, but it 
involves heavier and somewhat more complex 
equipment than the prism method. There is 



Figure 39. Mirror frame in aft position. 

little to choose as regards time and accuracy 
between the two methods. Either is entirely 
adequate in both respects. 

The mirror boresight method is strikingly 
satisfactory for the two upper combinations, 
and should be used whenever possible for these 
stations. In conjunction with the wire method, 
it constitutes the MFA method. If used in con¬ 
junction with the prism method, it would re¬ 
move the requirement for relatively level 
ground as far as 250 ft from the aircraft at 
bearing 150 or 210 degrees, when a 6-degree 
prism is used. 

The mirror frame method has not been 
worked out as fully as the other three methods, 
but its inherent simplicity and freedom from 
the need for critical adjustment of small in¬ 


strumental parts suggests the desirability of 
developing and testing it far enough to reach 
a definite conclusion as to its usefulness. 

The present quality of Plexiglas introduces 
errors larger than the desired accuracy of the 
harmonization equipment. Every effort should 
be made to improve the quality of the Plexiglas, 
particularly in the dome for the ringsight. The 
Plexiglas should be removed if further defini¬ 
tive tests of harmonization equipment are to be 
carried out. 

All the factors which disturb the harmoniza¬ 
tion of aircraft over a period of time are not 
clear. It is obvious that combat damage and 
rough landings will throw the level of various 
stations out of adjustment, and possibly even 
rotate them, so that releveling will not neces¬ 
sarily restore the harmonization. But there is 
little if any information to show whether there 
is a gradual warping of the fuselage which is 
sufficient to require periodic harmonization in 
the absence of accidents. The circular tracks on 
which the guns and the ringsight turn are not 
as rigid as they should be if harmonization is 
to be maintained to 1 mil. 

It seems likely that one of the principal 
causes of the development of errors in the har¬ 
monization over a period of time in the field is 
gradual unbalancing of the servo-amplifier. It 
would seem logical to check the balance of each 
amplifier before undertaking optical harmoni¬ 
zation, and then to verify the harmonization to 
see whether any changes in the setting of the 
selsyns are actually needed. It seems most un¬ 
likely that if the selsyns are set up tight they 
will rotate spontaneously, and so changes in 
harmonization should be sought elsewhere and 
corrected in other ways whenever possible. 
When turrets or sights are replaced, optical 
harmonization is of course required, but in 
most other cases it seems unlikely that it will be 
necessary to change the selsyn settings if dis¬ 
turbances in leveling of guns and sights are 
corrected, unless the fuselage bends over a 
period of time. 

A question of great importance is the extent 
to which harmonization of a plane in flight 
differs from that of an airplane on the ground, 
resting on its landing gear. The difference is 
probably much greater than the 1 mil accuracy 



RECOMMENDATIONS BY NDRC 


309 


requested for the harmonization procedure. It 
is of paramount importance that the difference 
be measured so that a systematic correction 
may be applied to harmonization carried out on 
the ground. Harmonization in the air could 
easily be checked by installing gun cameras 
with moderately long-focus lenses in the tur¬ 
rets tracking the sun or moon at various bear- 
ings and making several exposures at instants 
when the reticle dot of the sight is centered on 
the sun or moon. By averaging the measured 
positions of the images on the resulting films, 
and comparing them with similar measures 
made on middle distance yard targets when the 
aircraft is on the ground, it should be possible 
to determine the change in harmonization in 
flight for each station and for various bearings 
of the line of sight. 

The effect of temperature and wind on har¬ 
monization due to bending of the fuselage 
should be determined by observing middle dis¬ 
tance yard targets under varying conditions. 
Limited observations at Bedford have shown 
that when the sun shines at about 45 degree 
elevation in a clear sky, it causes on one side 
of the fuselage a separation in the line of sight 
of the two lower turrets amounting to about 2 
mils. The effect of moderate wind on bending 
of the fuselage as the result of pressure on the 
tail and elevators is not known, but there is a 
definite indication that it amounts to much 
more than 1 mil. 

Important improvements remain to be made 
in the equipment used for harmonization. 
Clamps provided with tangent screws to con¬ 
trol accurately motions in elevation and azi¬ 
muth should be designed for the sights. The 
present clamps are far from satisfactory. Im¬ 
provements should be made in the stands and 
in the translator used for the wire method. The 
MFA translator should have its horizontal run 
extended by about 3 in., not only to make the 
location of the stand less critical, but also to 
make it easier to use two instruments simul¬ 
taneously on the same stand when they must be 
located several inches apart in the horizontal 
direction. A boresighting telescope with about 
3X magnification should be designed and put 
into general use as soon as possible. At present 
the armorer finds it difficult to make settings 


accurately, even on the middle distance yard 
targets. Magnification should markedly in¬ 
crease both accuracy and ease of setting. 

68 RECOMMENDATIONS BY NDRC 

1. For immediate use in the field, it is sug¬ 
gested that a combination of the prism method 
for eight stations and the mirror boresight 
method for two stations be adopted whenever 
the terrain permits use of the prism method. 
When space is limited, it is suggested that the 
MFA method be used, namely, the wire method 
for eight stations and the mirror boresight 
method for two stations. 

2. A complete evaluation of the new methods 
should be carried out with a trained crew under 
the direction of a small committee made up of 
a physicist, an engineer, and a competent sta¬ 
tistical analyst. Two airplanes should be avail¬ 
able for a period of at least three months, with 
adequate space and shop facilities. The tests 
should be carefully planned to measure sepa¬ 
rately the various factors which combine to de¬ 
termine the overall accuracy of each method. 
The Plexiglas should be removed from the air¬ 
craft during these tests. 

3. Further studies and developments should 
be carried out on all of the new methods, giv¬ 
ing particular attention to the following: 

a. Prism Method 

(1) Determine the maximum angle 
(more than 6 degrees) that can be 
used without introducing other 
serious difficulties, so that the 
space requirements on all sides of 
the fuselage may be reduced to a 
minimum. 

(2) Make various minor improve¬ 
ments in equipment, including 
method for night illumination, 
means for clamping rotation of 
double-image prism, and other 
mechanical details. 

b. Wire Method 

(1) Compare use of left half of bin¬ 
ocular as a telescope (Mark II) 
with its use as a collimator (Mark 
III) under field conditions, after 
making all indicated improve- 



310 


HARMONIZATION OF B-29 GUNS AND SIGHTS 


ments in cross wires, illumination, 
and method for focusing at close 
distances with Mark II. Accuracy 
and ease of use should both be 
taken into account. 

(2) Improve stands and translating 
devices. Study advantages and de¬ 
ficiencies of all parts of Mark II 
and Mark III equipment. Develop 
best possible stand for conven¬ 
ience in operation, adjustment, 
and assembly. Eliminate need for 
the fork lift, if possible. Improve 
the Mark III stand by increasing 
width of steps of the ladder and 
place steps closer together. Ma¬ 
neuvering and assembly can be 
improved. 

c. Mirror Boresight Method 

(1) Increase brightness of lamps to 
facilitate sighting through plastic 
dome in bright sunlight. 

(2) Add protection for mirror to 
avoid damage during use. 

d. Mirror Frame Method 

(1) Measure variations of setting of 
mirrors under extreme tempera¬ 
ture changes. 

(2) Study possible improvement in 
choice of angle of bearing and ele¬ 
vation for which the frame is de¬ 
signed. 

(3) Complete development of peri¬ 
scope for right blister sight. 

(4) Mount a reflex sight on wind¬ 
shield of truck unless adjustment 
in azimuth is provided for the 
mirror frame. 

(5) Provide adjustment of the mirror 
frame in azimuth and elevation on 
the truck to facilitate settings. 

(6) Study selected plate glass to de¬ 
termine whether it is accurate to 
0.3 mil across the surface of 20-in. 
mirrors. If not, use Blanchard 
ground, polished %-in. plate glass. 

(7) Develop best procedure for mak¬ 
ing settings of the mirror frame. 

(8) Design targets for installation at 
sights and guns for harmonization 


by sighting on reflections in mir¬ 
rors. 

4. Study present variations in harmoniza¬ 
tion setting. Keep a harmonization log on sev¬ 
eral B-29 aircraft over a period of several 
months without changing any adjustment dur¬ 
ing this period. Measure (1) the setting of 
optical harmonization under as wide a variety 
of conditions as possible during the period, (2) 
the balance of the servo-amplifier at least once 
a week, and (3) the level of the turrets at fre¬ 
quent intervals and under different conditions. 
This should be done on at least two aircraft, 
one of which is flown often, the other held on 
the ground. The aim should be to separate and 
measure the magnitude of the following factors 
which affect harmonization: 

a. Unbalance of servo-amplifier (prob¬ 
ably a major factor). 

b. Change in setting of selsyns (probably 
a minor factor). 

c. Change in level of sights and turrets 
(probably a major factor). 

d. Effect of rough landings and violent 
. maneuvers in the air on the level of 

sights and turrets. 

e. Effect of jacks on harmonization set¬ 
tings, particularly side blister sights to 
tail mount and to lower aft turret. 
Check on middle distance targets. 

f. Effect of wind pressure on tail and 
elevators on harmonization settings. 
Check on middle distance targets. 

g. Effect of temperature on harmoniza¬ 
tion settings. This includes effect of 
changing uniform temperature and 
also the effect of different temperature 
on top and bottom and right and left 
sides, due to sun. Check on middle dis¬ 
tance yards. 

5. Test the effect on harmonization of main¬ 
taining in good adjustment over a period of six 
months the balance of the servo-amplifier and 
the level of the sights and turrets on two air¬ 
craft, one of which is flown often, the other 
kept on the ground. Find out whether, if these 
are maintained in adjustment, it is necessary 
to change the setting of the selsyns, and how 
frequently it is actually necessary to make opti¬ 
cal checks on harmonization. The results of this 




RECOMMENDATIONS BY NDRC 


311 


test can be used to guide the setting up of 
maintenance procedures. 

6. Measure the difference in harmonization 
setting for an airplane in flight, as compared 
with that for the same airplane on the ground, 
resting on its landing gear and jacks. Use gun 
cameras with moderately long focus lenses. 
While in flight, make exposures on the sun or 
moon and compare with exposures made on 
middle distance targets on the ground. Make 
observations on harmonization at various alti¬ 
tudes for each pair of stations at various bear¬ 


ings and elevations, and at various altitudes. 
Establish systematic corrections to be applied 
to harmonization settings made on the ground, 
so that harmonization in the air will be correct. 

7. Make improvements in present standard 
harmonization equipment: 

a. Design and make available slow motion 
tangent screw clamps for adjusting the 
setting of both pedestal sight and 
ringsights in bearing and elevation. 

b. Develop and make available for gen¬ 
eral use a 3x boresight telescope. 



Chapter 7 

OPTICAL FLUORITE 

By Sidney W. McCuskey and James G. Baker a 


7.1 INTRODUCTION 

T he continuous demand on the part of 
optical designers for materials having un¬ 
usual combinations of refractive index and dis¬ 
persive power, or of peculiar transmission 
characteristics, has led in recent years to the 
development of new glasses and of large trans¬ 
parent crystals of several kinds. The latter in¬ 
clude artificially grown crystals of lithium 
fluoride, sodium chloride, potassium bromide, 
and the like. These are of particular value 
when cut into prisms for use in infrared spec¬ 
trometers. Their transmission in the red end 
of the spectrum extends at a high level to 29 p 
(for KBr). The development of techniques for 
growing these crystals has already been dis¬ 
cussed 1 and they are now made commercially 
by the Harshaw Chemical Company of Cleve¬ 
land, Ohio. 

In the design of optical systems, particularly 
in lenses for aerial photography, it is highly de¬ 
sirable to eliminate as far as possible the chro¬ 
matism that remains after the lens system has 
been achromatized for two wavelengths. Crys¬ 
talline calcium fluoride, CaF 2 , has been long 
known as a material highly suitable for this 
purpose. It has a low refractive index, 1.4338, 
a high reciprocal dispersion (v value), 95.4, 
and, more important, the ratio of its disper¬ 
sion (g — dn/dx) to that of crown glass re¬ 
mains practically constant with wavelength. 
This means that a lens of crown glass and 
fluorite when achromatized for two wavelengths 
will be nearly achromatic for all. 

The striking reduction in secondary spec¬ 
trum obtainable by the use of a fluorite com¬ 
ponent in the optical system, however, is ac¬ 
complished at the expense of less rigid toler¬ 
ances in the other aberrations or else more com- 


a Warner P. Swasey Observatory of the Case School 
of Applied Science, and Harvard College Observatory. 
Dr. Baker is responsible for compiling the section of 
this chapter entitled, “Optical Applications of Fluorite.” 


plicated designs. This limits its usefulness in 
wide-field systems, since in all probability 
lenses with aperture ratios greater than about 
//8 cannot be fully corrected. Furthermore the 
low index of the material introduces difficulties 
into the design. For telescopes and collimators, 
however, the elimination of color is most im¬ 
pressive. The design of camera lenses and col¬ 
limators incorporating fluorite elements is de¬ 
scribed in Chapter 1. 

Large natural crystals of CaF 2 , sufficiently 
transparent for optical purposes, are exceed¬ 
ingly rare. Since camera lenses for aerial pho¬ 
tography usually require disks 3 to 6 in. in 
diameter, the only alternative if fluorite is to 
be used is to grow satisfactory crystals in the 
laboratory. 

The success of the Crystal Laboratory at the 
Massachusetts Institute of Technology [MIT] 
in growing large single crystals of LiF led the 
NDRC in 1941 to seek their help in developing 
methods for growing fluorite crystals of large 
size, under Project AC-11. During the ensuing 
four years, under Contract OEMsr-45, satis¬ 
factory methods for growing crystals as large 
as 6 in. in diameter were developed. Figure 1 
shows some of the crystals grown in the labo¬ 
ratory. About 1,100 specimens of various sizes 
were made. Although the homogeneity varied 
somewhat, and although the majority of the 
crystals were not single, many disks 4 in. in 
diameter were made into excellent components 
for aerial camera lenses. Large prisms for 
infrared spectrographs were made from two 
of the crystals. 

In the method developed under contract 
OEMsr-45, 2 artificial fluorite crystals are 
grown in a low-pressure atmosphere by con¬ 
trolled upward freezing of nearly pure molten 
CaF 2 . The charge of raw material is obtained 
from carefully selected and graded natural 
fluorspar. A small quantity of PbF 2 is added to 
the melt to prevent contamination by hydrol¬ 
ysis. With proper adjustment of the scaveng- 


312 



INTRODUCTION 


313 


ing agent, deleterious reaction products and the 
excess PbF 2 leave the melt by volatilization 
prior to crystallization. At times the melt is 
obtained from a chemically synthesized CaF 2 - 
PbF 2 mixture. The melting point of this charge 
is about 1400 C. 



Figure 1. Artificial optical fluorite crystals 
grown in elevator furnace. (The larger crystals 
are 4 in. in diameter.) 


Freezing takes place in a thermal field de¬ 
signed to produce an effective temperature 
gradient through the melt layer which is in 
contact with the advancing crystal surface. The 
freezing is controlled by moving the charge 
through the thermal field in the direction of 
decreasing temperature in an elevator furnace, 
or by changing the field in a pot furnace. De¬ 
partures from the thermal field conditions theo¬ 
retically most suitable for the growing of single 
crystals lead to multiple crystal formation, en¬ 
trapped bubbles, and poor crystallographic 
structure. 

The effective temperature gradient , which 
exists in the melt at the liquid-solid boundary, 
and which is normal to this boundary, controls 
the crystal growth in the following ways: 

1. It insures that all parts of the melt are at 
temperatures exceeding the highest to be found 
in the solid. New crystalline structures cannot 
originate at any considerable distance within 
the melt, hence the chance of multiple crystal 
production is minimized and the resulting 
single crystal grows continuously. 

2. The effective temperature gradient pro¬ 


duces confusion at the advancing surface of the 
crystal by thermal bombardment. Deposits of 
impurities are thus dispersed and the resultant 
crystal structure is more uniform. 

Closely interwoven with the effective tem¬ 
perature gradient in controlling the character 
of the final crystal is the working speed. This is 
defined as the rate at which the solid is per¬ 
mitted to increase in volume. It is closely con¬ 
nected with the time required for diffusion and 
for latent heat removal. Spontaneous speed, de¬ 
fined as the rate at which freezing would take 
place if it were not held in check, is approached 
when the rate of cooling is increased beyond a 
maximum permissible value, which obviously 
depends on the surrounding temperature. For 
a given crystal size and temperature gradient 
there is a critical speed beyond which control 
of crystallization is lost. When this takes place 
the inclusion of impurities and other defects 
is to be expected. Temperature fluctuations 
have the net effect of increasing the working 
speed. In the extreme, they can result in an 
actual working speed approaching the spon¬ 
taneous value even though the average speed, 
expressed in terms of mass per unit time, is 
numerically below the allowable maximum. 

The first crystals grown by the method out¬ 
lined were found on occasion to possess three 
major defects, namely, color, multiplicity, and 
a large amount of light scattering. The removal 
and control of these defects may be summarized 
briefly. 

Color. Objectionable color in the crystals 
grown early in the project was found to be 
caused by careless selection of raw fluorspar 
stock, by presence of air leaks in the furnace 
housing, and by improper design of the heater 
and baffling system. The latter caused in¬ 
sufficient ventilation of the charge during crys¬ 
tal growth. 

Yellow, orange, red, and brown crystals re¬ 
sulted from colorless stock when appreciable 
leaks existed in the furnace, even though the 
pressure was kept low by fast pumping. Im¬ 
purities in the stock likewise were found by 
experiment to give rise to deeply colored, muddy 
crystals. Of forty crystals (%-in. diameter) 
grown finally from practically colorless stock 
in an improved furnace, only three were notice- 





314 


OPTICAL FLUORITE 


ably colored. Two of these colored specimens 
were traced to faulty stock and one to poor 
vacuum in the furnace. 

Multiplicity. Impurities in the natural fluor¬ 
spar also resulted in multiple’ crystal growth. 
By properly designing the furnace, including 
crucible and seat, and by careful control of 
the operating temperature during the begin¬ 
ning of the crystallization, the yield of single 
crystals was increased considerably. 

The operating temperature at which crystal 
multiplicity was least likely to appear was con¬ 
siderably lower than that which favored the 
high degree of purification required by the 
CaF 2 stock. Experiment showed, however, that 
the temperature could be increased, after the 
crystal had been well started, without inducing 
multiplicity. 

When the rate of crystallization was reduced 
by a factor of 4, it was found possible to grow 
essentially single crystals in every trial. Four 
single crystals 4 in. in diameter were produced 
in four trials. The crystallization of each re¬ 
quired a week. 

Light Scattering. The scattering of light 
within a crystal is very detrimental to its use 
in an optical instrument. Much effort was ex¬ 
pended, therefore, in overcoming the rather 
serious scattering which appeared in the first 
crystals grown. 

The directional selectivity of the scattering 
particles suggested that they were oriented in 
definite crystallographic directions. Micro¬ 
scopic examination verified this and also re¬ 
vealed the hexagonal outline of the particles. 
Since the particles were observed in the 111 
planes, and because the hexagonal outline could 
be derived from an octahedral specimen of CaF 2 
by cleavage through one of these planes, the 
conclusion was drawn that the particles were 
thin negative crystals, perhaps containing gas¬ 
eous or solid impurities. Concentration of the 
scattering particles near the top of the crystal 
as well as in the “grain boundaries” of some 
multiple specimens confirmed this view. Nu¬ 
merous subsequent observations with the petro¬ 
graphic microscope led to the conclusion that 
the impurities involved are substances such as 
CaO and BaS. That the anions were responsible 
for the ex-solution was shown by adding known 


amounts of different compounds to high-quality 
melts. As little as 0.1 per cent of CaS pro¬ 
duced a marked increase in scattering, for ex¬ 
ample, whereas 1 per cent of BaF 2 produced 
no noticeable effect. 

In order to remove the deleterious light scat¬ 
tering, a scavenger was introduced. After care¬ 
ful consideration, PbF 2 was selected as a 
scavenger and the following reactions were 
predicted: 

PbF 2 + CaO = CaF 2 + PbO 

PbF 2 + CaS = CaF 2 + PbS 

PbO + C = Pb + CO 

2PbS + C = 2Pb + CS 2 

Tests were made with varying percentages 
of PbF 2 with the result that 1 to 2 per cent of 
the scavenger was sufficient to free the crystals 
from light scattering. The following contami¬ 
nants were introduced in varying percentages 
from 0.01 to 1: SrF 2 , BaF 2 , CaO, CaS0 4 , CaS, 
Si0 2 , Fe 2 0 3 . In all cases the light scattering in 
the crystal was reduced to a negligible amount. 
Further details concerning these and related 
experiments are given elsewhere. 2 

72 EXPERIMENTAL PROCEDURE IN 
CRYSTAL MANUFACTURE 

7-2,1 Preparation of the Stock 

Natural Fluorite. Two types of raw material 
have been used in the production of CaF 2 crys¬ 
tals, namely, natural fluorite and synthetic 
CaF 2 . Natural fluorite is a moderately hard, 
glassy, transparent or translucent mineral oc¬ 
curring most abundantly in ore deposits and 
sedimentary formations in close association 
with other minerals. The Ulinois-Kentucky 
region of the United States and a small area 
in New Mexico are two of the finest fluorite 
producing areas in the world. The mineral com¬ 
monly occurs in crystalline masses ranging in 
color from water white to black. 

As the material comes from the mine it con¬ 
tains varying amounts of impurities such as 
calcite, barite, silica, galena, and clay. Since 
only the best of CaF 2 stock can be used suc¬ 
cessfully for synthetic crystals, a field survey 
project was established in August 1942 under 
Contract OEMsr-563 with Princeton Univer- 



EXPERIMENTAL PROCEDURE IN CRYSTAL MANUFACTURE 


315 


sity, 3 to discover sources of raw fluorspar suit¬ 
able for synthesis. As a result of the survey, 
ore from both the Babb-Frazer mine and the 
Aluminum Ore Company mines in the Illinois- 
Kentucky district, and from deposits near Dun¬ 
can, Arizona was judged suitable for the pur¬ 
pose. Approximately 5 tons of good grade ore 
were shipped to MIT. 

Extreme care must be exercised in selecting 
fluorspar for recrystallization. Only clear color¬ 
less material can be used. The larger pieces are 
broken into %-in. pieces and immersed in 
water. Clear pieces are then readily distin¬ 
guished from those with minute solid inclusions 
which are rejected. The selected pieces are 
washed with water to remove surface clay and 
any soluble salts,- and are rinsed twice with 
alcohol to remove the water. Finally the fluor¬ 
spar is pulverized in a mineral crusher and 
stored in stoppered clear glass jars. This mate¬ 
rial is scavenged with 2 per cent PbF 2 just 
before it is loaded into the crucible. 

Chemically Prepared CaF 2 . On June 1, 1942, 
a special study was undertaken to determine to 
what extent it was feasible to synthesize the 
required raw material in the event that the 
natural fluorite supply became inaccessible or 
poor in quality. 

Only the highest grade chemical reagents 
were used in the synthesis of CaF 2 . In several 
instances it was necessary to repurify the 
reagent. Commercial aqueous HF solution was 
redistilled in a platinum still. Ca(N0 3 ) 2 was 
made in a way designed to eliminate as far as 
possible the impurities known to be contained 
in the calcite crystals which serve as the source 
of Ca++. The details of this chemistry are given 
elsewhere. 2 

In the first syntheses of CaF 2 , the mixing of 
the reagents Ca(N0 3 )2 and NH 4 F was brought 
about by slow diffusion. 4 After the solutions 
had stood for several days, small crystals of 
CaF 2 began to form in and around one of the 
dishes. These were coarse dendrites made up 
of tiny cubes piled on one another. The process 
required so much time, however, that it was 
rejected as impractical. 

Titration experiments were made in which 
dilute NH 4 F solution was allowed to run slowly 
into dilute CaCl 2 , while at the same time an 


equivalent amount of more concentrated CaCl 2 
was added. The milky precipitate which formed 
was far from crystalline. Final large scale titra¬ 
tion trials yielded completely erratic results. 
Even with minor changes in concentration, 
acidity, or treatment, the products varied widely 
in appearance. Efforts to crystallize the mate¬ 
rial in the furnace were unsuccessful in spite 
of the fact that spectroscopic analysis showed 
the synthetic CaF 2 to be equal in purity to the 
natural fluorspar. 

The use of PbF 2 as a scavenger when me¬ 
chanically mixed with these same synthetic 
products was not successful in preventing light 
scattering in the crystal. The dendritic char¬ 
acter of the precipitate apparently hindered 
the removal of last traces of water during the 
preparation, and rapid preliminary evacuation 
prior to heating was obstructed. By co-precipi- 
tating CaF 2 and PbF 2 , however, a more inti¬ 
mate mixture of the scavenger with the CaF 2 
was obtained and a satisfactory crystal mate¬ 
rial resulted. 

Since the synthetic mixtures of CaF 2 and 
PbF 2 when co-precipitated and crystallized re¬ 
sulted in a product equal to that obtained with 
natural stock, a simplification of the synthesis 
was undertaken. The procedure developed for 
the preparation of a fine powder containing 2.5 
per cent of PbF 2 , and having a density about 
half that of natural fluorspar, is as follows: To 
an aqueous suspension of cp calcium and lead 
carbonate powders in a lead receptacle, is added 
aqueous cp hydrofluoric acid in excess. The 
precipitate is washed by decantation, dried over 
a steam bath, and stored in HF resistant con¬ 
tainers. This material, being bulky, is ordi¬ 
narily preheated to the melting point in a 
vacuum furnace, then granulated and otherwise 
treated as spar. 

Four 1.5-in. crystals were successfully grown 
with this mixture of fluorides, indicating that 
such synthetic material may, if necessary, be 
used instead of natural stock. 

7 ' 2 * 2 Elevator Furnace Design 

Vacuum Tanks. It is necessary that crystal¬ 
lization of CaF 2 from the melt be carried out 
in vacuum to prevent chemical reaction of 



316 


OPTICAL FLUORITE 


the stock and other materials present in the 
furnace. Thus each furnace consisted of a steel 
or heavy brass tank with heavy bottom flanges 
which could be fitted into a machined base plate. 
The latter was tapped and bored to receive the 
tank flange screws, electrodes, pump connec¬ 
tion, and other necessary items. Both externally 
wrapped water tubes and helically baffled water 
jackets were used for cooling. Figure 2 shows 
the upper part of one of the steel bell jars with 
its water-cooled window, lifting rings, and the 



Figure 2. Upper part of steel bell jar. 


soldered rectangular copper tubing wound 
around the cylinder. Figure 3 shows the fur¬ 
nace bell jar raised so that the interior posi¬ 
tioning of electrodes and crucible is evident. 

The tanks were sealed to the base plates by 
either lead antimony gaskets compressed within 
the tank flange grooves or by rubber gaskets 
squeezed between plane metal surfaces. The lat¬ 
ter were found to be very satisfactory when 
kept cool. Machine screws maintained compres¬ 
sion between tank flange and base plate. 

Dimensions of the tanks ranged from 8 to 
16 in. in diameter and 20 to 48 in. in height, 


with walls from 3 / 16 to in. A thick disk welded 
to the top of each tube contained a water-cooled 
quartz observation window, shown in Figure 2. 

The large furnaces were evacuated in about 
an hour to the desired pressure, 10~ 3 mm of 
mercury or lower, by two glass mercury dif¬ 
fusion pumps in series. One of these was de¬ 
signed for a speed of 75 liters per second, the 
other for a speed of 30 liters per second. A 
Cenco “Hypervac 20” was used as a forepump. 
Vacuum systems for the smaller furnaces were 
identical with the above except that the fore¬ 
pump was a Welch Duoseal Model 1405 H. An 
essential feature of each evacuation system is 
a demountable trap, whose function is to collect 
corrosive gases such as HF which are produced 
during the scavenging action of the PbF 2 . 

Heaters. The relatively high melting point 
of calcium fluoride, about 1400 C, necessitated 
the production of temperatures far above those 
ordinarily encountered in work of this kind. 
Since a high temperature gradient had to be 
maintained through the melt-crystal boundary, 
the maximum temperature inside the furnace 
was about 1600 C. All heaters were designed 
to keep operating potentials below values which 
would cause gaseous discharges. 

Heaters of two types were used. The earlier 
ones were formed of molybdenum wire wound 
on Alundum grooved cores. It was found, how¬ 
ever, that the terminal potentials with this type 
of heater were so high that destructive gaseous 
discharges occurred between hot parts in the 
rarefied atmosphere. Also the CaF 2 vapor at¬ 
tacked the Alundum core so that it deteriorated 
rather rapidly. This type of heater was aban¬ 
doned, therefore, and replaced by helical or 
vertical bar type graphite heaters. 

In the graphite heating elements a helix, 
turned on a lathe from a solid rod of graphite, 
is surmounted by a grid of parallel graphite 
bars, electrically in series. Several of the helices 
are shown in Figure 4. To prevent thermal field 
distortion due to the terminal connections, 
graphite rods were used in place of the con¬ 
ventional water-cooled clamps. These were of 
such diameter that their resistance heating 
tended to compensate for conduction losses from 
the heaters. Terminal contact areas were made 
as large as possible and their resistances were 





.. 


EXPERIMENTAL PROCEDURE IN CRYSTAL MANUFACTURE 


317 



Figure 3. Elevator furnace with bell jar raised. 

















318 


OPTICAL FLUORITE 


stabilized by means of shrink fits or screw 
threads. The rods were held by clamps attached 
to water-cooled electrodes in the base plate. 

Figure 5 shows schematically a cross section 
of the elevator furnace. Positioning of heater 
and grid are evident from the numbered leg¬ 
ends. Dimensions of the heater for an 8-in. 
furnace are representative. Helices for growing 



Figure 4. Graphite helices used for heaters. 


%-in. diameter crystals were 1% in. in diam¬ 
eter, separated by an annular molybdenum 
gradient baffle (see Figure 4, middle helix). 
The helices were each 3% in* long with wall 
thickness y 16 in. and were threaded with %-in. 
pitch. 

Other furnaces employed a single helix (see 
Figure 5, for example) with cover heater in 
the form of a grid. In this type the separation 


gradient baffle previously described became the 
lower baffle (No. 32 in Figure 5). Such a heater 
proved superior to the double helix. 

The machining and other fabrication difficul¬ 
ties led finally to the adoption of cylindrical 
grids with the coils running lengthwise, as 
shown in Figure 6. Thermal field distortions 
due to the low-resistance massive terminals 
were minimized by placing these terminals at 
the top of the heater where they would be as 
far as possible from the freezing region. 

In all furnaces the system of baffles is much 
the same. The top baffle is a laminated circular 
plate supported by a graphite rod in a position 
directly above and close to the furnace. The 
laminations from bottom to top (No. 43 in Fig¬ 
ure 5) are 0.005-in. molybdenum sheet, %- or 
%-in. graphite plate, and either another molyb¬ 
denum or a 0.029-in. stainless steel sheet. The 
cylindrical baffles are composed of two sheets 
of molybdenum wrapped around a graphite 
tube and tied with molybdenum wire. Molyb¬ 
denum wire is placed between the two sheets 
to insure the small spacing required for effec¬ 
tive baffling. At the bottom of the crucible seat 
are two circular baffles of molybdenum spaced 
about % in. apart. The gradient baffle (No. 32 
in Figure 5) is a molybdenum annulus whose 
inside diameter is slightly larger than the out¬ 
side diameter of the crucible. 

At the present time no probe is provided to 
tell whether the crystal-melt boundary is in 
the plane of the gradient baffle. At this point, 
under ideal conditions, the heat flow lines are 
parallel and the temperature gradient is high. 
Actually the maintenance of these ideal condi¬ 
tions is only for short durations of time and is 
more or less fortuitous in the present furnaces. 
A comprehensive study of freezing levels and 
temperature control in the lower chamber of 
these furnaces is indicated in order to put the 


Figure 5. Cross section of elevator furnace. (1) base plate, (2) tank, (3) base ring, (4) water jacket, 
(5) cap plate, (6) window tube, (7) quartz window, (8) cooling tube, (9) sealing gasket, (10) clamping 
screw, (11) lifting ring, (12) pumping tube, (13) elevator bushing housing, (14) gasket seat, (15) neo¬ 
prene gasket, (16) thrust washer, (17) adjusting screw, (18) copper electrode, (19) mica insulators, 
(20) cooling chamber cap, (21) power lead binding nut, (22) electrode connector, (23) clamping ring, 
(24) insulated locating coupling, (25) graphite electrode extension, (26) clamping nut, (27) mica insula¬ 
tors, (28) insulated adapter, (29) molybdenum lower baffles, (30) graphite elevator rod, (31) spacers, 
(32) gradient baffle, (33) gradient baffle support, (34) crucible seat, (35) molybdenum side baffle, 
(36) crucible, (37) crucible lid, (38) cylindrical heater, (39) power lead baffle support, (40) graphite 
clamping screw, (41)heater support ring, (42) grid, (43) top baffles, (44) top baffle support plate. 






EXPERIMENTAL PROCEDURE IN CRYSTAL MANUFACTURE 


319 



A/o te • AIIcooling tubes 
so/dered or welded for 
good thermal contact 


Figure 5 


























































































































































320 


OPTICAL FLUORITE 


consistent manufacture of crystals on a firm 
basis. 

Crucibles. Highly purified graphite crucibles 
were used in these furnaces. They had the dis¬ 
tinct advantage that the crystal did not adhere 
to the crucible while cooling and hence strains 
likely to result from differential contraction 
were largely avoided. The crucibles were thin- 
walled so that thermal conduction was negli¬ 
gible in comparison with conduction in the 
fluorite melt. Otherwise the crucible contents 


seat at speeds ranging from 4.8 to 0.025 in. per 
hour. A 1 rpm synchronous motor drives this 
mechanism. Thus the crucible is lowered at a 
rate which causes proper solidification of its 
contents as each level passes the gradient baffle. 

Power Supply. To supply power at variable 
low voltage to the heaters, General Radio Type 
50B Variacs control the primary voltage to 
7 kva transformers whose divided secondaries 
can be connected in series-parallel combina¬ 
tions. The Variacs are equipped with clutches 
and chain sprockets for continuous power ad- 




Figure 6. Cylindrical heater grids. 

would be more or less thermally short-circuited 
and maintenance of a controlled temperature 
gradient would be impossible. 

The crucibles were turned on a lathe from a 
solid rod of highly pure, close-grained graphite. 
The bottoms are conical to increase the chance 
of forming single crystals. Wall thickness 
ranges from % 2 in. in the case of 1-in. crucibles 
to y 16 in. for those with diameters of 4 and 6 in. 
The angle of the cone-shaped base is 60 degrees 
for the smaller crucibles and 160 degrees for 
the larger ones. Several crucibles are shown in 
Figure 7. 

In the elevator furnace the crucible is sup¬ 
ported in a graphite seat, a thin-walled cylinder 
open at the top with a heavy bottom through 
which a graphite rod contacts the tip of the 
crucible cone. The upper end of the cone makes 
light contact with the top of the seat as shown 
in Figure 5 (No. 34). The graphite rod is an 
extension of a steel rod which is lowered by a 
gear box mechanism capable of lowering the 


Figure 7. Graphite crucibles. 

justments with a motor drive. Each heater cir¬ 
cuit has a wattmeter or ammeter or both. An 
overall view of an elevator furnace installation 
with its power supply is shown in Figure 8. 

Operation. Actual operation characteristics 
of such a furnace may be described in terms of 
the record for growth of a 4-in. diameter single 
crystal of excellent quality. It must be empha¬ 
sized that repetition of the cycle would be no 
guarantee of successful duplication of the crys¬ 
tal, but it is a typical record. 

The crucible was charged with 1,600 g of 
good quality stock and 2 per cent by weight of 
PbF 2 and placed in the furnace with the cruci¬ 
ble tip % in. above the gradient baffle. The 
elevator mechanism was adjusted to lower the 
crucible through the baffle at a rate of 1 in. in 
26.4 hr. Then the tank was bolted down, water 
valves were opened, and vacuum pumps were 
started. 

When the pressure had reached 1 cm of mer¬ 
cury, liquid N 2 was applied to the trap. Power 



EXPERIMENTAL PROCEDURE IN CRYSTAL MANUFACTURE 


321 



Figure 8. Elevator furnace installation. 


















322 


OPTICAL FLUORITE 


was then supplied according to the schedule of 
Table 1. 


Table 1. Power supplied to elevator furnace in 
one experimental run. 


Time 

Pressure 
in 10~ 3 mm 
Hg 

Power to 
grid (w) 

Power to 
heater (w) 

0 

0.4 

1,040 

2,040 

50 min 

1.1 

1,620 

2,700 

1 hr 20 “ 

4.8 

2,220 

3,780 

1 “ 50 “ 

4.8 

2,800 

5,800 

2 “ 15 “ 

1.6 

3,300 

7,800 

2 “ 30 “ 

1.7 

3,900 

9,800 


At the end of 2.5 hr the elevator was started 
down, at a rate of 1 in. in 26.4 hr. Approxi¬ 
mately 67 hr later, after the elevator had trav¬ 
eled 2.5 in., the rate of descent was changed 
to 1 in. in 13.2 hr. After the elevator had trav¬ 
eled % in* farther it was stopped and the Variac 
drives were engaged to decrease the power 
continuously at a rate which would bring the 
voltage to zero in about 12 hr. Air was then 
admitted to the cold furnace and the crystal 
removed. An example of this size of crystal is 
the larger specimen exhibited in Figure 1. 

7 * 2,3 Pot Furnace Design 

Crystal blanks of large diameter but small 
thickness, such as those needed for aerial cam¬ 
era lenses, have been grown in a pot furnace. 
This type of furnace is mechanically static, 
the freezing of the melt being controlled en¬ 
tirely by regulating the power input. The tem¬ 
perature gradient is provided by supplying heat 
independently above and sometimes below the 
crystal. No particular advantage accrues to the 
use of the pot furnace other than the mechan¬ 
ical simplicity. For thin (1 in.) crystals this 
type of furnace was found to yield good results 
very consistently. For thicker crystals the gra¬ 
dient is too low for satisfactory crystal growth. 

Figure 9 shows the essential parts of such a 
furnace. The heating element consists of a new 
type of grid, two spiral graphite ribbons in 
parallel suspended inside and at the top of the 
cylindrical heater. A guard ring surrounding 
the lower part of the crucible eliminates most 
of the lateral heat loss. The cylindrical heater 


is of the longitudinal coil type finally adopted 
for the elevator furnace. Its general character 
is indicated in Figure 9. In other respects the 
pot furnace resembles the elevator type. 

For small pot furnaces which accommodate 
crucibles up to 4 in. diameter, a 7 kva trans¬ 
former and General Radio 50B Variac supply 
power to each heater. For the larger furnaces, 
in which 6-in. crystals are grown, it was nec¬ 
essary to use several such transformers and 
controls. A cam and gear train decreased the 
power continuously at a rate which was in¬ 
tended to keep the crystal growth linear in 
time. Voltage regulators on the power supply 
responded to fluctuations of the order of 0.25 
per cent and served to stabilize the supply to 
the point where the cam drive was effective. 

In this type of furnace it is particularly nec¬ 
essary to have a probe of some kind to measure 
the progress of the freezing level in the melt. 
Such a probe was used in the largest pot fur¬ 
nace. A relatively rigid 50-mil tungsten wire 
was fastened to an iron rod diameter 

and 1 in. long. The iron rod slides smoothly in 
a Pyrex tube mounted at the top of the furnace 
tank. It was raised and lowered by an electro¬ 
magnet surrounding the tube. The tungsten 
wire extended down into the tank through a 
series of coaxial holes in the top baffle, top 
heater, and crucible lid. Electrical heating of 
the passageway through the top baffle was nec¬ 
essary to guard against clogging by sublimed 
CaF 2 . 

Crystal growth which is linear with time is 
approximated by using the cam and gear train 
previously mentioned. The probe readings pro¬ 
vide a record of crystal growth which serve 
as the basis for design of this cam. The ultimate 
desire in power control, of course, is to produce 
uniform vertical heat flow. Although many ex¬ 
periments have been performed to determine 
the proper ratio of grid to heater power, the 
best value still remains to be found. 

The operations required to make a 6-in. crys¬ 
tal will serve to illustrate the procedure, time, 
and power involved in a typical pot furnace 
run. The crucible was charged with 3,000 g of 
good quality stock and 2 per cent of lead fluoride 
by weight, and was then placed in the furnace 
so that the tip of the crucible was approximately 






EXPERIMENTAL PROCEDURE IN CRYSTAL MANUFACTURE 


323 



Figure 9. Cross section of pot furnace. (1) vertical bar type heater, (2) crucible lid, (3) crucible, 
(4) molybdenum baffles, (5) molybdenum cylindrical baffle, (6) crucible seat and baffle carrier, (7) grid 
clamping nut and probe guide, (8) stainless steel, (9) molybdenum baffle, (10) top heater, (11) connector 
and probe guide, (12) top heater electrode, (13) cylindrical heater electrode, (14) common electrode. 


i /2 in. above the bottom of the heater coils. Then 
the tank was bolted down, the probing device 
installed, and the vacuum pump started. Liquid 
nitrogen was applied to the trap after the pres- 


Table 2. Typical power schedule for pot furnace 
run. 


Time (hr) 

Pressure 
in 10~ 3 mm 
Hg 

Power input 
to grid (w) 

Power input 
to heater (w) 

0 

1.5 

1,135 

960 

1 

1.6 

2,840 

2,580 

1% 

7.1 

3,980 

3,830 

2% 

7.2 

5,820 

5,950 

2% 

3.2 

9,100 

7,970 

3 

3.1 

9,650 

6,520 


SV 2 Grid power started down with cam-gear drive. 


sure had reached 1 cm of Hg. The power sched¬ 
ule is shown in Table 2. 

When probe measurements indicated that 
crystallization was complete, the power was 
lowered to zero over a period of 1.5 hr. The 
crystal was allowed to cool in vacuum over 
night and was then removed from the furnace. 
The curve in Figure 10 illustrates the growth 
of the crystal as measured with the probe. 

724 Annealing 

Strains in crystals grown from the salt melt 
may, for convenience, be classified as (1) cool¬ 
ing and (2) locked or structural strains. The 


















































































































































324 


OPTICAL FLUORITE 


former evidently result from uneven or too 
rapid cooling through the plastic temperature 
range. The latter are usually associated with 
imperfect crystallographic structure. The usual 
treatment for strain is a recooling of the crystal 



Figure 10. Curve of growth of crystal in 6-in. 
pot furnace. 


through the plastic range at a speed and under 
thermal field conditions favorable for homog¬ 
enous contraction. This annealing process often 
eliminates cooling strains but seldom reduces 
the structural strains. Faulty isothermal con¬ 
tours, especially during early stages of growth, 
incorrect ratio of growth speed to temperature 
gradient, and entrapment of impurities are 
suggested as causes of locked strain. 

A special annealing furnace was designed 
and constructed for this work. It consists of a 
capped stainless steel tube 6% in. diameter, 
28 in. long, and 0.080 in. wall thickness, which 
is heated along the upper third of its length. 
The heat source is a thermally insulated 
Nichrome wound Alundum core whose inside 
diameter is 7 in., leaving a space of 5 / 16 in. be¬ 
tween tube and heater. It is powered by a trans¬ 
former controlled by a Variac. The voltage may 
be slowly decreased by a continuous drive mech¬ 
anism. Through a gasket seal, the tube may be 
bolted securely to a base plate and its interior 
evacuated. Vacuum-tight thermocouple connec¬ 
tions are provided in the base plate. 

Preliminary experiments indicated that an¬ 
nealing at a temperature of approximately 
1000 C in a pure nitrogen atmosphere at normal 
pressure to suppress sublimation would remove 
strain. Equally effective results were later ob¬ 
tained at a temperature of 800 C. The minimiz¬ 
ing of temperature gradients in the furnace is 
the essential requirement. At 800 C it was pos¬ 
sible to anneal the crystals in vacuum. 

Crystals to be annealed are placed in heavy- 


walled graphite crucibles in the chamber of the 
furnace. Eight 2-in. diameter crystals or two 
4-in. crystals can be annealed in one loading. 
A representative schedule for the annealing of 
a 4-in. crystal is as follows: 

Heating to top temperature (750 to 800 C) = 3.25 hr 
Time at top temperature = 9 hr 

Automatically controlled cooling to 75 C =35 hr 

At the latter point the power is zero. The fur¬ 
nace then was allowed to cool to room temper¬ 
ature (about 24 hr), the vacuum was broken, 
and the crystal removed. 

For a 2-in. diameter crystal, the controlled 
cooling time was about 15 hr instead of 35. 

Several crystals were satisfactorily annealed 
under conditions varying but slightly from 
those described while others showed bad strain 
even after treatment as nearly identical as pos¬ 
sible. Schedules involving longer times of cool¬ 
ing removed most of the strain from these 
crystals but cracks developed during subse¬ 
quent handling or processing, although all op¬ 
erations were performed cautiously. One may 
infer that the crystals having locked strains 
cannot be annealed satisfactorily by the stand¬ 
ard method developed. It is evident that more 
study of the annealing process is required to 
insure uniformity of product. 

The annealing furnace, as designed, was not 
strong enough to withstand evacuation at high 
temperatures. Air leaks through the welded 
seams appeared after short operation, followed 
by collapse of the stainless steel lining. A re¬ 
design of the furnace is indicated. Because of 
this failure, most of the annealing was done in 
the crystallizing furnace. The resulting crystals 
while not perfectly strain-free were sufficiently 
so for most optical purposes. They could be 
sawed, ground, and polished with reasonable 
care in handling. 

Summary of Crystallizations 

The results of crystallizations in the various 
types of furnaces are presented in Table 3. The 
table includes the majority of crystals grown 
during the most vigorous prosecution of the 
work but does not include results of runs made 
for determining suitable furnace design, proper 
operating conditions, or quality of raw material. 

The scarcity of large single crystals grown 























OPTICAL PROPERTIES OF SYNTHETIC FLUORITE CRYSTALS 


325 


in no way detracts from the importance of the 
development from the optical standpoint. Mul¬ 
tiple crystals, with proper care, have been 
worked into lenses and prisms, and the multi¬ 
ple structure appears to have no appreciable 
deleterious effects on the quality of the optical 
part. 


a spectrometer table at controlled air temper¬ 
atures near 15, 35, and 55 C by the method of 
minimum deviation. The measures were then 
corrected to refer to air at the temperatures 
listed and at a pressure of 760 mm of mercury. 
The probable errors are estimated as not in 
excess of 2 X 10~ G , with systematic errors prob- 


Table 3. Results of crystallizations. 


Type of furnace 

Element 

diameter 

(in.) 

Elements 

grown 

Single 

clear 

Multiple 

clear 

Crystals 

annealed 

Crystals 

grown 

with 

scavenger 

Crystals 
with no 
scattering 

Elevator type with mo¬ 

iy 2 

105 

2 

6 

11 

0 

4 

lybdenum wound 

1 

3 

0 

0 

0 

0 

0 

Alundum core 

1% 

15 

0 

0 

2 

0 

0 

heater 

2 

31 

0 

7 

0 

0 

0 



154 

2 

13 

13 

0 

4 

Elevator type with 

iy 2 

124 

23 

37 

32 

73 

43 

graphite spiral 

2 

214 

48 

93 

154 

207 

87 

heater 

1% 

1 

0 

1 

0 

0 

0 


% 

324 

59 

143 

14 

23 

52 


4 

52 

2 

37 

22 

52 

37 



715 

132 

311 

222 

355 

219 

Elevator type with ver¬ 

1 

11 

4 

5 

0 

11 

7 

tical bar type heater 








Pot furnaces 

2 

15 

0 

0 

0 

0 

0 


1% 

12 

0 

8 

0 

12 

0 


3 % 

216 

2 

185 

85 

215 

137 


6 

16 

0 

15 

4 

16 

11 



259 

2 

208 

89 

243 

148 

Grand totals 


1,139 

140 

537 

324 

609 

378 


7 3 OPTICAL PROPERTIES OF SYNTHETIC 
FLUORITE CRYSTALS 

Refractive Index 

Two fluorite crystals were made into 60-de- 
gree prisms and transmitted to the Bureau of 
Standards for refractive index measurement. 5 
The first of these, 1-224, was figured by the 
Bausch and Lomb Optical Company from a 
crystal grown with scavenger from colorless 
New Mexico stock which had been processed 
once previously through the furnace. It had 
faces 40 mm on a side. The second prism, IX-7, 
with faces 75 mm on a side, was figured by the 
Perkin-Elmer Corporation. It was grown from 
Butler Cave fluorspar with scavenger. 

The refractive indices of these 60-degree 
prisms were measured in a stirred air bath on 


ably not in excess of two or three times the 
probable error. 

Table 4 exhibits the refractive indices aver¬ 
aged for the two prisms. The results for the 
40-mm prism were, on the average, 5 X lO^ 6 
higher than the values given in Table 4, and the 
indices for the 75-mm prism were correspond¬ 
ingly lower. These values average 13 X 10 -6 
higher than the indices of fluorite as measured 
by Schonrock at the Reichsanstalt and 6 X 10 _G 
higher than some measurements of natural 
CaF 2 made at the Bureau of Standards. 5 

From these data we find a v-value of 95.2 at 
15 C, and an average temperature coefficient 
of refractive index of —11 X 10~ 6 per degree 
centigrade. 

The influence on refractive index of impuri¬ 
ties in the synthetic CaF 2 crystals is insignifi- 






326 


OPTICAL FLUORITE 


cant. Measurement of n D in a prism made from 
a crystal grown without lead fluorite scavenger 
yielded a value 1.43389 at 15 C, which is essen¬ 
tially the same as that recorded in Table 4 for 
crystals grown with the scavenger. 

Transmission 

Over the entire wavelength range from 0.4 \i 
to 6 p the transmission of the artificial fluorite 
crystals, uncorrected for surface reflections, is 
nearly uniform at 93 per cent for a path of 
31 mm. Figure 11 shows the transmission of 


The ultraviolet cutoff point of the material 
has been estimated from samples of crystal 
varying in thickness from 2 to 10 mm by ex¬ 
amining the spectrum of an iron arc with the 
vacuum spectrograph. The mean of seven esti¬ 
mates is 0.132 [i ± 0.002. The cutoff point for a 
plate of natural fluorite 1.9 mm thick has been 
given as 0.124 \i.° 

Homogeneity 

The optical quality of five large fluorite crys¬ 
tals as regards homogeneity was measured in 



WAVELENGTH IN MICRONS (EXPANDED SCALE) WAVELENGTH IN MICRONS 


Figure 11. Transmission of artificial fluorite. 


one sample as a function of wavelength for this 
path length. It should be noted that in the 
range of wavelength less than 1 \i the scale of 
abscissae has been considerably expanded. The 


Table 4. Refractive index 
crystals. 

of synthetic 

CaF 2 

Wavelength 

Indices of Refraction 

(A) 

15 C 

35 C 

55 C 

*7678.58 

1.430984 

1.430768 

1.430546 

7065.188 

1.431763 

1.431553 

1.431331 

6678.149 

1.432356 

1.432142 

1.431922 

6562.793 

1.432545 

1.432335 

1.432114 

*5892.62 

1.433893 

1.433683 

1.433465 

5460.740 

1.435023 

1.434813 

1.434593 

4861.327 

1.437106 

1.436894 

1.436680 

4358.342 

1.439555 

1.439354 

1.439143 

4046.563 

1.441573 

1.441378 

1.441168 


* Intensity-weighted mean of doublet. 


curve is compiled from measures made on a 
prism in the range from 0.4 \i to 10 \i. For the 
range below 0.4 \i, the measures were made with 
a vacuum grating spectrograph on various sam¬ 
ple plates of CaF 2 and the results must be con¬ 
sidered provisional. 


a preliminary way at the Mount Wilson Ob¬ 
servatory and more completely at the National 
Bureau of Standards. The report of the Na¬ 
tional Bureau of Standards 7 will be cited in 
some detail, since it furnishes the most exten¬ 
sive and quantitative data. 

Photographs of the fringe patterns produced 
in the Michelson-Twyman interferometer were 
made in unpolarized and in polarized light. The 
fringes, in general, appeared somewhat blurred 
because of double refraction. Because of rela¬ 
tively large deviations from homogeneity in 
the crystals it was not deemed necessary to 
photograph the fringes showing surface irreg¬ 
ularities. Instead, careful drawings were made 
of these fringes and in determining the magni¬ 
tude of heterogeneities within the cylinders, 
allowance was made for the deviations of the 
surfaces from true planes. 

For purposes of comparison, a series of pho¬ 
tographs was also made of a specimen of nat¬ 
ural fluorite which was available in the form 
of a 60-degree prism. The natural fluorite used 
was not entirely free from visible defects and 
does not represent the best quality that is some- 














































OPTICAL WORKING OF SYNTHETIC CRYSTALS 


327 


times available. The prism, however, has served 
satisfactorily for research work. 

The five cylinders of synthetic CaF 2 crystals 
were 100 mm in diameter and varied in thick¬ 
ness from 38 to 51 mm. 

The variation in optical path length from 
an assumed normal value through each speci¬ 
men was measured, allowance having been 
made for irregularities of both surfaces of the 
crystal. Maximum path-length differences, ex¬ 
pressed in wavelengths of green light over a 
central portion of the crystal 75 mm in diam¬ 
eter, were found to be: 1.4, 0.8, 0.8, 0.6, 1.0 for 
crystals numbered 1 to 5 respectively. These 
values are for a single path through the crystal 
and arise from heterogeneities within the ma¬ 
terial. For a path length of 2 in. in optical glass 
of the same aperture, one would not expect 
path differences due to inhomogeneity to exceed 
0.05 wavelength. 

Figures 12 and 13 show the fringe patterns 
for specimens No. 3 and 5, both in polarized 
and in unpolarized light. The abrupt curvature 
of the fringes at the edges of the crystal is 
caused by dubbing of the surface ends. The 
streaks on the third vertical fringe and across 
the two vertical fringes at the right side of 
specimen No. 3 are probably caused by twin¬ 
ning of the crystals. The whorled effect in No. 3 
resembles the markings caused by striae in 
glass. In specimen No. 5 there is a cleavage 
plane horizontally across three vertical fringes 
on the right. The smaller streaks, white spots, 
and apparent sharp displacement of fringes in 
this specimen are caused by scratches and 
marks on the surface. 

It is evident that the fringe patterns in 
polarized light are much sharpened. The pho¬ 
tographs taken with polarizer and analyzer 
crossed reveal considerable stress in the crys¬ 
tals. The birefringence was not measured quan¬ 
titatively but appears to be considerably more 
than is tolerated in optical glass. For the latter 
the tolerance is 10 mq per cm although most of 
the optical glass now produced shows a bire¬ 
fringence no greater than 5 mq per cm. In 
specimen No. 3 the crystal multiplicity shows 
plainly. 

Figure 14 shows the interference pattern 
photographed through a 60-degree prism of 


natural fluorite, the maximum path length 
being 51 mm. The variation in sharpness and 
regularity of fringes from one side to the other 
arises from the variation in path length through 
the prism and does not indicate any regular 
variation in quality of the specimen. Opaque 
spots in photographs made with the unpolar¬ 
ized light are apparently due to small facets or 
fractures in the interior of the crystal which 
have an iridescent appearance. In polarized 
light the pattern is mottled and much finer 
grained than is the pattern for the synthetic 
crystals. Presumably a finer grained stress pat¬ 
tern is indicated. 


71 OPTICAL WORKING OF SYNTHETIC 
CRYSTALS 

No matter how completely internal stresses 
have been removed from the fluorite crystal by 
annealing, a thermal shock will introduce a 
certain amount of stress temporarily. Sudden 
changes of temperature, such as would occur 
if the crystal were submerged in cool water 
after optical working, must be avoided. 

Cutting of crystals may be carried out with 
a diamond saw with proper precautions. If the 
crystal is washed in water which is 5 C cooler 
than the crystal, the latter will almost certainly 
crack. As a test to determine whether thermal 
shock alone is sufficient to cause shattering, a 
stream of cold tap water was allowed to fall 
on an annealed 3%-in. multiple crystal. It was 
shattered immediately. Furthermore one of the 
pieces which was crystallographically single 
quickly broke when it was placed in the cold- 
water stream. 

This result has been borne out by the experi¬ 
ence of optical workers who have been devel¬ 
oping methods of grinding and polishing flu¬ 
orite surfaces. Thermal shock has been found 
to be far more serious than mechanical shock. 
Size of disk has a great deal to do with resist¬ 
ance to thermal shock. No 2-in. disks were 
broken in experiments on grinding and polish¬ 
ing, but some thick 4-in. disks broke in spite 
of care. 

It is evident, therefore, that the crystal must 
be kept at room temperature as nearly as pos- 


, RESTRICTED 





328 


OPTICAL FLUORITE 



LIGHT UNPOLARIZED 
FRINGES HORIZONTAL 



LIGHT POLARIZED 

POLARIZER AND ANALYZER PARALLEL 
FRINGES VERTICAL 

Figure 12 . Interference patter 



LIGHT UNPOLARIZED 
FRINGES VERTICAL 



LIGHT POLARIZED 

POLARIZER AND ANALYZER PERPENDICULAR 
FRINGES VERTICAL 

—artificial fluorite, specimen No. 3. 
















OPTICAL WORKING OF SYNTHETIC CRYSTALS 


329 




LIGHT UNPOLARIZED 
FRINGES HORIZONTAL 


LIGHT UNPOLARIZED 
FRINGES VERTICAL 



LIGHT POLARIZED 

POLARIZER AND ANALYZER PARALLEL 
FRINGES VERTICAL 



LIGHT POLARIZED 

POLARIZER AND ANALYZER PERPENDICULAR 
FRINGES VERTICAL 


Figure 13. Interference pattern—artificial fluorite, specimen No. 5 

















330 


OPTICAL FLUORITE 


sible during optical working. It must be worked 
slowly and allowed to stand after working and 
before washing. The water in which it is washed 
must also be at room temperature. 

This sensitivity to thermal shock is not 
uniquely characteristic of synthetic fluorite. It 


NATURAL FLUORITE 60° PRISM 



LIGHT UNPOLARIZED 
FRINGES VERTICAL 


LIGHT UNPOLARIZED 
FRINGES HORIZONTAL 


\ 


LIGHT POLARIZED 
POLARIZER AND ANALYZER 
PERPENDICULAR 


Figure 14. Interference pattern—natural fluor¬ 
ite. 


has been demonstrated repeatedly that natural 
fluorite crystals crack when treated similarly. 

Techniques for figuring fluorite surfaces have 
been worked out by the Perkin-Elmer Corpora¬ 
tion, under Contract OEMsr-1177, and by Har¬ 
vard under Contract OEMsr-474. The general 


conclusions of the former may be summarized 
as follows: 

1. In the rough grinding stage mechanical 
shock is likely to arise if the roughing is done 
with coarser than No. 120 Carborundum. No. 
240 Carborundum or emery is recommended as 
a start; coarser grades cause chipping, flaking, 
and subsurface fracturing. 

2. In spite of all care in the polishing stage, 
the soft crystalline materials show a tendency 
to flake. These flakes are a source of sleeks and 
scratches. The elimination of difficulty from 
this source constitutes the major problem in 
working the synthetic crystals. 

3. It is advantageous to produce as fine a 
ground surface as possible before polishing. 
Very long polishing times are required, at best, 
to remove the subsurface fracturing caused by 
the grinding materials. The fractures appear to 
extend to a depth several times the size of the 
grains used in grinding. In the polishing proc¬ 
ess the crystal layers present a “grain” appear¬ 
ance not met in the polishing of homogeneous 
material like glass. 

4. The presence of a scratch interferes with 
figuring, presumably on account of the internal 
stress in the fluorite. The test plate shows a 
figure error amounting to several waves sur¬ 
rounding such a scratch and the error remains 
until the scratch is completely eliminated. The 
area affected depends on the depth of the 
scratch, extending sometimes as far as ^4 in. 
from the scratch. 

5. In samples figured from multiple crystals, 
it was found that on a face showing parts of 
several crystalline components, each of the 
parts persisted in standing at a different level 
(approximately a quarter-wave) from its 
neighbors. Each component part of the crystal, 
nevertheless, was flat to a tenth of a wave with¬ 
out rolled edges at the crystal boundaries. 

6 . The reaction products arising from the 
polishing of fluorite appear to produce a glaze 
over the lap which interferes with polishing. 
This is in sharp contrast to the polishing of 
glass with a pitch lap, where the reaction prod¬ 
ucts actually assist in the operation. 

These general conclusions are confirmed by 
the experience at Harvard in the grinding and 
polishing of fluorite. It was found there that 





OPTICAL WORKING OF SYNTHETIC CRYSTALS 


331 


when emery replaced silicon carbide as the 
principal grinding agent many of the fractures 
and cracks resulting from the latter did not 
appear. Bonded diamond wheels were used to 
generate the required curves and emery was 
used for the final grinding with very satisfac¬ 
tory results. 

After considerable experimentation, a tech¬ 
nique suitable for working these crystalline 
surfaces was developed by Perkin-Elmer. The 
experience was gained by working (1) two 
CaF 2 prisms of 60-degree angle, (2) five CaF 2 
disks 4 in. in diameter and 2 in. thick, (3) two 
small 60-degree prisms, one of BaF 2 and one 
of SrF 2 , (4) twenty-four disks of CaF 2 about 
50 mm in diameter and 12 mm thick, and (5) 
many small pieces for experimental purposes 
only. 

The technique recommended 8 for the surface 
working of these crystals follows: 

1. Blocking. Since fluorite cannot stand the 
heat of regular blocking pitch, a soft, nonflow¬ 
ing, adhesive wax was necessary for attaching 
hubs to the work. A very satisfactory compo¬ 
sition for the purpose is: 


Rosin 

200 parts 

Asphalt 

150 “ 

Beeswax 

100 “ 

Paraffin 

25 “ 

Rouge 

200 “ 


The hub is heated and smeared with a little 
of the wax. When this has cooled, it is exposed 
very briefly to a flame and then pressed against 
the fluorite. It takes very little heat to bring 
this wax to a point where it will adhere firmly. 
The hub is removed afterward with a light tap. 

Unless the pieces are small (1 in. or less in 
diameter), it does not seem worth while to 
block them in groups for polishing. The ease 
with which the material is scratched means 
that a high percentage of reworks would result 
from each plateful. The optical figure, further¬ 
more, cannot be corrected when several pieces 
are blocked at once, since each piece acquires 
a rolled edge. Multiple blocking is, however, a 
timesaver in the grinding stages. 

2. Grinding. For rough grinding, No. 240 
Carborundum or emery wet with water is used; 
coarser grades cause too much chipping, flak¬ 


ing, and subsurface fracturing, although they 
are unlikely to cause breakage. For fine grind¬ 
ing, the succession is No. 3F, No. 900, No. 1600, 
and (if the operator can manage it success¬ 
fully) No. 2600, all of these being wet with 
denatured alcohol or a large amount of water. 
Sodium oleate solution and ethylene glycol 
were tried as substitutes for alcohol, but were 
not so satisfactory. In working with alcohol, 
the fluorite should be taken off the lap at each 
wet, and run in a little by hand after the alco¬ 
hol and emery are added. After three or four 
wets, the fluorite and the tool should be wiped 
clean before proceeding, in order to avoid 
scratches. To use extreme care, the operator 
should do his wiping after every wet. He may 
avoid the necessity of running in by hand if 
the iron tool has approximately double the di¬ 
ameter of the work. With a slight amount of 
overhang in the stroke, there is then no part 
of the tool which is always against the fluorite. 
The abrasive can be introduced at the center 
of the tool and caused to spread evenly by the 
motions of the machine. 

3. Polishing. Both on cloth and pitch laps, it 
was found that rouge, cerium oxide, or Barne- 
site produced many scratches and sleeks. More 
success was had with Linde A and Linde B, 
which are finely divided sapphire, the particles 
in the former being less than 0.3 in diameter 
and in the latter less than 0.1 \i. These mate¬ 
rials work faster and cleaner when wet with 
saturated salt solution instead of water. The 
technique followed involves (1) running the 
work rapidly on a cloth lap with Linde A, and 
(2) on a soft pitch lap (Burgundy) with 
Linde B at a moderate running speed (150 
rpm). To remove the sleeks formed in this step, 
the operator then (3) goes back to a cloth lap 
and Linde A. A minimum of pressure is used 
in these steps, i.e., the weight of the work and 
the stroke arm. Since figuring must be done on 
pitch, it will be seen that the conditions of good 
figure and freedom from sleeks are very diffi¬ 
cult to obtain simultaneously. When glass¬ 
working methods are applied to fluorite, the 
result is a maze of fine scratches. The surface 
looks like a piece of glass which has been pol¬ 
ished on a lap very badly contaminated with 
grit. 



332 


OPTICAL FLUORITE 


The pitch lap is made of Zopharlac, and the 
cloth lap of standard white polishing cloth such 
as manufacturers of spectacle lenses use. 

4. Edging. Disks may be edged on a hori¬ 
zontal spindle with loose grit and a strip of 
brass. 


7.5 OTHER ARTIFICIAL CRYSTALS 

During the latter part of the experimental 
phase of the work under Contract OEMsr-45, 
crystals of barium fluoride and strontium flu¬ 
oride were grown. The technique followed was 
very closely the same as for CaF 2 . Measure¬ 
ments of the refractive index were made at the 


at present. The barium fluoride sample shows 
considerable absorption, as is evident by visual 
inspection of the crystal. Other crystals, how¬ 
ever, give a visual impression of being highly 
transparent, and so it will be extremely desir¬ 
able to measure the transmission of additional 
samples. The cutoff wavelength for a BaF 2 
crystal 2-mm thick was estimated at 0.139 \i, 
while that for a crystal of SrF 2 was 0.125 \i. 


7 6 PRESENT STATUS OF DEVELOPMENT 
OF ARTIFICIAL FLUORITE 

Artificial CaF 2 crystals, grown from the melt 
as described in the preceding pages, have been 


Table 5. Refractive index of BaF 2 and SrF 2 . 


Wavelength 

(A) 

15 C 

SrF 2 

35 C 

55 C 

15 C 

BaF 2 

35 C 

55 C 

*7678.58 

1.435136 

1.434898 

1.434652 

1.470533 

1.470233 

1.469925 

7065.188 

1.435916 

1.435672 

1.435431 

1.471469 

1.471167 

1.470869 

6678.149 

1.436508 

1.436268 

1.436031 

1.472199 

1.471892 

1.471591 

6562.793 

1.436703 

1.436470 

1.436226 

1.472444 

1.472130 

1.471835 

*5892.62 

1.438082 

1.437848 

1.437606 

1.474127 

1.473815 

1.473520 

5460.740 

1.439245 

1.439008 

1.438771 

1.475561 

1.475255 

1.474951 

4861.327 

1.441402 

1.441172 

1.440932 

1.478237 

1.477926 

1.477634 

4358.342 

1.443964 

1.443742 

1.443505 

1.481421 

1.481112 

1.480819 

4046.563 

1.446085 

1.445857 

1.445632 

1.484054 

1.483753 

1.483468 


* Intensity-weighted means of doublets. 


National Bureau of Standards on 60-degree 
prisms made from 2-in. cylinders of BaF 2 and 
SrF 2 . The data obtained are shown in Table 5. 

These refractive indices were measured 9 in 
a stirred air bath on a spectrometer table at 
controlled air temperatures near 15, 35, and 
55 C by the method of minimum deviation. 
The probable errors are estimated to be not 
over ± 2 X 10 -6 ; systematic errors probably 
do not exceed two or three times the probable 
error. 

The surfaces of these prisms were not as flat 
as might be desired and it was found necessary 
to diaphragm the prisms in order to exclude 
the especially defective areas within 3 or 4 mm 
of the edges. The remaining apertures were 
used symmetrically during all measurements. 

The transmission characteristics for these 
materials have not been accurately determined 


found quite satisfactory for lenses in highly 
critical instruments such as aerial cameras. 
Multiplicity of the crystal does not appear to 
be seriously objectionable in lens applications. 
The principal disadvantage lies in the extreme 
fragility of the material. If the fluorite lens 
component is cemented between glass elements 
its fragility is somewhat overcome and at the 
same time the tolerance on surface accuracy 
may be relaxed. Soft cement is used for the 
purpose in order not to set up objectionable 
stresses. The lenses (two of them), made with 
a CaF 2 component placed between appropriate 
glasses, were almost completely color free. The 
other aberrations were small and after proper 
design apparently were in no way enhanced by 
the fluorite component. Details of these lens 
systems are given in Chapter 1. 

Undoubtedly a considerable amount of de- 








OPTICAL APPLICATIONS OF FLUORITE 


333 


velopment in manufacturing methods will be 
required to place the production of these crys¬ 
tals on a commercial basis. Studies of power 
rates and elevator rates as a function of crys¬ 
tal size must be continued in order to insure a 
high yield of good quality crystals. Annealing 
studies and the design of a new furnace for 
this treatment are clearly indicated. 

For infrared spectroscopy, the prisms ob¬ 
tained from 4-in. CaF 2 cylindrical crystals are 
in great demand. Lithium fluoride prisms, now 
used for this work, usually have an objection¬ 
able absorption band due to HF, which the 
fluorite does not possess. 

At the conclusion of the research described, 
the principal defect observable in the synthetic 
CaF 2 crystals appeared to be the lack of ho¬ 
mogeneity due to stress in the material. If 
annealing methods can be developed to the 
point where a high percentage of the crystals 
can be relieved of this fault, the successful use 
of CaF 2 components in optical systems of some 
size for removal of secondary spectrum seems 
assured. 

The stockpile of crystals produced at the 
Crystallographic Laboratory of MIT under 
Contract OEMsr-45 has been transferred to 
the National Bureau of Standards. These crys¬ 
tals will be available for use by government 
laboratories and, under certain circumstances, 
for noncommercial experimental use in private 
laboratories. 


7 7 OPTICAL APPLICATIONS OF FLUORITE 

The success attained in producing large disks 
of synthetic fluorite carried with it the possi¬ 
bility that the resolution of aerial lenses might 
be improved by its use. In 1942, information 
as to what factor most seriously limits resolu¬ 
tion in the air was very meager. It was consid¬ 
ered reasonable that the color errors known to 
be present in all standard lenses might very 
well be one of the serious barriers to better 
results. For this reason Harvard was requested 
to accept a contract, OEMsr-474, for the pur¬ 
pose of initiating and developing the use of 
synthetic fluorite in an aerial camera lens. 


From the early days of the German optical 
industry natural fluorite has been used in apo- 
chromatic or semi-apochromatic microscope 
objectives. No one in practice considered fluo¬ 
rite for other purposes, owing to the drastic 
limitation on available diameters and to the 
great monetary value attached to even small 
pieces. 

The use of fluorite in combination with 
quartz for apochromatic doublets, particularly 
for ultraviolet work, is well known. Equally 
well known is the fact that fluorite combines 
well with crown glass for visual objectives es¬ 
sentially free of secondary spectrum. 

Reference to the comments on secondary 
spectrum in Section 1.2.1 will reaffirm that 
only for long spectral ranges, as in color pho¬ 
tography, or for large lens diameters, will sec¬ 
ondary spectrum constitute a serious limita¬ 
tion to the quality of aerial photography. In¬ 
deed, yellow and red resolution tests of the 
100-in., //10 lens (Figure 76 of Chapter 1) 
show little difference, in spite of the design 
correction at 6250 A and visually observed 
aberration in yellow and green. In 1942 the 
point of view was less clear-cut, but it was 
realized that fluorite should be used for the 
larger lenses. A Harvard report notes that it 
was a standing request to the project on 
fluorite at MIT to produce as large a disk as 
possible. 

The development of camera lenses employing 
fluorite has already been discussed in Section 
1.3 in connection with aerial equipment. At 
this point it would seem expedient to add a few 
summarizing general remarks likely to aid fur¬ 
ther work along these lines. 

The index of refraction of fluorite is much 
too low for general utility in the design of 
aerial lenses, being only 1.43385 for the D line. 
In combination with achromatizing elements of 
crown glass, the situation is slightly improved 
by the co-existence of a very high v-value, 95.4, 
which in turn tends to result in weaker inter¬ 
nal curvatures. In practice, the curves are 
made as strong as correction of the aberrations 
so introduced will stand. It would appear that, 
even with prolonged designing, speeds which 
are greatly in excess of //8 over the large an¬ 
gular fields now known to be imperative are un- 



334 


OPTICAL FLUORITE 


attainable with fluorite apochromatic systems. 

The secondary spectrum characteristics and 
the high v-value of fluorite demand that fluorite 
be used solely for positive elements, if an 
apochromatic correction is desired. Great dam¬ 
age is imparted to the Petzval sum, which can¬ 
not be overcome in many familiar types of all¬ 
glass lens designs altered for the use of fluorite. 

In the 36-in. apochromatic lens which em¬ 
ploys fluorite, the Petzval sum was made suffi¬ 
ciently low in spite of the use of fluorite, by 
means of a long lens barrel. The resulting 
higher order astigmatism constituted the limit¬ 
ing aberration in the corners of the 9x9 picture. 
It might be possible to obtain a larger coverage, 
but the design would no doubt become very 
complex. 

The device of lengthening the lens barrel for 
obtaining shallow curves in spite of the low 
Petzval sum required is expedient only for 
small angular coverages. The disadvantage of 
fluorite to the Petzval sum would have been far 
more marked had not the design made good 
use of the material in the form of an elaborated 
Cooke triplet which contained a cemented 
triplet for the negative control component. In 
the Cooke form, one usually gains considerable 
net power for a given barrel length. 

Two additional effects helped the success of 
the Harvard design. The use of the fluorite at a 
low relative height in the lens system dimin¬ 
ished its effect on the Petzval sum. Moreover, 
much of the achromatizing of the outer positive 
elements was accomplished by means of nega¬ 
tive glass elements of the same or similar glass 
type. Thus, the fluorite had only to overcome 
the difference, amounting probably to about 
half of the required overall correction. Finally, 
the curvatures required at the low relative 
height were in the most symmetrical position 
within the Cooke triplet form, and behaved 
with good consistency over the entire required 
field. 

The chief defect remaining in the final lens 
design is higher order astigmatism in the 
corners of a 9x9 picture, produced by the large 
air space between the first and second elements. 
At large oblique angles, the tangential focus 
is pulled forward, before the middle elements 
have had a chance to compensate. This distance 


effect becomes very pronounced in the corners 
of the picture, even to the extent that the varia¬ 
tion of astigmatism with color becomes notice¬ 
able. On the other hand, the design chosen 
minimizes the overall aberrations, and within 
the included circle of a 9x9 picture gives opti¬ 
mum resolution. 

Almost the sole advantage of fluorite is the 
elimination of secondary spectrum. Its use in¬ 
troduces abnormally large monochromatic ab¬ 
errations and errors in spherical chromatism 
which present a real difficulty for the designer 
to overcome. In an apochromatic system it is of 
no use to replace the normal color blur by a 
monochromatic blur of the same image cross 
section. Consequently, the fluorite apochromat 
requires the designer to obtain unusual quality, 
in spite of unusual obstacles. Obviously, a ma¬ 
terial of high index and high v-value, accom¬ 
panied by suitable secondary spectrum charac¬ 
teristics, would produce a drastic improvement 
in apochromatic aerial lenses in speed, angular 
coverage, and resolution. 

Fluorite is a relatively insoluble crystal. It is 
mechanically weak and has a thermal expan¬ 
sion coefficient of 19 X 10~ 6 per degree centi¬ 
grade, double that of ordinary optical glass. 
Consequently, large diameter elements of 
fluorite cannot be cemented tightly to glass 
mates. The Harvard lenses made use of plasti¬ 
cized Transil Oil with success, even at extremely 
low temperatures. Fluorite lens elements are 
almost always biconvex and steeply curved. 
The accompanying thin edge is stretched 
around a thick center. Any sudden cooling will 
almost inevitably start a crack in the lens. 
Ordinary handling by the optician is haz¬ 
ardous. 

Fluorite has also a great change of index of 
refraction with temperature, amounting to 
—0.00001 per degree centigrade rise in temper¬ 
ature. The ensuing change in focus of a large 
apochromatic lens is many times the tolerable 
depth of focus. It is therefore imperative that 
aerial lenses making use of fluorite be thermo- 
stated. 

Along with the application of synthetic fluo¬ 
rite to aerial lenses, Harvard (Contract 
OEMsr-474) completed several other types of 
apochromatic instruments. 



OPTICAL APPLICATIONS OF FLUORITE 


335 


7-7,1 1.8-in., f/20 Apochromatic Triplet 

Fluorite Objective 

The first completed optical instrument em¬ 
ploying synthetic fluorite was a cemented trip¬ 
let telescope objective, 1.8 in. in diameter. The 
design was corrected for both spherical aber¬ 
ration and coma, in addition to a very thorough 
color correction. The system made use of nega¬ 
tive elements of C-l glass enclosing the single 
positive element of fluorite. The internal ce¬ 
mented surfaces represent such a drop of index 
and such relatively steep curves that full cor¬ 
rection is possible for both coma and spherical 
aberration. 

Tests made with this objective confirmed the 
almost complete absence of color. According to 
design figures, when F and C (wavelengths 
4,861 and 6,563 respectively) are combined, 
the discrepancy in focus at 5,500 A amounts to 
only 0.000170, still of the nature of ordinary 
secondary spectrum. It is stated that only un¬ 
certainty in glass indices prevents the residual 
from being zero. Even so, the secondary spec¬ 
trum has been reduced to very closely one-third 
of the normal glass value, and at //20 has 
brought a very long spectral range within the 
Rayleigh limit. 


7-7-2 6-in. Dialytique Apochromat Objective 

In November 1942, disks of fluorite as large 
as 3% in. in diameter were available in ad¬ 
vance of a completed design for an aerial cam- 


apochromatic color correction is achieved by 
means of a hyperchromatic combination in the 
converging beam of the telescope. Figure 15 
shows a cross section of the test instrument. 

The performance tests were conclusive. It 
was found by autocollimation that the resolv¬ 
ing power obtained agreed closely with theo¬ 
retical requirements, and that the scattered 
light found around the image was very slight. 
These results were obtained in spite of the fact 
that tests through crossed Polaroids showed the 
presence of considerable strain in the two 
fluorite disks used in the design. 

The telescope has never been completed, 
although a mounting was finished to hold all 
lens elements securely in line for testing pur¬ 
poses. It should be noted that such a color- 
corrected instrument will have considerable 
lateral color which must be eliminated by 
means of a special eyepiece design. Figure 16 


\ 

.NORMAL 

. ACHROIv 

1AT 













-""d 

lALYTIQUf 

: APOCHI 

ROMAT 

TC 

(WARDS LI 

♦ 

ENS 











4000 4500 5000 5500 6000 6500 7000 7500 

WAVELENGTH IN A 


Figure 16. Color curve of dialytique apochro¬ 
mat. 


reproduces the color curve of the ordinary 
glass achromat and the calculated curve of the 
dialytique apochromat. 


* 


IT 1 




Y 


AA 



k fl 

Hi 



vf 

L, 


OVERALL LENGTH = 90'.'438 
f = 119"51 
TR = 0"757 

Figure 15. The 6-in. aperture, f/20 dialytique 
apochromat. 

era. In order to ascertain the probable per¬ 
formance of fluorite with large aperture and 
lenses of long focal length, it was proposed 
under the Harvard contract to construct a 6-in. 
aperture apochromatic telescope, in which the 


7-7-3 A Folded Apochromatic Telescope 
and Collimator 

Figure 17 shows a larger diameter objective 
made in accordance with the specifications of 
the cemented triplet form of objective. The 
focal length is 70 in. and the clear aperture 
3.5 in. The complete length of the folded sys¬ 
tem amounts to only 30 in. and the weight to 
20 lb. 

The eyepiece shown in the drawing can be 
replaced by either a draw tube with light 
source of any description for use as a collima¬ 
tor, or by a Leica camera attachment for use as 
an analyzing telescope. 






















































336 


OPTICAL FLUORITE 


Reference to Figure 17 will show that light 
trapping for outdoor use is not sufficient. There 
is direct illumination falling on a considerable 
length of the eyepiece drawtube. In daytime 
application, to which it seems best suited for 
military purposes, it will be necessary to add 
additional stops and to adopt a fiber-black in- 


ment. The system as given is designed for 
optimum performance in e (5,461 A) light. 
Calculations show that the lens yields three 
coincident foci, based on aperture and diffrac¬ 
tion considerations, that occur at 4,700, 5,500, 
and 7,000 A. The maximum deviation is less 
than 10 per cent of the normal achromat error, 



Figure 17. Folded fluorite apochromat telescope. 


ternal coating, at least in the drawtube. A suf¬ 
ficient gain in light trapping might be made by 
shortening or inverting the drawtube arrange¬ 
ment. 

The specifications of the //20 objective are 
shown in Table 6. 


Table 6. Optical data of an apochromatic triplet. 


Surface 

Radius 

(in.) 

Thickness 

(in.) 

Glass type 

1 

33.22 

0.772 

C-2 

2 

20.14 

0.772 

fluorite 

3 

—25.59 

0.772 

C-l 

4 

—83.52 


... 


It is of interest to give more data for this sys¬ 
tem, inasmuch as the computations were carried 
to nearly an ideal level. The indices as used are 
shown in Table 7. 


Table 7. Indices of materials for apochromatic 
triplet. 


Tie 

Wd 

nr 

Glass type 

1.50952 

1.51200 

1.51798 

C-2 

1.43250 

1.43385 

1.43706 

fluorite 

1.51993 

1.52260 

1.52883 

C-l 


Thick lens elements are used in order to 
overcome any possible flexure caused by ce- 


for which F and C are usually combined. 
Longitudinal Aberration: 

Rim ray at //20 (D — e) —0.000007 

Rim ray at //20 (F — e) —0.000053 

Paraxial (F — e) —0.000081 

The telescopic image was found to be excel¬ 
lent and quite free from visual color aberra¬ 
tions. At a power of 350X or 100X to the inch 
of aperture, only a slight trace of astigmatism 
was found. There proved to be no observable 
variation in quality over the field of 1 in. di¬ 
ameter permitted by the mounting. 

This folded system seems to be thoroughly 
practical for a medium aperture apochromatic 
system of general utility. It is recommended 
that this form of telescope be considered by the 
Armed Forces as suitable for tripod mounting 
in the field, and that it be further modified for 
field work by installation of an inverting sys¬ 
tem. Even a brief examination of the colorless 
image will impress an observer who is accus¬ 
tomed to ordinary achromatic objectives. 


7.7.4 Yerkes Apo-Periscope Objective for 
Mark IV Periscope 

A more complete discussion of the optical 
characteristics of this objective is given in 
Chapter 10. The optical work was carried out 































































ASSOCIATED MATERIALS 


337 


at Harvard. The fabrication of the four air- 
fluorite surfaces required several times as 
much careful work as in the case of the three 
glass lenses. The glass surfaces were worked 
with unusual care in order to place the burden 
of any residual errors on the fluorite elements. 
Every attempt was made to finish the fluorite 
lens elements to similar quality, but with no 
final success. They were marred by several 
small holes surrounding pits in the surface. 
Continued polishing produced new pits and 
new holes (see Section 7.4). The final objec¬ 
tive produced a slightly hazy image, owing to 
light scattered from the four surfaces. It is 
believed that continued experimentation by a 
skilled optician trained in physical concepts 
would produce flawless fluorite surfaces. 


7 8 ASSOCIATED MATERIALS 

The primary purpose of the development of 
fluorite under NDRC was to provide a new 
optical material for the improvement of image 
quality in aerial camera lenses and other opti¬ 
cal instruments. It is debatable in the light of 
war experience whether the effort devoted to 
the development of fluorite might not have been 
more valuable if used to produce materials of 
higher index. In defense of the work on syn¬ 
thetic fluorite it should be noted that its manu¬ 
facture represents a technical triumph of con¬ 
siderable magnitude and that there undoubt¬ 
edly will appear many uses of ultimate impor¬ 
tance to science and to the country. 

A number of optical materials other than 
fluorite were considered in connection with the 
design of aerial lenses. None of these reached 
the state of perfection acquired by fluorite, 
owing mostly, it is believed, to the lack of suffi¬ 
cient experimentation by qualified laboratories. 

Table 8 lists a number of optical materials 
that merit notice. Although rare-earth glass 
represents one of the most important advances 
in glass technology in recent years, it was not 
an NDRC development. Its use is described be¬ 
low in connection with general comments on 
optical materials. 

Spinel. Of the materials listed in Table 8, the 
most desirable from the point of view of optical 


design, although not from a practical stand¬ 
point, are spinel, for use as positive elements, 
and the alkaline halides, for use as negatives. 
Information at hand on spinel is too uncertain 


Table 8. Optical constants of important materials. 


Material 

Index 

n-o 

Recip¬ 

rocal 

disper¬ 

sion 

Design 

utility 

Prac¬ 

tical 

utility 

rare-earth glass* 

1.745 

45.7 

excel¬ 

lent 

excel¬ 

lent 

spinel 

1.723 

64 

excel¬ 

lent 

doubt¬ 

ful 

lithium fluoride 

1.3915 

83.3 

poor 

poor 

lithium chloride 

1.662 

39.6 

fair 

poor 

lithium bromide 

1.784 

28.4 

poor 

poor 

lithium iodide 

1.955 

20.5 

poor 

poor 

sodium fluoride 

1.3258 

83.5 

poor 

poor 

sodium chloride 

1.5441 

42.8 

good 

poor 

sodium bromide 

1.6412 

32.1 

good 

poor 

sodium iodide 

1.7745 

22.0 

good 

poor 

potassium fluoride 

1.361 

80.2 

poor 

poor 

potassium chloride 

1.490 

44.0 

good 

poor 

potassium bromide 

1.5594 

33.2 

good 

poor 

potassium iodide 

1.6670 

23.3 

good 

poor 

rubidium fluoride 

1.398 

81.2 

poor 

poor 

rubidium chloride 

1.4936 

43.7 

good 

poor 

rubidium bromide 

1.5528 

33.9 

good 

poor 

rubidium iodide 

1.6474 

23.5 

good 

poor 

caesium fluoride 

1.478 

75.9 

poor 

poor 

caesium chloride 

1.534 

46.0 

fair 

poor 

caesium bromide 

1.582 

34.2 

good 

poor 

caesium iodide 

1.661 

23.5 

good 

poor 

jS-magnesia 

1.738 

53.5 

good 

poor 

calcium fluoride 

1.4338 

95.4 

poor 

good 

barium fluoride 

1.4741 

81.7 

fair 

good 

strontium fluoride 
cyclohexyl- 

1.4381 

93.2 

poor 

good 

methacrylate 

1.5064 

56.9 

fair 

good 

styrene 

1.5916 

31.0 

good 

good 

diamond 

2.419 

45 

excel¬ 

lent 

even¬ 

tually 


* There are a number of other types. 


for any conclusion to be made as to its second¬ 
ary spectrum characteristics. However, meas¬ 
urements on a poor sample indicate that spinel 
would combine well with the various flint 
glasses. Similarly, data on the characteristics 
of the halides are not at hand, but inspection, 
as well as computations based on patent litera¬ 
ture, indicate that some reduction of secondary 
spectrum is obtainable in combinations of the 
halides with the barium crown glasses. Conse¬ 
quently, it is likely that combinations of spinel 
and the halides will lead to systems with im¬ 
proved color correction. 







338 


OPTICAL FLUORITE 


Spinel is a water-white crystal with a hard¬ 
ness equal to that of sapphire. Measurements 
made at Harvard indicate a v-value of about 64 
with a D-line index of 1.723. From a design 
point of view such optical properties are ad¬ 
mirable. From a practical point of view, 
samples of spinel produced up to 1945 seem to 
have a considerable variation of index, amount¬ 
ing to about one unit in the third decimal place. 
Spinel would be very difficult to fabricate into 
lenses. From brief NDRC experience it would 
seem that it can be more easily polished with 
Linde A or B than with rouge or barnesite. As 
a front element for a military lens, once prop¬ 
erly made, it would be nearly ideal because of 
its nearly complete mechanical and chemical 
resistance. 

The Alkaline Halides. These materials can 
be grown synthetically and a few of those listed 
are available commercially. All the halides are 
water-soluble to some appreciable degree. How¬ 
ever, freshly polished surfaces untouched by 
fingers or deleterious atmospheric gases will 
survive for a long time in ordinary indoor use 
if protected against dampness and circulated 
air. 

The halides are unusual, compared to optical 
glasses, at least in part since many of them 
combine an unusually low v-value with a fairly 
average index of refraction. Thus, potassium 
bromide has the v-value of an extra dense flint 
glass, but the index of a light barium crown. 
Many photographic lens designs are in need of 
just such materials. 

The halides offer many practical disadvan¬ 
tages. Polished surfaces do not resist abrasion, 
and, exposed to military use, would soon de¬ 
teriorate beyond repair. Although it may prove 
possible to coat the surfaces with hard, non¬ 
reflecting films in spite of the obviously diffi¬ 
cult cleaning problem, it would seem more 
practical from a military point of view to pro¬ 
tect the halide elements by surrounding ele¬ 
ments of glass, preferably cemented. Owing to 
a relatively high thermal expansion, the ce¬ 
ment used would have to permit differential 
expansion without strain. 

Magnesium Oxide. Magnesium oxide or 
/3-magnesia has desirable optical properties, 
except that its secondary characteristics are in 


the wrong direction for a positive material. 
Apochromatic doublets of quartz and /8-mag¬ 
nesia have been designed in which the MgO is 
used as the negative material. Practically, the 
material is available (as of 1945) only in small 
sizes. Polished surfaces are short-lived in a 
moist atmosphere. 

The Fluorides. Calcium fluoride or fluorite 
has already been described in detail. Barium 
fluoride has been mentioned under Section 1.3 
in the description of an apochromatic telephoto 
lens. Its optical properties are sufficiently su¬ 
perior to those of fluorite to warrant substitu¬ 
tion in further apochromatic design work. 
Attention is drawn, however, to the need for 
further studies of the transparency of barium 
fluoride. The only existing data, although very 
uncertain, indicate that the transmission is too 
low for use in normal lens thicknesses. Before 
new design work is accomplished, the trans¬ 
mission curve of barium fluoride should be well 
established. 

In addition to calcium fluoride and barium 
fluoride, measurements were made on stron¬ 
tium fluoride. The secondary spectrum charac¬ 
teristics proved to be intermediate between 
fluorite and barium fluoride. Strontium fluoride 
is therefore superior to fluorite, and would 
serve as a substitute for barium fluoride, 
should the latter be found inadequate in trans¬ 
parency. 

Plastics. Any endeavor to design a fully cor¬ 
rected photographic lens of normal field angle, 
making use of only cyclohexylmethacrylate 
[CHM] and styrene, is fraught with many dif¬ 
ficulties. The two materials are essentially an 
old style pair and for many applications are in 
violent conflict with the requirement of the 
Petzval sum. It is possible to overcome the ob¬ 
jection by employing designs with large lens 
thicknesses, field flatteners, and perhaps 
aspheric surfaces. But whatever is possible 
along these lines with plastic is less difficult 
with optical glasses, which in turn would lead 
to better glass lens designs. In general, the de¬ 
signer’s work is materially increased when only 
plastics are used. 

Viewed in combination with glass types it is 
quite possible that plastic lenses have more to 
offer. Styrene is exceptionally valuable as a 



ASSOCIATED MATERIALS 


339 


negative material because of the low v-value 
for its index. In many applications styrene will 
excel any available glass. The several alkaline 
halides with similar optical properties are in¬ 
ferior to styrene from a practical point of view. 
Styrene has a large dependence on tempera¬ 
ture, relative to change of index and volume ex¬ 
pansion. Because of the unusual dielectric 
strength of styrene, any unprotected surface of 
styrene soon collects large quantities of dust 
and lint. A coating of a slightly conducting ma¬ 
terial will remove this difficulty. 

Cyclohexylmethacrylate has unusually good 
secondary spectrum characteristics to the ex¬ 
tent that combined with medium flint glass 
secondary spectrum in the visual range is 
almost completely removed. For systems where 
plastic optics are applicable, this favorable 
color correction ought not be overlooked. It 
would be possible to design an aplanatic, nearly 
apochromatic, cemented triplet of CHM and 
DF-2 that would serve well for low power in¬ 
struments covering a long spectral range. 

In the preceding comments it will be under¬ 
stood that the range of requirements of optical 
designs is so great that many useful applica¬ 
tions of any one material might be found and 
emphasized. 


7,81 General Considerations of the Optical 
Properties Needed in Photographic 
Lens Design 

It is worth while to point out here a few 
generalities gained from NDRC experience, 
likely to be valuable in further work. There 
exists no single rule governing the utility of a 
given optical material. However, if comments 
can be confined to the subject of photographic 
lens design, it becomes possible to isolate a 
few guiding principles. 

Quite frequently in wide-angle systems it is 
necessary to use strong meniscus lenses, curved 
around a central stop. The smaller the field 
angle, the less important it becomes to curve 
the lens elements around the stop, and the more 
important it becomes to maintain a short lens 
barrel relative to the equivalent focal length. 
Then for relatively small angular fields, where 


higher order astigmatism is of no special con¬ 
sequence, diminishing to the microscope re¬ 
quirements for negligible fields for which even 
primary astigmatism is usually neglected, the 
lens barrel once more can become lengthened 
for the sake of additional lens speed and opti¬ 
mum spherical correction. In the wide-angle 
type of lens, particularly where it is desired to 
obtain increased speed, it is important to use 
high-index positive materials and to use as thin 
lens elements as practicable from a design 
point of view. It should be noted that there ex¬ 
ists a fairly uniform progression from wide- 
angle systems strongly curved around a stop to 
microscope systems, whose surfaces more 
nearly conform to advancing wave fronts, in 
order that the sine theorem be satisfied at ex¬ 
treme convergence angles. 

For lens systems corrected for distortion, it 
is necessary to make numerous compromises 
for the purpose of maintaining speed and angu¬ 
lar field. There is no doubt that rare-earth glass 
or any material of similar index is of consider¬ 
able importance in such an application. The 
standard Aero-Ektar //2.5 lens makes good 
use of rare-earth glass elements to obtain coin¬ 
cidence of tangential and radial field curva¬ 
tures to the very corner of its 5x5 picture size. 
This type of field correction is very much to be 
desired and represents a distinct advance over 
the usual correction involving astigmatic dif¬ 
ferences for the purpose of flattening the mean 
field. The chief aberration of the Aero-Ektar is 
oblique spherical aberration which is charac¬ 
teristic of designs based on the Opic or Biotar 
prototypes. 

The v-value of rare-earth glass is usually still 
too low to please a lens designer. The use of a 
positive material like spinel would make avail¬ 
able a whole variety of indices for the negative 
elements, capable of overall color correction 
with favorable curves. 

Any positive element in a lens system occu¬ 
pying a position of large incident ray height 
will profit by having relatively high index. If 
at the same time its v-value is high, it becomes 
more possible with existing negative materials 
to obtain satisfactory color correction along 
with low index for a favorable Petzval sum. As 
the relative height of the ray becomes smaller 



340 


OPTICAL FLUORITE 


in the system, the importance of the high in¬ 
dex diminishes. In any given situation the 
high-index material for a positive element is 
generally better, but the degree of improve¬ 
ment depends on the ray height and the re¬ 
quirement. Similar comments obtain for nega¬ 
tive elements. At large ray heights, consider¬ 
able gain for the Petzval sum is obtained by use 
of low index. At small ray heights, it is gen¬ 
erally more profitable to use high-index nega¬ 
tive materials. 

Very often, thickness or air space in a 
strongly converging or diverging beam is of 
greater advantage than the high index, par¬ 
ticularly for fast lenses of small coverage. Then 
again, use of a low-index negative material 
generally near the stop in a symmetrical sys¬ 
tem with its resulting steep curves, produces 
better field corrections than a high-index ma¬ 
terial. For lenses of wide angular coverage, 
great lens thickness for the purpose of gaining 
power is usually a source of large astigmatism 
of higher order. In turn, the introduction of 
negative materials like the alkaline halides 
with their very low v-values will make color 
correction fairly easy when used with ma¬ 
terials of intermediate v-value. 

In systems requiring an exceptionally good 
correction for spherical achromatism, it is 
often necessary to adjust the v-values of posi¬ 
tive and negative materials according to the 
various vergences of the system. No fixed rule 
can be applied, inasmuch as the monochromatic 
correction can lead to such a variety of forms. 
It is obviously advantageous in such a problem, 
and in fact in all problems, to have at hand nu¬ 
merous materials of high quality covering the 
diagram of index versus v-value. It is by no 
means imperative that positive elements have 
high indices of refraction. Very many success¬ 
ful designs developed under NDRC in the war 
have inverted pairing and obtain correction of 
the Petzval sum by air spacing and lens 
powers. The Zeiss Telikon and the NDRC 36-in. 
wide-angle telephoto make use of such normal 
or old-style achromats. It is of interest to note 
that many successful NDRC lenses might have 
been constructed in 1885, in so far as use of 
glass types is concerned. 

Generally speaking, at low relative heights in 


the lens system, the index of the negative ele¬ 
ment must be high, rather than low for best 
results. Also, the flatter curvatures that often 
accompany high-index negative elements tend 
to favor zonal spherical correction in narrow- 
angle systems. Finally, the paired old-style 
achromat often keeps oblique spherical aberra¬ 
tion and higher order astigmatism under con¬ 
trol. 

One of the very remiss features of the optical 
industry in the United States is the universal 
failure to describe the physical properties of 
various types of filter and optical glasses. Even 
in German catalogs the ultraviolet and infra¬ 
red transmissions for optical glass are omitted. 
Except for usual experience, the optical de¬ 
signer either works by analogy with German or 
English literature, or by guess. For military 
purposes, data required are transmission, pre¬ 
cise indices of refraction at all accessible wave¬ 
lengths, hardness, and durability. Less impor¬ 
tant are striation and bubbles, except for spe¬ 
cial applications, but these properties should 
be given also. The thermal properties, includ¬ 
ing volume expansion and change of index 
should be noted. Finally, some idea of relative 
costs, reproducibility, availability, and volume 
of production should be given. Too much detail 
is far better than too little in optical design 
work. It is to be regretted that no project under 
NDRC existed for the determination and re¬ 
porting of such data to all optical designers. It 
is strongly recommended that the Armed Serv¬ 
ices sponsor the procurement of such data, in¬ 
cluding experiences in the field, preferably by 
a laboratory divorced from commercial con¬ 
siderations. 

79 RECOMMENDATIONS BY NDRC 

1. A method for growing single crystals 
should be developed. This can probably be done 
without any considerable change in present 
technique. Single crystals would be desirable, 
partly to reduce strain which may be impos¬ 
sible to remove by annealing, and partly to 
facilitate the production of a uniform figure on 
a polished surface. 

2. Larger crystals of fluorite should be 
grown as soon as possible, since there are nu- 



RECOMMENDATIONS BY NDRC 


341 


merous applications for which they could be 
used to advantage. 

3. The technique for growing crystals of 
barium fluoride should be developed further, in 
view of the fact that this material will prob¬ 
ably be superior to fluorite for optical appli¬ 
cations. Its transmission throughout the spec¬ 
trum should be measured accurately as soon as 
possible, since preliminary measurements in¬ 
dicate considerable absorption, even in the 
visible part of the spectrum. Quite possibly 
these measurements are not representative of 
average crystals free from impurities. 

4. The most effective annealing schedule for 
fluorite crystals should be fully investigated. 
Particular attention should be given to the 
question whether it is possible, by any schedule 


of annealing, to relieve strains set up at the 
interfaces of individual crystals. These studies 
should be carried out on a quantitative basis, 
with measurements of strain made in various 
parts of the crystal before and after each run 
in the annealing furnace. 

5. Further studies should be made to deter¬ 
mine whether synthetic calcium fluoride can be 
used as the raw material. Particular attention 
should be given to various methods for elimi¬ 
nating adsorbed water. 

6. Programs should be undertaken that aim 
at making other crystals available in large 
sizes for optical use, including spinel in par¬ 
ticular and some of the alkaline halides. It is to 
be hoped that rare-earth glass will soon be 
available for general use in large sizes. 



Chapter 8 

OPTICAL PLASTICS 

By Sidney W. McCuskey a 


si INTRODUCTION 

T he heavy burden imposed on the optical 
industry by the demand for fire-control in¬ 
struments in great numbers led the NDRC in 
1940 to investigate plastic substitutes for glass. 
The objectives of the program, undertaken by 
the Polaroid Corporation, may be summarized 
as follows: 

1. To supplement the supply of optical glass 
with improved optical plastics. 

2. To devise methods of manufacture of op¬ 
tical parts using unskilled or semi-skilled labor 
not drawn from the optical industry. 

3. To synthesize new plastics in the hope of 
overcoming objectionable features of those cur¬ 
rently in use. 

Optical designers are constantly on the look¬ 
out for materials which provide unusual com¬ 
binations of index of refraction and reciprocal 
dispersion (v- value). Furthermore, in the de¬ 
sign of many instruments lens surfaces of high 
curvature and of aspheric shape are highly de¬ 
sirable. Limitations of glass in meeting these 
requirements are well known and the use of 
plastics was considered worth investigating. 

Plastic materials available in 1940 for opti¬ 
cal purposes, primarily methyl methacrylate 
and styrene, had many disadvantages. Among 
these the most important were: inhomogeneity, 
high water-absorption, surface inaccuracy, im¬ 
perfect retention of figure, high temperature 
coefficients of refractive index and of expan¬ 
sion, low softening temperatures and low abra¬ 
sion resistance. Some of these disadvantages 
have been overcome by the use of new materi¬ 
als and others have been circumvented by new 
design. Optical plastic elements for instru¬ 
ments with moderate magnification and aver¬ 
age field of view can now be made with a qual¬ 
ity which is at least comparable with that of 
corresponding optical glass systems. 

a Warner and Swasey Observatory of the Case School 
of Applied Science. 


CHM and Styrene. The chemical and fabri¬ 
cation research to be described in the follow¬ 
ing pages yielded two plastics having proper¬ 
ties suitable for precision optical parts, namely, 
polycyclohexylmethacrylate [CHM] and poly¬ 
styrene [styrene]. All fire-control instruments 
designed and manufactured by Polaroid em¬ 
ployed one or both of these plastics in the opti¬ 
cal system. They are well adapted to the pro¬ 
duction of achromatized doublets, as is evi¬ 
denced by their optical constants: CHM, 
n D = 1.50645, v-value 56.9; styrene, v , D 
— 1.59165, v-value 31.0. Thus CHM corre¬ 
sponds to crown and styrene to flint in a glass 
combination. 

Both of these plastics are linear polymers. 
CHM is very similar to methyl methacrylate, 
commercially known as Lucite. Polystyrene as 
used for optical elements is a highly purified 
form of the styrene which has been found use¬ 
ful for many other purposes. CHM is superior 
to its predecessors in having low water-absorp¬ 
tion, as well as a shrinkage on polymerization 
of only 12.5 per cent against 22 per cent for 
methyl methacrylate. Its high boiling point, 
210 C, makes possible polymerization at a 
higher temperature without causing bubbles 
due to boiling. As a result, CHM can be cast 
with satisfactory internal and surface homoge¬ 
neity. It has a low softening temperature, how¬ 
ever (about 70 C) and is easily scratched. 

Although styrene has somewhat higher 
shrinkage (16 per cent) and has a yellowish 
tinge when cast, it is the best high-index plastic 
thus far found. 

The details of research in the chemistry and 
fabrication of optical plastics as carried on by 
the NDRC contractor will be given in the fol¬ 
lowing pages. 

82 CHEMICAL RESEARCH 

Little has been published on the possibilities 
of methacrylic compounds and styrene as sub- 


342 



CHEMICAL RESEARCH 


343 


stitutes for glass in the manufacture of optical 
elements. It is known, however, that the excel¬ 
lent optical properties of both the methacry¬ 
lates and styrene were recognized. A. H. 
Pfund 1 of Johns Hopkins University made dis¬ 
persion measurements on a 30-degree prism 
ground and polished from methyl methacrylate 
polymers prepared by the E. I. Du Pont de 
Nemours Company. The refractive index n D 
was found to be 1.4893, and the dispersion 
n F — n c was 0.0085. The v-value was calculated 
as 57.6. However, the results of any other in¬ 
vestigations which may have been made on the 
optical properties of this material were not 
stated. 

The possibility of polymerizing methyl 
methacrylate at high temperatures, especially 
under pressure, in glass molds was recognized 
as early as 1936. 2 A similar molding process 
was developed in 1937, 3 wherein monomeric 
methyl methacrylate is heated with a polymer¬ 
izing catalyst, such as benzoyl peroxide, to 
form a partially polymerized ester which is 
subsequently completely polymerized under heat 
between glass plates. 

However, the specific application of methyl 
methacrylate and styrene to the manufacture 
of optical elements had been mentioned but 
little, if at all, prior to 1940. Ellis' comprehen¬ 
sive article on polystyrene, 4 which discusses a 
large variety of uses of the material, does not 
specifically mention the possibility of its use in 
optical elements. 

821 Glasses—Inorganic and Organic 

Optical Properties of Matter 

The optical properties of matter are depend¬ 
ent upon the interaction of the atoms and ions 
of a given material with light. The theory of 
dispersion of light by matter connects this in¬ 
teraction with the electronic structure of the 
atoms and ions involved and with their geo¬ 
metrical arrangement in the substance under 
consideration. The complex index of refraction 
combines the two interactions, which are usu¬ 
ally considered separately as absorption and 
diffraction. Optical systems, in general, should 
exhibit as little absorption as possible for the 
wavelengths of light (ultraviolet, visible, or in¬ 


frared) for which they are designed, except 
in the case of filters in which an exactly con¬ 
trolled absorption is required within a certain 
spectral range. Absorption takes place if cer¬ 
tain electrons of the atoms, ions, or molecules 
in the material are raised to a higher energy 
level by absorbing a light quantum of the inci¬ 
dent radiation. Diffraction is the scattering of 
light by the elementary particles of the sub¬ 
stance under consideration and depends upon 
their polarizability. 

Inorganic Optical Materials. Inorganic 
glasses of manifold chemical composition have 
served for optical purposes successfully for a 
long time. They consist in principle of a frame¬ 
work of silicon and oxygen atoms, in which are 
embedded ions of various metals, mostly be¬ 
longing to the alkalis or alkaline earths. The 
silicon-oxygen skeleton furnishes a solid back¬ 
bone of the whole structure, particularly for 
hardness and high softening range, and the 
ions which are distributed in the framework 
permit the adjustment of the refractive index 
and the dispersion of the system within certain 
limits. 

The energy of dissociation of an Si-0 bond 
in silica is comparatively high, namely, 89.3 
kcal per mol and, because of the small atomic 
(or ionic) volume of silicon and oxygen (the 
ionic radii are 0.39 A and 1.32 A respectively), 
the specific spatial bond density is very high. 
The consequences are high softening point, low 
coefficient of thermal expansion, low compres¬ 
sibility, and great surface hardness, all factors 
favorable for the use of such systems in the 
construction of precision optical elements. On 
the other hand, the tetravalency of silicon and 
the divalency of oxygen results in the existence 
of a dense three-dimensional network of strong 
bonds, which is not favorable for the release of 
stresses and for the quick dissipation of me¬ 
chanical energy within the system. Therefore, 
very careful annealing or tempering is neces¬ 
sary to obtain glass optical elements free of 
strain and even these still exhibit a consider¬ 
able degree of brittleness. 

Organic Optical Materials. The technology 
of organic high polymers has recently been de¬ 
veloped to the point of yielding materials which 
possess a number of properties valuable for the 



344 


OPTICAL PLASTICS 


production of optical elements. These polymers 
consist of a backbone of carbon-carbon bonds, 
which have a dissociation energy of about 72 
kcal per mol, and, if three-dimensionally ex¬ 
tended throughout space, lead to the very dense 
and tough lattice of diamond. If two of the four 
valences of each carbon atom are not saturated 
by another carbon atom, but are occupied by 
hydrogen or other substituents, such as OH, 
NH 2 , Cl, CH 3 , C 6 H 6 , long chain molecules re¬ 
sult which are held together internally by co¬ 
valent bonds of high dissociation energy (70 to 
90 kcal). The cohesion of the chains with each 
other is effected by van der Waals forces, hav¬ 
ing dissociation energies between 4 and 10 kcal 
per mol. Because two kinds of forces are re¬ 
sponsible for the cohesion of such systems and 
because the hydrogen atoms or other substit¬ 
uents along the chain molecules have a certain 
degree of bulkiness or space requirement, the 
specific bond density in such organic polymers 
is not as high as in inorganic glasses. There¬ 
fore, organic glasses have lower softening tem¬ 
peratures, higher coefficients of thermal expan¬ 
sion, higher compressibility, and lower surface 
hardness but, because of the absence of ele¬ 
ments of a high atomic number, are much 
lighter than their inorganic counterparts. 

These organic glasses can be made from 
many easily available materials and can be 
polymerized in various ways. They can be cast 
or molded with a considerable degree of accu¬ 
racy, have little residual color, and can be ob¬ 
tained in a high degree of clarity. The absorp¬ 
tion and refraction of such organic polymers 
can be varied over a wide range by introducing 
either elements of a high atomic number or by 
distributing along the main valence chains 
organic groups or residues of high polarizabil¬ 
ity, such as conjugated double bonds or con¬ 
densed aromatic ring systems. Typical of these 
plastics are polystyrene, polymethyl methacry¬ 
late, and polyvinyl acetate, which have been 
used for many kinds of optical elements of low 
precision, such as goggles, cheap camera lenses, 
and similar products. 

Aims of the Research 

In the research directed at the use of these or 


analogous materials in the production of pre¬ 
cision optical elements, the basic tenets thought 
mandatory for success were: 

1. Use of raw materials of the utmost 
purity. 

2. Careful exclusion of dust and dirt from 
every part of the chemical and fabrication 
process. 

3. Adjustment of the elementary optical 
properties such as refraction and dispersion to 
the required qualities by incorporating the 
proper chemical elements or groups into the 
monomer or monomers. 

4. Regulation of the mechanical and thermal 
properties of the material by obtaining the 
proper bond density and the most favorable 
nature of the covalent backbone by controlling 
the degree of cross-linking in the system. 

5. The obtaining of a sufficient chemical re¬ 
sistance and dimensional stability against 
moisture absorption by avoiding the presence 
of too many hydrophilic groups or by protect¬ 
ing such groups with the aid of large lipo¬ 
philic residues, such as cyclohexyl, isobutyl, 
and the like. 

6. The carrying out of the polymerization 
reaction in such small and controlled steps that 
there result optical elements containing a mini¬ 
mum of frozen-in strains. 


8 ' 2 ' 3 Properties Demanded of Optical 
Plastics 

The following are the requirements which 
any plastic must fulfill to be satisfactory for 
casting into precision optical elements: 

1. Homogeneity. Refractive index constant 
throughout a given sample within 2 X 10 -6 . 
Striae, bubbles, and other local defects substan¬ 
tially absent. 

2. Transparency. High transmission (for 
most applications, 90 per cent or more per 
10-cm path). 

3. Freedom from haze. 

4. Reproducibility with respect to refractive 
index and v -value. 

5. Freedom from color. Nonselective trans¬ 
mission of light, at least over all the visible 
wavelengths, and in the ultraviolet. 



CHEMICAL RESEARCH 


345 


6. Stability. Stable to heat, light, and ultra¬ 
violet radiation. 

7. Toughness. Shatter resistant. 

8. Hardness. Capable of receiving and hold¬ 
ing optical finish and the desired figure. High 
scratch resistance desirable but not absolutely 
necessary. 

9. Loiv polymerization shrinkage. Low 
shrinkage during polymerization contributes 
to the avoidance of strain. 

10. Ease of polymerization, (a) Rapid poly¬ 
merization, (b) homogeneous polymerization, 
which requires that the polymer be soluble in 
the monomer, and (c) high boiling point of 
monomer, to allow rapid polymerization at 
comparatively high temperatures without boil¬ 
ing and the consequent occurrence of bubbles. 

11. Ease of synthesis. Methods of chemical 
synthesis amenable to quantity production. 

12. Availability of raw materials. Raw ma¬ 
terials preferably readily available in current 
market. 

13. Water absorption. Absorption must be 
low enough to have a negligible effect on the re¬ 
fractive index of the material. 


8,2,4 The Problem of Inhomogeneity 

Visible inhomogeneity was associated with 
variations of refractive index within the resins 
and was readily observed at the peripheries of 
samples of methyl methacrylate which were 
polymerized in cylindrical bottles. Three fac¬ 
tors probably account for most of the lack of 
homogeneity. 

Monomer Loss 

Since polymerization is a reaction which 
does not go entirely to completion, it is possible 
that some monomer remains to plasticize the 
polymers. The inhomogeneity of the resins may 
be caused by a more rapid evaporation of the 
residual monomer from the surfaces of the 
polymers than by diffusion through the masses 
to the surfaces. If the evaporation at the sur¬ 
faces proceeds at a faster rate than diffusion 
from the center to the surface, then the ma¬ 
terial at the surface will differ from that at the 
center of the mass. 


Many efforts were made to rid the castings 
of residual monomer. Monomeric methyl meth¬ 
acrylate was polymerized through heating 
under high vacuum, but this method proved 
slow and awkward and never fully succeeded 
in ridding the sample of monomer. An attempt 
was then made to vacuum and heat dry a fine 
molding powder, with the idea that the residual 
monomer could be driven more readily from 
the increased surface area of the powdered 
polymer. Successive weight measurements of 
the treated powder indicated substantial free¬ 
dom from monomer. However, it was found im¬ 
possible to mold the powder into a completely 
homogeneous mass; complete fusion of the 
particles was never obtained. 

Absorption of Water 

Since the atmosphere in which the material 
exists is constantly changing in temperature 
and relative humidity, the plastic is, with re¬ 
spect to its water content, seldom at equilib¬ 
rium with its surroundings. Where water is 
absorbed in the plastic, a change in refractive 
index may occur. This change is a complicated 
phenomenon; one cannot predict even its direc¬ 
tion. In the case of methyl methacrylate, we 
might expect the refractive index of the final 
product tp be decreased, since the refractive in¬ 
dex of water is lower than that of the com¬ 
pound. Curiously, the index often increases in¬ 
stead. The material becomes denser and serious 
inhomogeneity arises. 

The absorption of water either from the 
vapor phase or from liquid water can be caused 
by two different phenomena. 

1. If there are hydrophilic groups such as 
OH, NH 2 , COOH, CONH distributed through¬ 
out the polymer, a certain amount of true solu¬ 
tion of water takes place. In such cases a re¬ 
versible equilibrium is established between the 
water in the vapor phase outside the polymer 
and the water inside it. Any change of the par¬ 
tial pressure outside is followed by a change of 
the equilibrium moisture content of the poly¬ 
mer. Because of the slowness of diffusion, there 
is usually a considerable lapse of time before 
the equilibrium moisture content is reached. 
Polymers having high solvent power for water, 
therefore, change such properties as density, 



346 


OPTICAL PLASTICS 


refractive index, modulus of elasticity, slowly 
and uncontrollably as the outside conditions 
change. Hence, high water solubility is un¬ 
favorable to the use of a polymer for optical 
purposes. 

The ultimate cause of high moisture content 
at equilibrium is the capacity of the atomic 
groups mentioned above to bind water with 
considerable strength. In many cases the bond 
between the polar group of the polymer and the 
water exceeds considerably the normal van der 
Waals attraction, and in the case of cellulose 
and water can be as high as 16 kcal per mol. 
As it is sometimes impossible to avoid com¬ 
pletely the presence of such polar groups, be¬ 
cause they contribute to the refractive index 
of the material, the working hypothesis has 
been formulated that one might successfully 
protect such polar groups in a polymer against 
the access of the water molecules by a hydro¬ 
carbon shield. This “shield” hypothesis has 
been very useful in achieving lower moisture 
content, and, in particular, has led to the de¬ 
velopment of cyclohexylmethacrylate for opti¬ 
cal purposes. Its guidance in any continuation 
of this work is recommended. 

2. In many polymer systems of colloidal na¬ 
ture there exists another mechanism for water 
absorption, namely, the capillary condensation 
of vapor inside the submicroscopic cavities of 
the polymerized substance. This type of water 
binding is of great importance for fibers and 
films, but there is no indication that in highly 
consolidated systems any such capillary con¬ 
densation has taken place. 

Variations in Cross-Linking 

Another cause for inhomogeneities in the 
molded polymer, which seems to be particularly 
important in the case of cross-linked resins, 
should be mentioned. The polymers ordinarily 
associated with plastics of the type discussed 
here consist of long chains of molecules. They 
are the so-called linear polymers. Cross-linked 
polymers, on the other hand, are those in which 
the chainlike structures are bound together by 
interlocking molecules at various points. If not 
very carefully controlled, cross-linking may 
proceed too rapidly within certain parts of the 
polymerizing mass and may produce material 


of higher density than in the surrounding 
region. Variation in the refractive index of the 
optical element may result. Such localized for¬ 
mation of highly cross-linked fractions has 
been observed in the polymerization of buta¬ 
diene and other conjugated monomers and has 
offered considerable difficulty in making com¬ 
pletely homogeneous polymers. 


82 5 Cyclohexylmethacrylate 

Cyclohexylmethacrylate 5 was used to test the 
foregoing theory of shields. Disks 1 in. in 
diameter and approximately Vs in* thick, of 
polymerized methyl methacrylate and of CHM 
respectively, were weighed, immersed in boil¬ 
ing water for 1 hr, removed, and weighed 
again. The methyl methacrylate disks absorbed 
approximately 1 per cent of their weight in 
water, indicating a comparatively high degree 
of water absorption, while those of CHM ab¬ 
sorbed one-tenth as much. This is approxi¬ 
mately the same as the value for styrene, which 
had previously been considered unique in its 
property of low water absorption. There was no 
observable inhomogeneity at the periphery of 
the CHM cylinders. 

This result would seem to indicate that the 
main factor correlated with inhomogeneity is 
water absorption. At least two supplementary 
explanations exist, however, to account for the 
superior homogeneity of CHM. The first is that 
the volatility of CHM monomer is considerably 
lower than that of monomeric methyl metha¬ 
crylate. This may mean that any residual 
monomer in the polymer vaporizes from the 
surface at a rate sufficiently slow to allow the 
monomer to diffuse evenly throughout the en¬ 
tire mass. 

A possible supplementary explanation is that 
CHM polymerizes more completely than does 
methyl methacrylate under the conditions 
available for promoting polymerization. It is 
not always easy to prove that one compound 
polymerizes more fully than another, and with 
CHM proof is lacking. 

Whichever explanation is correct, theory and 
practice require that it be possible to synthesize 
a monomer which has a high boiling point (and 



CHEMICAL RESEARCH 


347 


low volatility) and which is capable of casting 
a nonpolar shield about the carboxyl group in 
the molecule. The methacrylates fulfill these 
conditions better than other types of resins and 
hence the chemical research has been concen¬ 
trated primarily upon their production. Among 
methacrylates other than methyl and cyclo¬ 
hexyl, which achieve these two conditions, may 
be mentioned phenyl, menthyl, and benzyl 
methacrylate and their substitution products. 

To predict low water absorption, a generali¬ 
zation which seems well sustained in practice is 
that the ester will be much less soluble, and its 
polymer for all practical purposes insoluble, 
when the alcohol from which the ester is syn¬ 
thesized has low water solubility. 

Other advantages which accrue with an ester 
of the type just described include a compara¬ 
tively low shrinkage during polymerization. 
Shrinkage is smaller because the reactive part 
of the monomer ester is a smaller fraction of 
the monomer molecule. It may be noted that the 
molecular shrinkage of all the methacrylates 
is approximately the same, but that the molecu¬ 
lar weight of CHM is 1.68 times that of methyl 
methacrylate. The shrinkage of CHM (12.5 per 
cent) is thus proportionately less than that of 
methyl methacrylate (22 per cent). A low 
shrinkage during polymerization results in 
simplification of the problems of fabrication 
because of less physical distortion and shrink¬ 
age of the resin in the mold. 

A further advantage of esters with high 
molecular weights, such as CHM, lies in their 
high boiling points (210 C for CHM as com¬ 
pared with 100 C for methyl methacrylate). A 
high boiling point makes it possible for poly¬ 
merization to take place at higher tempera¬ 
tures without causing bubbles due to boiling. 

As a result of these advantages, when CHM 
is cast, local inhomogeneities, such as striae 
and bubbles, and premature separation from 
the mold, can be largely overcome. Styrene, 
when employed in a purified form, likewise can 
be cast with satisfactory homogeneity. In the 
forming of polymeric masses, the manufacture 
of organic resins achieves greater efficiency 
than is possible with inorganic glass, where the 
percentage of glass which must be rejected is 
very high, especially with large masses. 


826 Styrene 

CHM must be supplemented for optical de¬ 
sign purposes by another resin. For simple 
achromats and for Porro prisms, a high index 
of refraction is required. 

Styrene possesses a relatively high index 
(n B = 1.59165). It is readily available in quan¬ 
tity at low cost, with no prospect of ultimate 
shortages. Its water absorption is about the 
same as that of CHM. It is tough and trans¬ 
parent. Shrinkage during polymerization 
amounts to 16 per cent. While this is a fairly 
large shrinkage, styrene has relatively low 
viscosity for a given concentration of polymer 
in monomer, and this permits successful fabri¬ 
cation. 

For these reasons, styrene was selected as a 
high-index material. Unpurified styrene unfor¬ 
tunately possesses certain defects. It is softer 
than methyl methacrylate and CHM, and even 
in small masses has a faint yellowish tinge and 
a small amount of haze. When fabricated, it 
tends to craze or crackle at the surface. Its rate 
of polymerization is slow, thus lengthening the 
time cycle required per unit to prepare the 
optical elements. 

Purified Styrene. The use of purified styrene, 
when combined with special polymerization pro¬ 
cedures, successfully avoids serious yellowish 
tinge and reduces haze considerably, although 
residual amounts are noticeable in large-sized 
optical elements. A method for purifying 
styrene will be described later. 


8 2 7 Synthesis of Other Plastics 

While CHM and styrene formed the combi¬ 
nation of plastics most suitable for optical 
systems, a large amount of chemical research 
was devoted to the synthesis of other materials. 
In general the objectives of this research were: 

1. To find plastic materials having unusual 
combinations of refractive index and v-value. 
A plastic having a high index and at the same 
time a high r-value would have distinct advan¬ 
tages in the design of optical systems. Some¬ 
thing comparable to dense barium crown glass 
was sought. 



348 


OPTICAL PLASTICS 


2. To replace styrene with a material having 
less shrinkage and haze and which would poly¬ 
merize more rapidly so that a shorter baking 
cycle could be employed. 

3. To provide a plastic with unusually low 
y-value (less than 25), a characteristic desir¬ 
able in the use of the material for achromatiza- 
tion of wide-angle eyepieces. 

4. To find plastics with low thermal expan¬ 
sion coefficients and more resistance to abrasion 
than those in current use. 

In the course of the research, 113 compounds 6 
were synthesized and their physical properties 
studied. Of these, 78 were obtained in a condi¬ 
tion permitting their indices of refraction and 
^-values to be measured. They range in index 
from 1.65 to 1.44 and in v-value from 21 to 59, 
ranges comparable to those for optical glass. 
In fact the same nearly linear relationship ex¬ 
ists between v-value and refractive index for 
plastic materials as for glasses. It is interesting 
to note that the refractive indices of the metha¬ 
crylates can be predicted prior to synthesis 
from the density of the material and its atomic 
refractivity. 6a 

High-Index, High-v Materials 

An attempt to produce materials having un¬ 
usual optical properties was begun by intro¬ 
ducing other chemical elements either directly 
in the monomer molecule or as homogeneous 
plasticizers. It was thought that since most 
common organic resins tend to follow an aver¬ 
age refractive index-v curve very closely, a dif¬ 
ferent class of elements might affect the optical 
properties of the resulting compounds. The 
following investigations were undertaken: 

1. Silicon. Silicon was introduced in the form 
of triethoxy silicol methacrylate. This element 
lowered the index for a given v-value rather 
than raising it. 

2. Heavy Metals. Attempts were made to in¬ 
troduce heavy metals, first through a nonpolar 
linkage in a polymerizable molecule. An exam¬ 
ple of such an attempt is the lead derivative of 
methacryl acetoacetate. This type of compound 
failed to polymerize. 

In addition to polymerizable molecules, one 
nonpolar linkage of the plasticizer type was 
investigated. A lead derivative of diethyl dithio- 


phosphate was introduced into the monomer. 
This lead derivative, however, proved to be 
insoluble in the monomer. 

A plasticizer type which was soluble in the 
monomer was lead diethyl dithiophosphate. 
About 10 per cent of this compound dissolved 
in common monomers. Unfortunately it proved 
to be an effective polymerization inhibitor. 

In addition to attempting to introduce vari¬ 
ous heavy metals through nonpolar linkages, 
one polar-linked, heavy metal compound has 
been synthesized. This is a lead methacrylate 
compound which shows a high index, but is not 
markedly off the average index-v curve. 

3. Sulfur. A third attempt consisted in in¬ 
troducing sulfur into monomers. Sulfides and 
vinyl sulfonic esters were considered. In some 
compounds, such as vinyl phenyl sulfide, the 
introduction of this element has resulted in a 
high index without much change in the v-value, 
so that the resulting compound lies well off the 
index-v curve. 

4. Nitrogen. Another element introduced into 
monomers is nitrogen. In the form of nitro 
compounds, it lowered the v-value for a given 
index. An example of this is nitromethyl-propyl 
methacrylate. In the form of amino compounds 
it raised the v-value for a given index. An ex¬ 
ample of this is /?-amino-ethyl methacrylate. 
Other examples are the N-substituted metha¬ 
crylamides. With these, the index is raised over 
that of the corresponding ester by 0.03 to 0.04 
but the v-value is relatively unaffected. 

5. Ether Linkages. Monomers containing 
ether linkages were investigated in seeking to 
raise the v-value for a given refractive index. 
Until very recently, results with ether linkages 
have proved disappointing. One or two causes 
may have contributed to this failure: either 
an inhibitor effect or a lowering of the density 
of the compound or both. In general, it was 
found that systems containing ether linkages 
lead to soft and rubbery polymers. Whether 
this is due to incomplete polymerization be¬ 
cause of an inhibitive action of the ether link¬ 
age or whether this is an intrinsic property even 
of very highly polymerized samples cannot yet 
be stated. The materials investigated would be 
too soft for use as lenses, prisms, or optical 
flats. Ether linkages, in general, lower the 



CHEMICAL RESEARCH 


349 


density of the compound considerably, with a 
consequent lowering of index. Thus, while the 
r-value may be raised considerably, it is accom¬ 
panied by a lower refractive index. 

6. Increasing Alicyclic Rings. Another ap¬ 
proach toward raising both index and v-value 
was made through increasing the number of 
alicyclic rings contained in the alcohol part of 
the methacrylic ester. CHM has an index higher 
than that of methyl methacrylate with no ap¬ 
preciable change in r-value. Following this 
trend further, cyclohexyl cyclohexylmethacry- 
late was synthesized, and here the index was 
raised somewhat with little change in v-value. 
Cyclohexyl cyclohexylmethacrylate appears to 
mark the practical limit to which this proce¬ 
dure can be carried, however. Bornyl metha¬ 
crylate was synthesized and, although it has 
three alicyclic rings, it showed no improvement 
over CHM in departing from the index-r curve. 

7. Other Methods. Three other methods of 
producing a high-index plastic with high-y value 
also yielded negative results. Substitution of 
other radicals in the a-position of acrylic acid 
and copolymerization of two different monom¬ 
ers were tried without success. Inclusion of 
halogens in the molecules resulted in two com¬ 
pounds approaching the desired index and 
y-values. These were methyl a-bromo acrylate 
(w D = 1.567, v — 46.5) and 2, 3-di-bromopropyl 
methacrylate (% = 1.57, v = 44). Unfortu¬ 
nately, however, these materials were unstable 
and turned yellow very quickly. 

It must be concluded, in summarizing the 
search for a plastic having unusual optical 
properties, that none was found in spite of the 
intensive program of research. 

Replacement of Styrene 

Simultaneously with the study of high-index, 
high-v materials, an investigation of materials 
which might replace styrene was undertaken. 
The objectives were to discover a material 
harder than styrene, with less shrinkage, and 
which would polymerize more rapidly in order 
to shorten the baking cycle in production. 

Materials investigated include styrene deriv¬ 
atives, some of the higher index methacrylates, 
and higher index vinyl esters. So far, no mate¬ 
rial of this group has proved sufficiently supe¬ 


rior to styrene to warrant placing it in pro¬ 
duction. 

An example of styrene derivatives is ortho¬ 
methyl styrene. While this substance exhibits 
less shrinkage, greater hardness, and a higher 
y-value than does styrene, it is somewhat diffi¬ 
cult to synthesize, and it is not easy to handle, 
since it cannot be polymerized as fully as sty¬ 
rene before gelling. 

An example of a high-index vinyl ester is 
vinyl benzoate. Its index and v-value are nearly 
the same as those of styrene, and this material 
might well be substituted for styrene. It is com¬ 
paratively easy to synthesize. It has not been 
successfully polymerized in mass, however, be¬ 
cause it does not produce a sufficiently hard 
polymer when the temperature is kept low, and 
bubbles arise when the temperature is raised. 
A possible explanation of the bubbles is that at 
high temperature there is some decomposition 
of this ester to produce acetaldehyde. 

Low-i/ Materials 

Certain problems of instrument design, such 
as the achromatization of wide-angle eyepieces, 
can be simplified by the use of a material with 
a v-value of 25 or lower. In the course of the 
investigation of high-index materials, several 
compounds with low-r values have been syn¬ 
thesized. They are listed in Table 1. 


Table 1. Plastic materials with low rvalue. 


Material 

Index 

v-value 

a-naphthyl methacrylate 

1.641 

20.5 

N-vinyl phthalimide 

1.619 

24.1 

Vinyl carbazole 

1.683 

18.8 

a-naphthyl carbinyl methacrylate 

1.63 

25.0 

9-fluorenyl methacrylate 

1.63 

23.1 


The first of these plastics is the most suitable 
for optical purposes, in spite of difficulties in¬ 
volved in its fabrication. The yield of pure 
material is about 15 per cent in synthesis and 
there are at times variable amounts of yellow 
color in the polymer. Lack of homogeneity 
in the polymers of the remaining entries of 
Table 1 preclude their use on a large scale. 

Hard and Thermally Stable Plastics 

In an effort to increase the abrasion resist¬ 
ance of plastics and to remove objectionably 






350 


OPTICAL PLASTICS 


high thermal expansion coefficients, research 
on cross-linked polymers has been undertaken. 
The use of cross-linked or 3-dimensional poly¬ 
meric materials as optical plastics offers sev¬ 
eral advantages over linear polymers. Among 
the most important properties they possess are: 

1. A relatively low coefficient of expansion 
(about one-third that of linear polymers). 

2. High softening temperature (above 150 C). 

3. High resistance to abrasion (the abrasion 
resistance of some cross-linked polymers, as 
measured in the falling silicon carbide test, is 
higher than that of glass). 

4. High hardness (the hardness of ethylene 
dimethacrylate [EDM] for example, measures 
about Z-126, that of CHM about Z-109 by the 
Rockwell test). 

In general, the synthesis of cross-linking 
monomers may be performed by the techniques 
developed for linear monomers which involve 
esterification of the alcohol. In the case of the 
cross-linking material, a polyhydroxy or an 
unsaturated alcohol is usually esterified with 
an unsaturated acid such as methacrylic acid 
in the presence of a suitable catalyst. However, 
the cross-linking materials usually have higher 
molecular weights than the monomers which 
give linear polymers, and, therefore, their boil¬ 
ing points are in general higher. This means 
that if the monomer is a liquid, distillation 
should be carried out under greatly reduced 
pressure (about 2 mm of Hg) in order to pre¬ 
vent polymerization in the still pot. This must 
be avoided at all cost, since even slight poly¬ 
merization causes gelling and the eventual 
formation of insoluble, infusible material. In 
large-scale production of EDM, it is possible 
to work out conditions so that polymerization 
during distillation is prevented. It is felt that 
similar conditions could be devised for any 
such material. The yield of pure EDM by this 
synthetic method has been of the order of 50 
per cent, which is comparable to the yield ob¬ 
tained in the preparation of CHM. Small 
amounts of cross-linking substances may also 
be obtained conveniently by esterification, using 
methacrylic anhydride in the presence of an 
aromatic base. It is also possible to prepare this 
type of compound by means of an ester inter¬ 
change. For example, EDM may be prepared 


from methyl methacrylate and ethylene glycol, 
using sodium methoxide as a catalyst. 

For other types of cross-linking materials, 
such as p-divinyl benzene, it is necessary to 
develop special syntheses. In general the syn¬ 
thetic methods employed are similar to those 
used for preparing the monofunctional analogs, 
for example p-divinyl benzene is prepared from 
terephthaldehyde. Cross-linking materials which 
contain unlike functions, such as allyl metha¬ 
crylate, have been prepared by the ester inter¬ 
change method. 

No unusual difficulty has been encountered 
in the storage of cross-linking plastics provided 
they are kept at a temperature below 3 C. EDM 
and similar materials cannot be stored at room 
temperature. Batches of such materials have 
been kept for periods up to 3 months without 
decomposition or polymerization, except for 
methacrylic anhydride. If methacrylic anhy¬ 
dride is stored at —50 C, however, no poly¬ 
merization seems to occur. It is good practice 
to store the cross-linking monomer without 
catalyst or inhibitor and to catalyze just before 
using. 

EDM and Allyl Methacrylate. The two cross- 
linking plastics which have been investigated 
most extensively in connection with this prob¬ 
lem are ethylene dimethacrylate and allyl meth¬ 
acrylate. These materials were chosen as being 
representative of the general classes suitable 
for optical purposes, with the idea that although 
they may not be suitable in all respects, they 
would at least indicate the general character¬ 
istics of the class. Since both compounds are 
relatively easy to synthesize, small pilot-scale 
syntheses have been developed. 

When properly polymerized, EDM has a 
hardness of Z-126, a coefficient of expansion of 
2.3 X 10^ 5 per degree centigrade, and does not 
distort in softening tests below 170 C. The 
specific gravity of the polymer is approximately 
1.25 at 25 C. The main disadvantages of EDM 
are its rather high polymerization shrinkage 
(15.7 per cent) and high refractive index 
change upon absorption of water (about 0.005 
in the 100 C 1-hr boiling test). 

Allyl methacrylate, when polymerized prop¬ 
erly, has a hardness of Z-126, a coefficient of 
expansion of 2.6 X 10^ 5 per degree centigrade, 



CHEMICAL RESEARCH 


351 


and a distortion point above 170 C. The polymer 

on these optical plastics are given in 

detail 

has a specific gravity of 1.18 at 25 

C. It has 

elsewhere. 65 






a polymerization shrinkage of 21.6 

per cent, 








which is high compared to EDM, but may 

be 

Allyl Methacrylate as an Example 


reduced by partially polymerizing to 

a soluble 

The significance of 

an appraisal of a plastic 

linear polymer, dissolving in 

more 

monomer 

on the rating scheme shown 

in Table 2 can be 

and then finishing the polymerization. Although 

judged best from an example showing the der- 

allyl methacrylate absorbs about the i 

same per- 

ivation 

of the 

numbers for 

one 

plastic. 

Con- 

centage of water as 

EDM, its index change 

is 

sider, for example, allyl methacrylate: 




Table 2. Evaluation ratings of optical plastics. 










R-amino 


N-allyl 


Ethyli- 

p,p'- 






Vinyl 

ethyl 

Ethylene 

Allyl 

metha¬ 

Vinyl 

dene 

xylylenyl 


Perfect 



carba- 

metha 

.- dimetha- 

metha¬ 

cryl¬ 

metha- dimetha- 

dimetha¬ 

p-divinyl 

Characteristic plastic 

CHM 

Styrene 

zole 

crylate crylate 

crylate 

amide 

crylate 

crylate 

crylate 

benzene 

1. Homogeneity 

2. Resistance to water 

15 

15 

14 

6 

5 

12 

12 

10 

9 

14 

12 

10 

absorption 

10 

9 

9 

9 

1 

6 

6 

5 

6 

5 

6 

10 

3. Hardness 

4. Softening tempera¬ 

10 

8 

6 

9 

8 

10 

10 

8 

10 

10 

10 

10 

ture 

8 

6 

6 

8 

7 

8 

8 

7 

8 

8 

8 

8 

5. Toughness 

6 

3 

5 

3 

4 

3 

3 

4 " 

2 

4 

4 

4 

6. Absence of color 

7. Ultraviolet 

8 

8 

5 

5 

6 

5 

7 

2 

7 

7 

6 

6 

stability 

5 

5 

3 

5 

4 

4 

5 

2 

4 

5 

5 

5 

8. Ease of 













polymerization 

5 

5 

3 

5 

3 

5 

3 

2 

4 

5 

5 

5 

9. Monomer boiling 













point 

5 

5 

3 

4 

5 

4 

3 

4 

4 

4 

3 

4 

10. Low polymeriza¬ 













tion shrinkage 

8 

7 

6 

7 

5 

6 

1 

3 

3 

7 

5 

5 

11. Degree of 













polymerization 

8 

5 

8 

5 

5 

1 

1 

2 

1 

1 

1 

1 

12. Ease of monomer 













manufacture 

12 

8 

11 

10 

7 

8 

8 

7 

3 

11 

6 

2 

100 

84 

79 

76 

60 

72 

67 

56 

61 

81 

71 

70 

refractive index 


1.5064 

1.5916 

1.683 

1.537 1.5063 

1.5196 

1.5476 

1.5129 

1.4831 

1.5559 

1.6150 

v-value 


56.9 

31.0 

18.8 

52.5 53.4 

49.0 

47 

46 

52.9 

37 

28.1 


considerably less, about 0.0008 in the 100 C 
1-hr boiling test. 


828 Evaluation of the Plastic Materials 

In order to compare the substances synthe¬ 
sized, on a semi-quantitative basis, a rating 
scheme was devised on the basis of which a 
perfect plastic would be 100. These character¬ 
istics and their perfect ratings are shown in 
Table 2 together with the evaluations of CHM, 
styrene, and the nine plastics which at the 
conclusion of Contract OEMsr-70 showed the 
greatest promise for future utility. Methods of 
synthesis and suggestions for further research 


1. Its homogeneity is high, but not as good 
as that of CHM or of styrene. Hence it was 
given a grade of 12 out of a possible 15. 

2. Its resistance to water absorption is poor 
since 0.5 per cent by weight is absorbed upon 
boiling for 1 hr. The rating on this character¬ 
istic is therefore 6 out of 10. 

3. The material is extremely hard; its rating 
is Z-125. Hence a maximum score of 10 is re¬ 
corded for this characteristic. 

4. Since the polymer decomposes before it 
softens, it was given a maximum rating 8 on 
this score also. 

5. A property common to linear polymers, 
which cross-linked polymers do not possess to 
the same degree, is toughness or resistance to 









352 


OPTICAL PLASTICS 


shattering upon sudden impact. Hence, only 
3 out of 6 points were given to this material. 

6. Since this material has not yet been ob¬ 
tained as free from color as CHM, only 7 points 
out of 8 were assigned to it. This is presumably 
no intrinsic property of the plastic and might 
be considerably improved by further work. 

7. The substance is as stable as any other 
organic resin to ultraviolet light, and therefore 
was assigned the highest rating. 

8. Since allyl methacrylate has not yet been 
successfully partially polymerized in order to 
reduce polymerization shrinkage in the mold 
and must, at present, be cast directly from the 
monomer, it was rated only 3 out of a possible 5. 

9. The substance boils at 55 C under 30 mm 
Hg pressure, and it is more difficult to prevent 
the escape of monomer than with CHM which 
boils at about 66 C at 3 mm Hg. Hence it was 
only rated 3 out of 5. 

10. The shrinkage of this plastic upon poly¬ 
merization amounts to not less than 21.6 per 
cent. This is exceedingly high and hence only 
1 out of 8 points was assigned for this property. 

11. Since only a very low molecular weight 
polymer can be prepared before casting, the 
material was given 1 out of a possible 8 points. 
This appeared at first to be a rather serious 
drawback, but improvement in technique finally 
led to the possibility of casting even this ma¬ 
terial without too much difficulty. 

12. Two satisfactory methods for synthesiz¬ 
ing allyl methacrylate have been worked out. 
But since the yields of pure product are only 
about 50 per cent, only 8 out of 12 possible 
points have been assigned for this character¬ 
istic. 

8 3 CURRENT MANUFACTURING PRACTICE 

The fabrication of plastic optical parts in¬ 
volves the techniques of solidifying the liquid 
monomers by polymerization and of imparting 
optical surfaces to the polymers. Two general 
methods have been tried in the fabrication re¬ 
search. The first consists in polymerizing a 
mass of material of roughly the shape desired 
for an optical part, and then grinding and 
polishing the blank. The second consists in 
casting the polymer in precision molds so that 


when the molds are removed the plastic sur¬ 
faces are optically accurate and have a high 
surface polish. 

Fabrication of blanks by the first of these 
methods leads to relatively strain-free and ho¬ 
mogeneous optical units. Grinding and polishing 
by the usual methods of the glass industry are 
not very satisfactory when applied to plastic 
materials. The harder polymers such as allyl 
methacrylate, and the medium-hard materials 
such as CHM, can be ground with the com¬ 
pounds used for glass. Styrene, on the other 
hand, is soft and since it does not chip, stand¬ 
ard cutting operations on a lathe or shaper are 
preferable. 

None of the plastic surfaces, however, has 
been very successfully polished. It has been 
found that the quality of the final surface of 
an allyl methacrylate blank is definitely im¬ 
proved when the allyl methacrylate is copoly¬ 
merized with 10 to 20 per cent of CHM. The 
CHM itself is too tough to polish well, although 
satisfactory surfaces have been achieved with it. 

Press polishing has been tried, but the suc¬ 
cessive heating of the polymer to make it flow 
against the mold, and the cooling to separate 
it from the mold, results in strains and pitting 
which render the part useless from an optical 
standpoint. 

Attempts to form optical elements by build¬ 
ing up sections of relatively thin sheets of ma¬ 
terial and improvement of surfaces by the 
addition of thin surface layers of polymerized 
material have not been very successful. 

In view of the difficulties involved in blank¬ 
ing and finishing optical elements and the time 
involved in the process, a casting technique has 
been developed in which the final surface and 
form of an optical part result directly. Experi¬ 
ments to this end are described elsewhere. 60 
The following pages will describe in some detail 
the methods of producing CHM and styrene 
optical elements. 

Chemical Production of CHM 
Monomer 

Materials for Synthesis 

The materials for synthesis of CHM consist 
of 86 parts by weight of methacrylic acid, 100 



CURRENT MANUFACTURING PRACTICE 


353 


parts cyclohexanol, 6 parts p-toluene sulfonic 
acid, about 5 parts pyrogallol, and about 80 
parts benzene. The p-toluene sulfonic acid 
serves as esterification catalyst, the pyrogallol 
as polymerization inhibitor. The benzene car¬ 
ries off the water of reaction as formed. The 
quality of the materials required is as follows: 

1. Methacrylic acid: Index of refraction be¬ 
tween 1.4306 and 1.4315 at 20 C; contains 
0.1 per cent by weight pyrogallol inhibitor, 
added at the time of synthesis by the manu¬ 
facturer. 

2. Cyclohexanol: Barrett's commercial grade, 
minimum purity of 99.25 per cent. 

3. p-toluene sulfonic acid, monohydrated: 
Eastman organic chemicals purest grade. 

4. Benzene: Technical grade; no special pre¬ 
cautions to guard against thiophene or other 
sulfur-bearing compounds. 

5. Pyrogallol: Chemically pure. 

For a representative synthesis the quantities 
required are 38.01 kg methacrylic acid, 44.23 kg 
cyclohexanol. 

Preparation of the Charge 

A charge of the foregoing materials, after 
being thoroughly mixed, is reacted at atmos¬ 
pheric pressure for 15 to 18 hr during which 
time the pot temperature of the charge rises 
from 105 to 115 C. Crude CHM results. 

The reactor is a 30-gal copper-jacketed kettle 
containing copper coils both inside the tank 
and in the jacket through which water, heated 
by a boiler and controlled by an aquastat at 
127 C, circulates. Above the copper reactor is 
a column 6 in. in diameter packed with copper 
mesh. Vapor from the top of the column is led 
to a total condenser and then into a separator, 
which retains the water distilled over in the 
reaction, but permits the benzene distilled over 
to return to the reactor. 

Water is removed periodically from the sep¬ 
arator in order to follow the stage of the reac¬ 
tion. Esterification is judged to be complete 
when the amount of water of reaction approx¬ 
imates the theoretical yield, which, for the 
above charge, is 8,250 cu cm. 

An excess (1.02 kg) of cp sodium carbonate, 
monohydrated, is then added to the reaction 
mixture to neutralize the esterification catalyst, 


p-toluene sulfonic acid. During this neutraliza¬ 
tion process, the heating system is turned off 
and the reactor vessel is allowed to cool. The 
heating system is again started and the water 
of neutralization distilled off. 

The benzene (the vehicle for carrying over 
the water of reaction) is now removed by dis¬ 
tillation ; first, at atmospheric pressure and then 
at decreasing pressure as the concentration of 
benzene decreases, until an absolute pressure 
of 25 mm Hg is obtained. At this point, the 
charge in the reactor is cooled by circulating 
cold water through the jacket and coils previ¬ 
ously used for heating. (The heating system 
is so designed that if the reactor temperature 
goes above a predetermined level, cold water 
can be circulated to check the reaction). 

After the liquid cools, the charge is pumped 
through a standard Sweetland pressure filter 
into a 20-gal copper reservoir. Additional pyro¬ 
gallol (0.454 kg) is added at this point. The 
material is then filtered and distilled until the 
proper fraction, which has a boiling point of 
60 C at 2 mm Hg, is obtained. 

CHM can be stored at 4.5 C, probably indefi¬ 
nitely, if the fraction of the pyrogallol inhibitor 
which distills over with the product is not re¬ 
moved. It has been stored at this temperature 
for 18 months without observable ill effects. 

Before use, the product is further purified by 
removal of the inhibitor by extraction with a 
sodium hydroxide solution. The charge flows 
by gravity from the reservoir into a 20-gal 
copper kettle equipped with coils and jacket, 
similar to the reactor. Above the kettle is a 
packed column consisting of a 5-ft section of 
glass pipe 3 in. in diameter, with cups of stain¬ 
less steel screening fitted tightly within the 
glass pipe. Above*the column are located a 
thermometer and a tap for measuring the ab¬ 
solute pressure of the system at the thermom¬ 
eter well. A condenser is attached, composed 
of 1.5 in. glass pipe, equipped with a cooling 
jacket and a glass coil within the pipe. Two 
5-gal glass receivers are used for the conden¬ 
sate. The vacuum lines are so arranged that 
the lower receiver may be opened to the atmos¬ 
phere in order to discharge the product while 
the rest of the system is under vacuum. 

The charge is distilled under vacuum, until 



354 


OPTICAL PLASTICS 


the boiling point reaches 57 C at an absolute 
pressure of 2 mm Hg, at which stage collection 
of the pure material is commenced. At an ab¬ 
solute pressure of 20 mm Hg, the true boiling 
point of pure CHM is 60 C. If the boiling point 
of the material exceeds the true boiling point by 
2 C, collection is stopped, and the material re¬ 
maining in the kettle is rejected. The resulting 
CHM is stored in 5-gal glass bottles, tightly 
sealed, at 4.5 C. 

Removal of Inhibitor Prior to Use 

Forty liters of monomer are put into a 20-gal 
stainless steel mixing tank and washed with 
15-1 batches of 5 per cent aqueous sodium hy¬ 
droxide solution until the aqueous layers of two 


tains enough impurities, principally ethyl ben¬ 
zene, to preclude its use for plastic optical 
elements. Furthermore, 10 parts per million of 
p-tertiary butyl catechol are added by the 
manufacturer as an inhibitor to prevent poly¬ 
merization. 

The purification of this material is illustrated 
schematically in Figure 1. Forty liters of sty¬ 
rene are placed in a 20-gal stainless steel mix¬ 
ing tank, and washed three times with 15-1 
batches of a saturated aqueous solution of 
sodium bisulfite to remove oxidation products. 
The styrene is then washed three times with a 
5 per cent aqueous solution of sodium carbonate 
to remove the polymerization inhibitor and 
other impurities. After the sodium carbonate 



Figure 1 . Schematic diagram showing steps in the purification of styrene. 


consecutive washings are colorless. The mon¬ 
omer is then likewise washed with water to 
remove traces of the sodium hydroxide, until 
the pH of the wash water is approximately 7. 
The monomer is next transferred to 5-gal car¬ 
boys, and one pound of cp* anhydrous sodium 
sulfate is added to remove all traces of water. 
The pure monomer is decanted, filtered, and 
placed in tightly sealed glass flasks at 2 C. The 
purified material is stored no longer than one 
month before being used. 


Purification of Styrene 

Styrene, as purchased from the Dow Chem¬ 
ical Company, while 99.5 per cent pure, con- 


is extracted by triple washing with water the 
material is distilled. The distillation apparatus 
consists of a 22-1 round-bottom Pyrex flask to 
which a glass column 3% ft high is attached. 
Connected to the column is a total condenser, 
followed by a Leighton fractionator and a 5-1 
receiving flask. The individual pieces of equip¬ 
ment are joined with tapered ground glass 
connections. Heat is supplied to the distilling 
flask by an electric heating mantle placed below 
the flask. 

Fifteen liters of the material are placed in 
the 22-1 distillation flask. Distillation is carried 
out at a temperature of 50 C and a pressure of 
25 mm Hg until only a small viscous residue 
remains in the distillation flask. The resulting 
pure styrene is stored at 4.5 C in glass flasks, 


















CURRENT MANUFACTURING PRACTICE 


355 


which are tightly sealed to keep out air and 
water vapor. The maximum storage period be¬ 
tween purification and use is 3 days. 


8 3,3 Casting Technique 

Casting precise optical parts of plastic con¬ 
sists in polymerizing the liquid plastic-forming 


2 min, the flasks are removed and the air within 
is displaced with filtered C0 2 . The gas is intro¬ 
duced in such a way that no bubbles are formed 
in the monomer. 

The flasks are returned to the hot plates and 
the contents heated at approximately 250 C for 
15 min. During this time the CHM is not al¬ 
lowed to boil longer than 3 min. At the end of 
this heating, the flasks are removed and sealed 


PURIFIED 

CYCLOHEXYL 

METHACRYLATE 




1ST 

2ND 

[water BATH 


STABILITY 




FINISHED 

BAKE 

BAKE 

|MQLD REMOVAL 


BAKE 


MACHINING 


ELEMENT 


_L_ 

AEROSOL 

MOLD 

LUBRICANT 


Figure 2. Polymerization and fabrication sequence for CHM. 


material against accurately surfaced mold 
walls. The procedure involves four basic steps: 
(1) partial polymerization, (2) preparation of 
the mold and injection of the polymer, (3) bak- 


airtight with cellophane and brown masking 
tape. 

Thorough mixing of the partially polymer¬ 
ized mass is accomplished by rotating the flasks 



Figure 3. Polymerization and fabrication sequence for styrene. 


ing, and (4) removal from the mold. These are 
illustrated schematically in Figures 2 and 3. 

Partial Polymerization 

The processing of the pure CHM monomer 
begins with polymerizing it until the viscosity 
is about 50,000 centipoises. The monomer, in 
600 cu cm batches in wide-mouth Erlenmeyer 
flasks, is inspected for foreign matter and is 
then heated on hot plates at 330 to 340 C. After 


for 2 hr on specially constructed turntables 
and conveyor belts. A portion of the mixing is 
done in a cool water bath at 25 C. 

After the mixing is complete, a solution con¬ 
taining a polymerization catalyst and mold lu¬ 
bricant is added. The solution consists of 1.8 g 
of benzoyl peroxide (0.3 per cent by weight) 
and 3 g of aerosol O.T. in 30 cu cm of CHM 
monomer. The benzoyl peroxide is purified 
from technical grade by dissolving it in acetone, 






























































356 


OPTICAL PLASTICS 


precipitating in water, filtering and drying 
under vacuum. The material is then mixed 
again for about 1 hr until all striations dis¬ 
appear. The flasks are sealed and stored in a 
refrigerator at 2 C until the partial polymer is 
to be cast. 

The partial polymerization of styrene is car¬ 
ried out in much the same way as described 
above for CHM. More polymerization catalyst 
is necessary, however, for styrene. For prisms, 
3 g (0.5 per cent by weight), and for lenses 
6 g (1 per cent by weight) of benzoyl peroxide 
is added, together with 1.2 g of stearic acid as 
a mold lubricant. More catalyst is used in ma¬ 
terial for lenses because the greater ratio of 
surface to volume for these elements permits 
control of exothermic reaction at a greater 
polymerization rate. The mixing time is ap¬ 
proximately 14 hr for the styrene containing 
1 per cent benzoyl peroxide and 7 hr for that 
containing 0.5 per cent. It is preferable to cast 
styrene elements immediately after the thick¬ 
ening process has been completed, since over¬ 
night storage increases the haze in the cast 
product. 

Preparation of Molds 

Molds for casting lenses and prisms are de¬ 
signed to impart an accurate optical surface 
of high finish to the molded elements and to 
permit quantity production. Surface accuracy 
in the product depends to a considerable extent 
on the accuracy, surface finish, and alignment 
of the molds. Quantity production of optical 
elements requires that the molds be designed 
for long life, for reduction of unit cost, and for 
easy and quick assembly and disassembly. 

The molds for lenses consist of solid, ground, 
and polished glass disks, one for each face of 
the desired lens. Pyrex glass is employed, its 
advantages over other kinds of glass residing 
in its strength, its resistance to scratches, its 
resistance to the dissolving action of water in 
the final bath in which the finished lenses are 
disengaged from the glass molds, its resistance 
to thermal shock, its low coefficient of expan¬ 
sion, and its availability. Because Pyrex is con¬ 
siderably harder than most glass, the manufac¬ 
ture of molds from it is somewhat more expen¬ 
sive than from other glass such as crown, which 


might be used. More abrasive is required, wear 
on diamond laps is greater, and polish is slightly 
more difficult to obtain. 

Lenses for which spherical molds have been 
made range in diameter from % to 5 in. Mold 
diameters are from 8 to 25 per cent larger than 
the lenses to be cast in them so as to allow for 
radial shrinkage and for poor alignment. The 
walls are 1 in. thick to preclude distortion dur¬ 
ing the final polymerization process. A locating 
groove is accurately ground in the circumfer¬ 
ence of the cylindrical side of each mold, the 
back side of the groove being accurately located 



Figure 4. Assembly of lens molds. 


at an established distance from the curved 
molding surface. 

The mold surfaces are ground and polished 
to spherical form by the usual methods of 
the optical industry. Aspherical surfaces are 
ground on a specially constructed grinder which 
transfers a curve from a cam through a grind¬ 
ing wheel to the surface of the glass. 

The grooved mold halves are mounted face 
to face in an assembly jig, which maintains 
them at the proper distance apart by register¬ 
ing the backs of the ground grooves against 
the stops of the jig. This assembly is shown in 
Figure 4. The stops are separated by a distance 
calculated to include the center thickness of 





CURRENT MANUFACTURING PRACTICE 


357 


the desired lens plus the amount allowed for 
shrinkage during solidification (which depends 
on the material used). Allowance for shrinkage 
is y 10 in. or less for thin (% in.) CHM lenses. 

In order to complete the mold cavity, flexible 
tape is wrapped around the opening between 
the two mold halves and fastened to the pol¬ 
ished edges. The tape provides a wall which is 
sufficiently rigid to hold the distance of sep¬ 
aration set by the stops but is not too rigid to 



Figure 5. Assembly jig for rhombs. 


accommodate shrinkage during polymerization. 

The flexible tape consists of Jonflex Indus¬ 
trial Adhesive Tape combined with polyvinyl 
alcohol [PVA] sheeting. This sheeting is ap¬ 
plied with a tape-rolling machine to the center 
portion of the adhesive surface of the Jonflex 
tape, sufficient adhesive being exposed on each 
side to produce a tight seal at the mold edges. 
The width of the PVA sheeting is slightly 
greater than that of the edge opening of the 
mold cavity. In this way the sheeting, which 
does not affect polymerization of the plastic 
material, prevents contact between the ad¬ 
hesive, which inhibits polymerization, and the 
material to be polymerized. The end of the tape 
is pulled back % to 1 in. to form an opening 
for filling the mold cavity. 


Prism molds consist of selected plate-glass 
side walls mounted on glass bases with solder 
applied at all joints. To make prisms for 6x30 
binoculars, glass plate is selected having an 
accuracy of 2 fringes for a piece 11 / 2 x 1 % in. 
and 4 fringes for a piece 1 %xl% in. The pieces 
with the broader tolerance are selected for 
Porro prisms where the plate on the hypotenuse 
requires more accuracy than on the legs. The 
difficulty of obtaining flat glass by selection 
increases with the size of the pieces required. 
The accuracy of the surfaces of plastic prisms 
molded with glass selected to 2 fringes is as 
high as can be attained at present. Specially 
ground and polished glass mold faces do not 
afford greater accuracy. 

The thickness of the glass employed for small 
prism molds is % in. Thinner glass bends 
slightly during the polymerization process. 
Quarter-inch plate is satisfactory for molding 
prisms or rhombs employed at unit power in 
offset devices. 

The plate-glass walls are assembled upon jigs 
as shown in Figure 5; the shapes are those of 
the required prisms and rhombs. Jigs are made 
from steel blocks milled to the proper angles. 
For very accurate surfaces, the jigs are ma¬ 
chined to as close a tolerance as possible, then 
are hardened, and ground and polished by hand 
to final shape. Measurements of angles in the 
last stages are made on a spectrometer. Jig 
surfaces have grooves to act as dust traps when 
the glass is slid into position, since the pres¬ 
ence of dust can cause deviation in angles 
between faces greater than the allowed toler¬ 
ances. Where tolerances of the angles must be 
held closely, say to 3 min of arc or less, as with 
Porro prisms or with the Offset Attachment 
Mark 8, closeness of contact is tested before 
soldering by examining interference fringes 
between glass and jig. 

The glass walls are soldered at the corners 
while mounted on the jig, care being taken to 
keep the glass clean. To the upper edges of the 
vertical mold faces is soldered a horizontal 
piece of glass to serve as base of the mold when 
it is removed and inverted. The tops of the 
finished molds are left free to allow for shrink¬ 
age during polymerization. The solder employed 
is the fusible metal alloy, Belmont No. 255, 




358 


OPTICAL PLASTICS 


applied in a molten state with a soldering iron. 
It has a melting point (124 C) which is above 
the temperature maintained during polymer¬ 
ization of the plastics and since it neither con¬ 
tracts nor expands upon solidification, it im¬ 
parts no strain to the mold walls. It can be 
removed readily from the glass. The procedure 
for assembling the molds while on the jigs 


ings are narrow, such as molds for double- 
convex lenses, is accomplished through cone 
funnels prepared by wrapping PVA sheeting 
around wooden forms and cementing with PVA 
solution. An amount of 500 to 600 cu cm of 
material is put into a cone, and the top is folded 
over so that no air is trapped. The bottom of 
the cone is snipped off with a pair of scissors 



Figure 6. Rhomb and prism molds ready to receive plastic. 


makes it possible to hold angles within 1.5 min 
of arc. 

A group of prism molds ready for use is 
shown in Figure 6. After the prisms have been 
fabricated, the solder is pried off the mold with 
a knife, the mold walls are disengaged from 
the finished prisms, and both solder and glass 
are ready for use again. 

Injection of the Polymer 

The injection of the partially polymerized 
material into mold cavities whose edge open- 


and this end is inserted in the opening down 
to the bottom of the mold cavity. The operator 
then continues to fold over the top of the cone, 
squeezing the material into the mold and at 
the same time retracting the cone. Dust is kept 
out by surrounding the injection table with 
acetate sheeting covers. 

When the mold cavities have wide edge open¬ 
ings, as molds for prisms and concave lenses, 
a simple pouring spout is inserted into the 
1,000 cu cm flask containing the partial poly¬ 
mer and filling is done by gravity pouring. 




CURRENT MANUFACTURING PRACTICE 


359 


Glass tubes permit the material to enter at the 
top of the mold and flow to the bottom in such 
a manner as to prevent the entrapment of air. 

The prism molds containing styrene are left 
unsealed during the solidification of the prisms. 
Those containing CHM are covered with a 
sheet of cellophane because air inhibits its 
polymerization. 

In sealing lens molds, the flap of tape is 
pulled down across the opening and sealed with 
a roller. This procedure extrudes all excess ma¬ 
terial, and at the same time prevents air from 
being caught within the mold. Surplus material 
is then wiped off, and the flap is fastened se¬ 
curely with Scotch or Jonflex tape to prevent 
sucking in air bubbles when the material 
shrinks in baking. 

With the filled mold assemblies positioned on 
simple V blocks (V-shaped troughs formed by 
two flat pieces of wood), the desired thickness 
of the final lenses can be realized within a tol¬ 
erance of ±0.010 in. without further equip¬ 
ment. 

Alignment of the mold faces of spherical 
lenses need not be very accurate, because the 
lenses are optically centered and trimmed on 
a lathe. An alignment of from 1/2 to 2 degrees 
is maintained, depending on the curvatures and 
on the diameters of the untrimmed lenses. 

For aspherical lenses, on the other hand, 
molds must be as well aligned as the finished 
lens is required to be. Where tolerances on 
wedge or thickness are closer than can be met 
by the use of wooden V blocks, molds are held 
in jigs during baking. Tolerances of 0.002 in. in 
thickness and 0.002 in. in wedge over a 3-in. 
diameter have been met. 

The Baking Cycle 

The manufacture of accurate optical elements 
is completed by placing the molds, filled with 
partially polymerized material, in constant 
temperature ovens and baking at two successive 
temperatures over a period of several hours or 
days. The material is solidified at the first 
temperature, and polymerization is brought 
substantially to completion at the second higher 
temperature. Both baking temperatures are 
below the softening point of the polymer. The 
bake times and temperatures required to com¬ 


plete the finished elements depend on the plas¬ 
tic, the size of the molded elements, and the 
quantity of polymerization catalyst employed. 
Table 3 shows typical baking times and tem¬ 
peratures for various optical elements. Poly¬ 
merization slows down after solidification, and 
very little takes place in the last few hours of 
baking. It is important that styrene be as com¬ 
pletely polymerized as possible during the final 
bake in order to prevent crazing. 


Table 3. Baking times for typical optical elements. 




Per 


Baking cycles 




cent 

I 

II 



cata¬ 

Temp. Time 

Temp. Time 

Element 

Plastic 

lyst 

(C) 

(hr) 

(C) 

(hr) 

Lens 

styrene 

1.0 

40-70 

2-30 

83 

15-24 

Lens 

CHM 

0.3 

25-70 

2-24 

80 

12 

Prism 

styrene 

0.5 

40 

24-48 

80 

25-40 

Prism (thin) 

CHM 

0.3 

50 

2 

80 

5 


During the experimental studies of homo¬ 
geneity in plastic optical elements the effect of 
polymerization rates was studied for CHM, 
methyl methacrylate, allyl methacrylate, metha- 
crylic anhydride, benzyl methacrylate, and sty¬ 
rene. For a given substance, the rate of poly¬ 
merization was found to depend on catalyst 
concentration, temperature, pressure, and, with 
copolymers on composition. Some of these ef¬ 
fects are shown graphically for typical plastics 
in Figures 7 and 8. 

Figure 7 shows the polymerization rates 
measured calorimetrically for two monomers 
and for a mixture. The increase in polymeriza¬ 
tion rate when two monomers are mixed is 
striking. The methacrylic anhydride apparently 
functions as a detonator in initiating chains. 
Control of the temperature in such a mixture 
would be extremely difficult. 

Catalyst concentration produces a marked 
effect on the polymerization rate and hence on 
the rate at which the liberated heat of polymer¬ 
ization must be dissipated. 

The result of autocatalytic heat polymeriza¬ 
tion due to the poor conduction of heat by the 
polymer is shown graphically in Figure 8. 
Measurements were made by thermocouples 
placed at the center and at the edge of a mass 
of methyl methacrylate. The region of high 







360 


OPTICAL PLASTICS 


temperature was observed as a small, nearly 
spherical, “hot spot” of higher refractive index 
in the center of the mass, rapidly growing until 
it covered the entire mass. The cooling of the 
outer portion by a water bath resulted in in¬ 
homogeneities which were clearly visible. The 
rapid changes present at the critical points also 
lead to bubbles and other evidences of strain. 

Removal from the Mold 

The molds containing polymerized optical 
elements are taken from the ovens after final 



Figure 7. Polymerization rates for two monomers 
and for a mixture. 


scraped from old edges, the molds are cleaned 
with ethyl acetate and lens tissue, polished with 
ethyl alcohol, and are then ready for re-use. 

When prism assemblies are removed from the 
oven, the metal cement is pried off with a blade 
and the base of the mold is knocked off. The 
prism and the mold side walls are then placed 
in a water bath at 80 C for 10 min. At the end 
of this time, the mold walls fall off easily. The 
prisms are blown dry with clean, dry, filtered 
air at room temperature. 

After the elements have been separated from 
the molds, they are placed in a 60 C oven and 
baked for 5 days to prevent crazing. 

Centering and Trimming 

Machining of spherical lenses is accomplished 
on a lathe. Lenses are centered on a vacuum 



baking, and the mold walls are removed from 
the formed elements. Removal is accomplished 
by stripping the tape from mold edges and im¬ 
mersing the molds in four successive filtered 
water baths maintained at progressively lower 
temperatures of 80, 70, 60, and 50 C. 

The adhesion of the elements to their molds 
is usually destroyed before the assemblies leave 
the first water bath. The assemblies, however, 
are carried intact through the four step-down 
baths before elements and molds are actually 
disengaged from each other. This procedure 
insures complete disengagement and is believed 
necessary to prevent cracking of mold and ele¬ 
ment from thermal shock. 

After the assemblies are taken from the last 
separating bath, the elements are removed 
from the molds, rinsed in warm water, and 
blown dry. At this stage, the elements are com¬ 
pleted optically. After plastic particles are 


440 
400 
£ 360 
| 320 
1 280 
co 240 

UJ 
UJ 

<r 200 
o 
w 

Q 160 
120 

0 12 3 

TIME IN HOURS 

Figure 8. Temperature changes during poly¬ 
merization. 

chuck by means of a dial indicator to within 
±0.005 in. A perforated sheet of PVA 0.001 in. 
thick, placed between the chuck and lens, serves 
to protect the lens face. 

Aspherical Schmidt corrector plates, the cen¬ 
ters of which are not used optically, are centered 

















PHYSICAL PROPERTIES OF CHM AND STYRENE 


361 


by means of a small circle (% 2 to % 6 in. diam¬ 
eter) formed from a groove cut in the mold. 

With rough cuts of 0.020 in. and finish cuts 
of 0.002 in., the lenses are turned to the proper 
diameter by advancing the cutting tool radially 
(never parallel to axis of rotation) while it is 
still in contact with the material. Machining 
of prisms may be done on either a lathe or a 
shaper. 

Yield 

In concluding the discussion of manufactur¬ 
ing technique, a few remarks about the per¬ 
centage yield of elements may be made. The 
yield varies with the size and shape of the ele¬ 
ment. For a representative month, out of 421 
lenses (2.8-in. diameter) cast, 243 were accept- 


pressure of 760 mm of mercury and are given 
in Table 4. 

The indices of Table 4 are the mean values 
for the three prisms measured. In the case of 
the styrene prisms, the individual values ex¬ 
hibited a range of 0.00002 to 0.00006, while 
for the CHM prisms the range was considerably 
larger, namely 0.00014 to 0.00016. Both ranges 
refer to the average for the three temperatures 
involved and for all five wavelengths. The prob¬ 
able error of the experimental determinations 
is estimated at less than ±1 X 10^ 5 , but the 
systematic errors may be three or four times 
that amount. 

It is necessary for optical designers to know 
the partial dispersions of the materials with 
which they are working. These are listed in 


Table 4. Index of refraction of plastic prisms as a function of temperature and wavelength. 


Wavelength 

(A) 

Styrene 

15 C 

CHM 

35 C 

Styrene 

CHM 

Styrene 

55 C 

CHM 

A 7678.58 

1.58116 

1.50165 

1.57853 

1.49923 

1.57581 

1.49640 

C 6562.793 

1.58696 

1.50448 

1.58429 

1.50208 

1.58158 

1.49924 

D l 5892.62 

1.59232 

1.50705 

1.58966 

1.50464 

1.58694 

1.50181 

F 4861.327 

1.60616 

1.51343 

1.60343 

1.51099 

1.60063 

1.50811 

G' 4358.342 

1.61760 

1.51842 

1.61482 

1.51596 

1.61198 

1.51309 


able, a yield of 58 per cent. The greatest cause 
of rejection was pullaway from the mold. 
Bubbles in the element accounted for the next 
highest rejection factor. 


8 4 PHYSICAL PROPERTIES OF 
CHM AND STYRENE 

An evaluation of the optical quality of plas¬ 
tic lenses, prisms, and flats is given in some 
detail in Section 8.7. In this section a summary 
of the pertinent physical properties of CHM 
and styrene is presented. 

Index and Dispersion. The refractive indices 
of three 60-degree styrene prisms and three 
60-degree CHM prisms were measured by the 
National Bureau of Standards. These prisms 
were measured by the method of minimum devi¬ 
ation, in a stirred air bath on a spectrometer 
table at controlled air temperatures near 15, 
35, and 55 C. All results have been corrected 
to refer to air at these temperatures and at a 


Table 5. Corresponding reciprocal dispersions 
(r-values) are 56.9 and 31.0 for CHM and 
styrene, respectively. 


Table 5. Partial dispersion of plastic prisms. 



Styrene 


CHM 


n F 

— nc 

0.01920 

n F 

- nc 

0.00895 

nit 

— nc 

0.00536 

n d 

— nc 

0.00258 

n F 

— Wd 

0.01384 

n F 

— nD 

0.00638 


Figure 9 shows the variation of index of 
refraction measured on 412 styrene prisms 
representative of the fabrication technique. Al¬ 
though production tolerances have been placed 
at ±0.0015, 75 per cent of the plastic elements 
produced fall within a range of ±0.0004 on 
either side of the mean. Sixty-five CHM prisms, 
measured with the precision refractometer in 
the same manner as those of styrene, showed 
a total spread in index values comparable with 
that for styrene. 

The refractive index of plastic elements can 
be duplicated within production tolerances over 










362 


OPTICAL PLASTICS 


a period of months. Measurements made on 
styrene polymer taken from production lots 
over a period of 8 months and on CHM polymer 
over a period of 5 months indicate an extreme 
range of 0.0012 in refractive index, the aver¬ 
age fluctuation being one-half this amount. 



Figure 9. Variation of index of refraction in 412 
styrene prisms. 


The index of both CHM and styrene optical 
elements undergoes change with time. Index 
measurements of 7 styrene elements and 6 CHM 
elements, made at 1-month intervals, indicated 
decreases on the average of 0.00069 and 
0.00052, respectively, over a period of 8 
months. 

Homogeneity. The internal homogeneity of 
styrene and CHM optical flats has been tested 
by double transmission in an interferometer 
at the Mount Wilson Observatory. The disks 
used were approximately 1 in. thick and 6 in. 
in diameter, and were molded between a pair 
of Pyrex flats. 

The surfaces of the disks were first tested 
for flatness with a test plate. They showed 
uniform curvatures amounting to between 6 
and 20 fringes over a central circle 3% in. in 
diameter. Although one surface was convex and 
the other concave on each disk, the two curva¬ 
tures were nearly equal in amount, showing 
that the disks had warped as a whole while the 
faces remained nearly parallel. Homogeneity 
over 3-in. diameter areas at the centers of the 
disks varied between 1 and 4 fringes for the 
double transmission with the interferometer. 


A part of this apparent inhomogeneity might 
have been due to the observed curvature of the 
surfaces. 

The interferometer test is time-consuming. 
Direct examination of a sample under the po- 
lariscope can often afford a rapid and reliable 
indication of its homogeneity, especially in the 
case of styrene, because lack of homogeneity 
is associated with strain. The amount of re¬ 
tardation present in most cast styrene prisms 
amounts to about three-quarters of a wave per 
114 in., over all the area except at the very 
corners. Resurfaced prisms made from annealed 
styrene blanks have slightly lower retardations, 
about one-half wave or less, and variations are 
uniformly distributed. 

Resurfaced prisms, as viewed in the inter¬ 
ferometer, appear to have better homogeneity 
than do those cast in a single operation, and 
they are nearly strain free. This may indicate 
that surface strain is largely responsible for 
whatever inhomogeneity is found in styrene 
prisms. 

Surface Accuracy. Flatness of the surfaces 
of plastic optical elements is tested with a mono¬ 
chromatic light source against a glass flat. To 
secure accurate readings, the temperature must 
be uniform throughout the sample; the ambient 
temperature must be constant within 0.25 C 
over relatively long periods of time. 

With respect to surface flatness, prisms of 
CHM are in general less accurate than those 
of styrene, although of comparable homogene¬ 
ity. Recent tests of CHM prisms have shown 
3 to 4 fringes over an area approximately 
114 in. square. Styrene prisms, such as those 
used with the 6x30 binocular, having faces 
approximately 1 in. square, have proved to be 
flat within 1 to 3 fringes. Curvature in most 
cases is concave. A further critical examination 
of the surface and interior homogeneity of fin¬ 
ished optical elements will be given in the last 
section of this chapter. 

Strain. Birefringence in inorganic glasses is 
always a sign that stress, either external or 
internal, is present; to obtain the glass in an 
optically isotropic state, it is only necessary to 
relieve all the stresses. Organic resins likewise 
exhibit stress birefringence, but of magnitudes 
which vary greatly with the nature of the resin, 







PHYSICAL PROPERTIES OF CHM AND STYRENE 


363 


and, in addition, they have another kind of 
birefringence which persists in the absence of 
mechanical stress and which may be called 
plastic birefringence. 

Varying widely, but generally of a higher 
order of magnitude than stress birefringence, 
this effect has long been recognized and has 
lately been put to use in photoelastic studies. It 
is doubtless of the same nature as the stream¬ 
ing birefringence observed with some liquids 
(most strongly with solutions of high poly¬ 
mers), and may be ascribed to the orientation 
of birefringent structural units by viscous 
flow. Plastic optical parts of CHM and styrene, 
cast according to current techniques, exhibit 
birefringences up to 95 and 162 m/x per cm, 
respectively. These are believed to result from 
plastic deformations introduced by the volume 
contraction on polymerization under conditions 
of adhesion to rigid mold walls. 

Porro prisms of styrene, 1 %e x l 1 A in. thick 
leg, were bound in pairs to form squares and 
were examined without making optical con¬ 
tact across the air gap. A single examination 
made through the central area of the prisms 
showed a birefringence of 130 m ^ per cm. 

Both CHM and styrene have positive bire¬ 
fringence in tensile elastic stress and negative 
birefringence in tensile thermal plastic stretch. 
A complete analysis would probably show all 
of the observations to be consistent with a pic¬ 
ture of plastic deformation consequent upon 
volume contraction during polymerization, and 
adhesion of the cast to the rigid mold walls. 
Similar birefringences are found in thin sheets 
of some polymers prepared by casting solu¬ 
tions on glass plates and allowing the volatile 
solvent to evaporate, the film adhering to the 
glass during drying. 

Hardness. Hardness measurements of sty¬ 
rene and CHM, as well as of allyl methacrylate, 
EDM, and Lucite, were made on a Rockwell 
hardness tester with a %-in. ball, a 20-kg 
major load, and a 10-kg minor load. This modi¬ 
fication of the usual Rockwell hardness test 
was made because the 60-kg major load ordi¬ 
narily used always cracked CHM samples and 
usually cracked those made of styrene, owing 
to the short average chain length of the poly¬ 
mers and their consequent brittleness. 


Tests with this modified instrument can be 
made rapidly and, on the whole, results ob¬ 
tained from various runs are consistent. While 
not quantitative as measured on the Rockwell 
Z scale, the results are clearly indicative of the 
relative hardness of the materials measured, 
as follows: 


Polymer Hardness 

styrene 103-105 

CHM 107-109 

allyl methacrylate 124-125 

EDM 126 

Lucite 99-100 


Density. Optical plastics are from one-third 
to one-half as heavy as inorganic glasses of 
corresponding optical characteristics. The den¬ 
sity of CHM is 1.0951 g per cu cm; the density 
of styrene is 1.0493 g per cu cm. 

Water Absorption. CHM and styrene differ 
strikingly in water-absorption characteristics 
from plastics heretofore proposed for optical 
applications. Under ordinary conditions, so 
little water vapor is absorbed that it has no 
measurable effect on optical performance. 

Elements of both CHM and styrene have 
been desiccated for 24 hr in an atmospheric 
desiccator filled with calcium chloride, after 
which the elements were boiled in water for 
1 hr. This extreme test showed water absorp¬ 
tion of 0.121 per cent for CHM and 0.087 per 
cent for styrene, with index changes of 
—0.00244 and —0.00058 respectively. A test 
made under the same conditions on Lucite 
(methyl methacrylate) showed water absorp¬ 
tion of 0.801 per cent with an index change of 
-0.00176. 

Although the index change shown for CHM 
would affect the focus of a simple lens by ap¬ 
proximately 1 per cent, it is apparent that the 
index change under normal conditions would 
be negligible. 

Transmission. The transmission of both 
CHM and styrene is approximately 90 per cent, 
uncorrected for reflection, over that region of 
the spectrum from 10,000 A to 3,900 A for a 
path length of 1 cm. CHM is almost invariably 
water white. There is a slight yellow tinge in 
styrene at times, caused by the catalyst used 
during polymerization. 



364 


OPTICAL PLASTICS 


Transmission curves for CHM and styrene 
are shown in Figure 10. The measures are un¬ 
corrected for surface losses. 

Softening Temperatures. The temperatures 
at which the various plastic materials soften 
depend on such factors as the catalyst used 
and the degree of polymerization attained. 
However, the following data are indicative of 
the softening-point temperatures of styrene and 
CHM. 

Softening Point 

Polymer Degrees Centigrade 

styrene made with 0.25 per cent 

acetyl benzoyl peroxide 82-83 

styrene made with 1 per cent 
benzoyl peroxide about 78 

CHM partially polymerized by 

boiling 61-62 

CHM partially polymerized at 

95 degrees 67-71 

The temperatures given above are those at 
which the samples sagged not more than 



WAVELENGTH IN MILLIMICRONS 

Figure 10. Spectral transmission of CHM and 
styrene. 


0.0005-in. after 14 to 16 hr when tested in the 
following way: 

Flat strips, SxV&x 1 /^ in., of the materials to 
be tested were placed with their ends on shoul¬ 
ders. Steel balls, */2 in- in diameter, fixed in 
place by guides, were placed on the upper sur¬ 
faces of the flats. Several samples of the same 
material were heated for about 15 hr at dif¬ 
ferent temperatures and the sag was then 
measured with a dial indicator. The softening 
temperature was taken to be that at which a 
sample sagged not more than 0.0005 in. in the 
time interval stated. 


Thermal Expansion. Organic resins have un¬ 
desirable thermal properties. The high coeffi¬ 
cient of thermal expansion of both CHM and 
styrene leads to many problems in the design 
of optical instruments, chiefly in the manner of 
mounting the plastic elements. 

The coefficient of linear expansion of CHM is 
76 X 10 -6 per degree centigrade. The coeffi¬ 
cient of linear expansion of styrene is 
71 X 10~ 6 . In comparison, that for glass is of 
the order of 9 X 10 -6 . 

The thermal index coefficient for both ma¬ 
terials has been calculated from National 
Bureau of Standards measurements. The mean 
change in index per degree centigrade rise in 
temperature, based on the index change from 
15 to 55 degrees is: 

styrene —0.000136 at the sodium D line 

CHM —0.000131 at the sodium D line 

Thermal Conductivity. Samples of CHM and 
styrene were tested on a special thermal trans¬ 
mission tester. The thickness of the sample was 
0.25-in. and the mean temperature of the 
sample was 70 F. The resulting thermal con¬ 
ductivities were: 

Styrene: 

0.64 Btu per hr per sq ft per degrees 
Fahrenheit per in. 

2.21 X 10 -4 calorie per sec per sq cm per 
degrees centigrade per cm 

CHM: 

0.67 Btu per hr per sq ft per degrees 
Fahrenheit per in. 

2.31 X 10 -4 calorie per sec per sq cm per 
degrees centigrade per cm 

Scratch Resistance. On the Mohs scale of 
hardness both CHM and styrene are between 2 


Table 6. Tensile, impact, and flexural strength 
of CHM and styrene. 



CHM 

Styrene 

Tensile strength (psi) 

1255 

1980 

Impact (ft-lb per in. of notch) 

0.14 

0.24 

Flexural (psi) 

2280 

3850 












PLASTIC OPTICAL SYSTEMS 


365 


and 3. This means that both materials are 
scratched by calcite and all harder materials, 
but not by talc and gypsum and materials with 
corresponding abrasive qualities. 

Tensile, Impact, and Flexural Strength. Five 
samples each of CHM and styrene were exam¬ 
ined for tensile strength, for impact strength 
in accordance with ASTM Standard D256-43, 
and for flexural strength in accordance with 
ASTM Standard D650-42T. The results are 
shown in Table 6. 


PLASTIC OPTICAL SYSTEMS 


Optica] Characteristics of 
Plastic Systems 

A number of optical instruments have been 
designed and constructed to explore the utility 
of plastic elements. Representative of these in¬ 
struments are the seven whose characteristics 
are listed in Tables 7a and 7b. Optical proper- 


Table 7a. Calculated performance of optical systems. 



T-108 antitank 
telescope 

T-118 antitank 
telescope 

6x40 inverting 
telescope 

Magnification 

3X 

5X 

6 X 

Exit pupil 

lin. 

0.8 in. 

0.3 in. 

Field of view 

6 ° 

6 ° 

10 ° 

Eye relief 

6 V 2 in. 

4 n /^ 2 in. 

% in. 

Spherical aberration 

10 sec 

8 sec 

12 sec 

Curvature of field (objective only) 

0.09 diopter 

0.18 diopter 

3.5 diopter 


5 in. radius 

6 V 2 in. radius 

3 in. radius 

Coma 

20 sec 

16 sec 

24 sec 

Distortion 

0 

0 

slight barrel distortion* 

Axial color 

7 sec 

6 sec 

6 sec 

Resolving power 

5 sec at c.o.f.f 

5 sec at e.o.f. 

5 sec at e.o.f. 


20 sec at e.o.f.J 

20 sec at e.o.f. 


Equivalent focal length of 




objective 

381.0 mm 

507.2 mm 

240 mm 

Transmission 

31% 

35% 

60% 

Clear aperture 

75 mm 

100 mm 

40 mm 

Astigmatism 

less than V 2 diopter 

less than V 2 diopter 

V 2 diopter 


♦Not computed, 
t Center of field. 
$ Edge of field. 


Table 7b. Calculated performance of optical systems. 



//0.7 Schmidt 
system 

//2.8 Aerial 
camera lens 

//1.6 Reflex 
sight 

Offset wedge 
Mark 7 

Magnification, permissible 



. . . 

7X 

Field of view 

28° 

47° 

12 ° 

70 

Spherical aberration 

10 sec 

50 sec 

1 min 


Curvature of field 

2 % in. radius 

flat 

IV 2 in. radius 


Coma 


8 sec 

IV 2 min 


Distortion 

0 

0.36% at 19%° 

0 at 23 V 2 0 

* 


Axial color 

none 

8 sec 

45 sec 


Resolving power 

1 min at c.o.f.f 

30 lines per mm 

IV 2 min 

8 sec 

Equivalent focal length 

2 % in. 

7.38 in. 

5% in. 


/-number 

// 0.6 

// 2.8 

// 1.6 


Transmission 

82% 

81% 

90% 

92% 

Clear aperture 

3% in. 

2 % in. 

3V 2 in. 

2 in. 

Back focal length 


2.3 in. 

•.. 

... 


♦Not computed. 

t Center of field. 











3 66 


OPTICAL PLASTICS 


ties of three low-power telescopes are given in 
Table 7a, while Table 7b presents data for more 
specialized visual and photographic systems. 

In order to interpret the table, the following 
definitions or usages must be borne in mind: 

1. Spherical aberration: The angle sub¬ 
tended at the second principal point of the ob¬ 
jective lens by the diameter of the circle of 



Figure 11. T-108, 3x75 telescope for antitank 

guns. 

confusion containing at least 75 per cent of the 
light. 

2. Curvature of field: The figures in Table 7a 
represent the diopter differences in focus be¬ 
tween the center and full field of the objective. 

3. Coma: The angle subtended at the second 
principal point of the lens by a circle of such 
diameter that it includes 70 per cent of the 
light in the comatic image. 

4. Axial color: The angle subtended by the 
diameter of the circle of confusion which in¬ 
cludes the F, C, and D rays. (F and C as a rule 
are coincident). 

5. Astigmatism: Given in diopters at the 
edge of the field. 

6. Resolving power: The angle subtended by 
adjacent discernible lines on a U. S. Bureau of 
Standards resolving-power chart when viewed 
through the instrument. 

7. Distortion: Expressed in terms of per¬ 
centage of linear distance from the center of 
the field for the stated field angle. 

The T-108 telescope shown in Figure 11 is a 
3X direct-sighting telescope for use on anti¬ 
tank guns. The unusual features of the instru¬ 
ment are its large eye relief (6.5 in.) and its 


large exit pupil (1 in.). A cast mirror-erecting 
system includes a reticle. By the use of a com¬ 
bination of glass and plastic elements in the 
objective the change of focal length with tem¬ 
perature has been kept as small as possible. 
The optical design is shown in Figure 12. Spec¬ 
ifications and details of the T-108, of which 
1,178 were manufactured, are given elsewhere. 6 " 1 

Optical tests of a typical T-108 telescope 
showed the resolving power to be 3 sec at 
center of field, 7 sec at half field, and 45 sec at 
the edge of the field. Tests with a dioptometer 
of 11%-in. focal length indicated negligible 
field curvature and longitudinal chromatism. 
Parallax, with the aperture stopped to 1.5 cm 
was undetectable with 30 X magnification both 
at half and at full field. 

The T-118 telescope is very similar in design 
and performance to the T-108 but has a larger 
aperture and higher power. 

The 6x40 telescope is an inverting instru¬ 
ment designed primarily for testing the per¬ 
formance of a plastic optical system against a 
corresponding glass system (the Mark 15 gun- 
sight). Such a comparison for the T-108 tele¬ 
scope is described in Section 8.7.1. 

In the Mark 7 offset wedge attachment for 
binoculars, illustrated in Figure 13, a double 
image is obtained by means of holes through 
which a direct image of an object is superposed 
on a deviated image produced by the prism. 
The optical design of the prism system is 
shown in Figure 14. The wedges resolve 7 to 10 
sec throughout the entire field. Longitudinal 
chromatism is too small to be measured. 

An aerial camera lens design (focal length 
7.38 in., jf/2.8) is shown in Figure 15. The 
effects of temperature change on the focal 
length have been minimized by inserting a 
glass element (D) in the system. 

The //1.6 reflector sight was designed for 
fixed gunnery in aircraft. It consists of a folded 
optical system with a spherical reticle. Over the 
entire field, the reticle rings are fixed in direc¬ 
tion in space within % mil. 

The small, fast Schmidt system (//0.7) is 
composed of the usual spherical reflecting mir¬ 
ror and a correcting lens mounted in a plastic 
tube which affords temperature compensation 
for the optical parts. At the center of the field 




PLASTIC OPTICAL SYSTEMS 


367 


80 per cent of the light is concentrated in a 
50-/1 circle. Production instruments (some 
5,000 have been made) resolve clearly 3 min of 
arc at the center of the field and 6 min at the 
edge over the entire temperature range, —40 C 
to 55 C. Figure 16 shows the optical design. 


Other Instruments Designed 

In addition to the representative instruments 
described above, numerous others have been 
designed and some of these constructed. Their 
diversity is shown by the following list: 

Galilean binoculars, 3x60 and 5x80 

//1.9 illuminated sight collimator 


85-3 Athermalization 

One of the principal considerations in the 
design of plastic optical instruments is the 
elimination of changes in focal length due to 
temperature fluctuations. Refocusing the in¬ 
strument to offset changes in focal length is 
obviously inconvenient and in many cases im¬ 
practical. An instrument designed to minimize 
these temperature effects is said to be ather- 
malized. 

Athermalization can be accomplished for a 
plastic lens system by the addition of one or 
more glass lenses; these have a negligibly low 
coefficient of thermal expansion and their opti¬ 
cal properties are substantially unaffected by 



-d6- 


-ii h 



LENS DIA 
IN 

INCHES 


MM INCHES 


MM INCHES 


l=|nn =1.523 
Ed*' =58 

j^np *1.5062 

FIELD = 6® 

EXIT PUPIL = 1" 

EYE DISTANCE = 6^" 

OBJECTIVE E FL =15.033 
EYEPIECE EFL = 4.86” 


GLASS LENS 


LENS DIA 
TOLERANCE 
+ .000" 
-.003“ 


381.84 MM 
123.65 MM 


isj 

| 3.125 

R9 

273.92 

10.433 

di 

8.25 

0.325 


RIO 

706.70 

27.822 

d2 

6.75 

0.266 

A 

3.000 

R1 

706.7 

27.82 

d3 

10.125 

0.399 

B 

3.000 

R2 

124.7 

4.909 

d4 

0.13 

0.005 

C 

2.843 

R3 

424.0 

16.693 

d5 

7.5 

0.295 

D 

2.843 

R 4 

223.2 

8.787 

d6 

363.137 

14.297 

E 

3.000 

R5 

70.7 

2.783 

d7 

84.61 

3.331 



R6 

103.4 

4.070 

d8 

7.33 

0.289 



R7 

146.2 

5.765 

d9 

33.33 

1.312 



R8 

192.7 

7.586 

dio 

0.19 

0.007 






1 dii 

17.33 

0.682 


Figure 12. Optical design for T-108 telescope. 


//2.5 anastigmat lens with field flattener, 
focal length 7 in. 
standard 6x30 binocular 
7x50 prism binocular 
140-mil reflecting offset, Mark 8 
//3 aerial camera lens 
3x75 prism telescope 
//0.7 camera lens 

//0.7 parabolic reflector, focal length 1 in. 
3x75 prism telescope 


changes in temperature. The glass element in 
the T-108 telescope, for example, decreases the 
temperature effects and at the same time pro¬ 
tects the plastic lens elements against scratch¬ 
ing. However, freedom from change in focal 
length can be realized, even in athermalized 
systems, only when all elements are in thermal 
equilibrium. 

It is also possible to athermalize a plastic 
lens system by the use of a housing composed 








































368 


OPTICAL PLASTICS 


of alternate layers of metal and plastic. The 
effect of a cumulative contraction adequately 
compensates for change in the optical elements. 
This procedure does not appear to be practical, 
however, because of the large number of layers 
required—four plastic and three metal—if the 
composite housing extends all the way from 
the lens to the focal surface. 

Partial athermalization for focal distance 
can be attained quite simply by using an 
aluminum housing. 

A plastic mirror system may be completely 
athermalized by making the connection be- 



Figure 13. Mark 7 offset wedge for binoculars. 


tween the mirrors, and from the mirror to the 
focal surface, of the same plastic material as 
the mirror. This is also essentially true in such 
a system as the Schmidt, in which the refract¬ 
ing element has negligible power. 

The relatively large coefficient of thermal 
expansion of plastics (about 71 X 10 -6 per de¬ 
gree centigrade for styrene, and 76 X 10 -6 per 
degree centigrade for CHM) necessitates cer¬ 
tain precautions in mounting which are not 
necessary for glass elements. 

Plastic lenses are usually mounted in a ma¬ 
terial of approximately the same coefficient of 
expansion. If this procedure is followed, any 
of the commonly accepted techniques for 
mounting glass lens components can be used 
with plastic lenses: snap rings, threaded re¬ 
taining rings, push fit rings, and so forth. 
Lenses have also been cemented to plastic 
sleeves with a special cement which forms a 


bond between the housing material and the 
optical material without injury to the plastic 
elements. 

It may be necessary to center the plastic 
sleeve containing the lenses in a metal tube, as 
was done in the T-108 and T-118 telescopes, 
and in the //2.8 camera. In such cases, the 
radial expansion of the sleeve may conven¬ 
iently be taken up with an elastic material, such 
as the rubber rods used as bushings in the //2.8 
camera lens mount. For the accurate centering 
necessary in boresighted instruments, the plas¬ 
tic sleeve may be made to ride on a conically 
machined metal outer sleeve. This was done in 
the T-108 and T-118 telescopes. 

In some instances it may be desirable to 
mount the lenses directly in an outer housing 
made of a material with a considerably lower 
thermal coefficient than the lenses. This is per¬ 
fectly feasible wherever the centering tolerance 



MM _ MM 


WEDGE 

PRISM ANGLE 

n D v 

PC 1 

l7°-20'-l9" 

1.5060 57.5 

PC 2 

8°- 4' -30" 

1,5911 31.5 


Figure 14. Optical design of offset wedge at¬ 
tachment. 

of the lenses is large enough to permit the 
clearance necessary for the full range of tem¬ 
perature to be encountered in use. Under these 
conditions, thermal compensation can be 
achieved by the use of metal springs, rubber 
bushings, gaskets, and other compression de¬ 
vices placed directly between the lenses and 
the housing. 

Rapid changes in temperature injure the re- 

























PLASTIC OPTICAL SYSTEMS 


369 



CA1 

±0.1 

R1 

105.97*023 

4.173 

d 1 

14.50 

0.571 

CA2 

±0.5 

R2 

537.2 2* 4 - 40 

21.151 

d2 

0.29 

0.012 

C A3 

±0.1 

R3 

65.38 ±0 * 09 

2.574 

d3 

14.50 

0.571 

CA4 

±0.5 

R4 

127.40 ±0 ' 40 

5.016 

d4 

8.56 

0.337 

CA5 

±0.5 

R5 

237.59* 0,90 

9.354 

d5 

3.48 

0.137 

CA6 

+0.5 

-o 

R6 

65.38* 0,09 

2.574 

d6 

28.62 

1 .127 

CA7 

+ 1 
-o 

R7 

124.81 ±0 ' 26 

4.914 

d7 

11.80 

0.464 

CA8 


R8 

118.40*018 

4.662 

ds 

64.79 

2.551 

CA9 

±0.3 

R9 

60.19±0.11 

2.370 

d9 

4.45 

0.175 

CAIO 

± 1 

RIO 

145.30* 0 * 53 

5.721 

dio 

59.30 

2.335 


A 

3.11 7 +.000 
5,11 7 -.003 

B 

2.629 + -°°° 

-.003 

C 

2.629 + * 000 

-.003 

D 

2.723+.000 

-.004 

E 

3 Q 7ft +.° 00 

3.978- 003 


47° FIELD 
190.5 MM = E FL 


Figure 15. Optical design of //2.8 aerial camera lens. 


solving power of plastic lens systems more 
than they do a glass system. Therefore, where 
the system is subject to such changes, it is nec¬ 
essary to insulate thick massive plastic lenses 
from the outer walls of a metal housing. This 
may be done by mounting the lenses in a plastic 
sleeve which bears upon the metal outer hous¬ 
ing at two narrow bands. The dead air space 
thus formed provides adequate insulation. This 
construction was used in the //1.6 sight. 

Current Limitations 

In concluding the discussion of instrument 
and design characteristics, it may be well to 
summarize the limitations imposed on the op¬ 
tical designer by the use of the two available 
plastics (CHM and styrene). Design possibili¬ 
ties with all plastic elements and with plastic 
elements combined with glass have been more 
fully discussed elsewhere. 15 There are four 
limitations: 

b See Chapter 1 and Section 10.7 of this volume and 
reference 6 of Chapter 8. 



REFLECTOR 3.6 0Z COEFFICIENT OF THERMAL 

CORRECTOR 2.00Z EXPANSION = 8x10' 5 /C° 

Figure 16. Optical design of //0.7 Schmidt sys¬ 
tem. 

1. The first arises from the high coefficient 
of thermal expansion, in each case nearly 10 
times that of glass. A small change in focal 
length caused by a change in temperature may 
be corrected in a focusing instrument or auto¬ 
matically compensated in an athermalized in¬ 
strument. However, the rate at which the plas¬ 
tic elements will come to equilibrium may still 





















































































370 


OPTICAL PLASTICS 


present a serious problem since the high spe¬ 
cific heat and low heat conductivity of plastics 
combine to encourage steep temperature gradi¬ 
ents within the optical elements with a corre¬ 
sponding reduction in performance. 

2. The low scratch resistance of the plastics 
now in production is another limiting factor, 
for most plastic optical systems have had to be 
protected by flat glass windows. 

3. While CHM and styrene are vastly more 
homogeneous than other synthetic resins 
hitherto developed, they are not as homoge¬ 
neous and haze-free as the best glass elements, 
nor do they have the same surface accuracy. 
For these reasons the optical designer must re¬ 
strict himself to instruments of 3 to 5 power, 
in which slight inhomogeneity and departure 
from perfect figure do not have serious conse¬ 
quences. 

4. Although optical constants of CHM and 
styrene produce some advantages, no known 
plastic combination is as advantageous for cer¬ 
tain types of camera lenses as some of the new 
high-index high-v crown glasses. A plastic of 
sufficiently high index and high v-value and a 
plastic of sufficiently low v-value are still much 
to be desired. 


tered light produced by scratches. In telescope 
systems, this exposure may be avoided to a 
great extent by making the leading element of 
glass, which serves as a protective surface in 
addition to helping stabilize the system ther¬ 
mally. But in other cases a protective window 
is usually required which may interfere with 
the optical performance of the instrument. 
This objectionable feature in plastic optical 
elements may be minimized either by the syn¬ 
thesis of a plastic which is inherently hard or 
by the addition of a coating which hardens the 
surface of the ordinary plastic. 

Cross-linked polymers, such as ethylene di¬ 
methacrylate and allyl methacrylate, have 
shown some promise of being the answer to 
the demand for an intrinsically harder plas¬ 
tic. 6 ® These are 25 per cent harder on the Rock¬ 
well test than either CHM or styrene. But they 
have a tendency to craze and to be highly water 
absorbent, and hence a considerable amount of 
research on their synthesis and fabrication will 
be required to make them acceptable. 

The application of surface coatings to ordi¬ 
nary plastics to make them abrasion resistant 
has been more successful. Two principal meth¬ 
ods will be described: (1) the evaporation of a 



Figure 17. The abrader, used in testing the resistance of a surface to abrasion. 


86 SURFACE-HARDENING COATINGS 
FOR OPTICAL PLASTICS 

The principal objection to the extensive use 
of plastic elements in optical systems is the low 
scratch resistance of the material. Where the 
optical surface is exposed to dust, it rapidly 
deteriorates in efficiency due to increased scat¬ 


coating by the techniques used in deposting 
metallic films, and (2) exposure to silicon tetra¬ 
chloride vapor in the presence of ozone. 

Evaporation Process 

The evaporation technique used at the Polar¬ 
oid Corporation 6 * is the same as that custom- 






























SURFACE-HARDENING COATINGS FOR OPTICAL PLASTICS 


371 


arily employed in depositing metal films on 
plastic elements. The deposition is made at 
pressures of the order of 5 X 10~" 5 mm of mer¬ 
cury. Tantulum filaments or boats are used to 
heat the material. 

Twenty-three materials were tried in the 
evaporation process. These were selected for 
adequate hardness, low solubility in water, 
good adhesive powers, index of refraction 
lower than that of the plastic, and a thermal 
coefficient of expansion such that the coating 
would not be likely to crack when subjected to 
large temperature changes. A few high-index 
materials were also tried. The films were de¬ 
posited on styrene flats, and the coating hard¬ 
ness was estimated semiquantitatively by rub¬ 
bing the surface with a cloth. The estimates of 
the scratchability of the surface were then 
correlated with the haze index, as determined 
in the standard ASTM falling silicon carbide 
test for mar resistance, D673-42T. 

Of the 23 materials tested, only grossularite 
[Ca 3 Al 2 (Si0 4 )3] and phenacite (2Be0*Si0 2 ) 
produced a satisfactorily adhesive coat with 
hard surface (haze index, 0 to 20). Some 
others, such as calcium triorthophosphate and 
calcium silicate, were hard and adhered to the 
surface, but turned dark in the evaporation 
process. 

Grossularite, in addition to being easily pro¬ 
curable, shows less tendency to crystallize than 
does phenacite. Phenacite is scarce and expen¬ 
sive, but shows an extremely low haze index 
on both CHM and styrene surfaces. 

It is evident that considerably more experi¬ 
mentation will be required before the evapora¬ 
tion technique can be considered wholly suc¬ 
cessful in producing hard surface coatings. It 
is, however, definitely promising. 


862 Silicon Tetrachloride—Ozone Process 

Progress in vaporization of surface-harden¬ 
ing materials on plastics, notably Lucite, has 
been made at the California Institute of Tech¬ 
nology under OSRD Contract OEMsr 657. 7 In 
brief, the procedure consists of exposing the 
plastic surface alternately to air of controlled 
humidity and to a mixture of silicon tetra¬ 


chloride vapor and ozone. Two exposures to the 
mixture containing silicon tetrachloride are 
sufficient to coat a plastic. It will resist abra¬ 
sion by normal cleaning 20 or 30 times as long 
as will the untreated plastic. A more detailed 
description of the process, and of the tests de¬ 
vised to measure quantitatively the abrasion of 
the surface, follows. A knowledge of testing 
methods is necessary for an evaluation of the 
success of this hardening technique. 

Methods of Testing 

In order to measure quantitatively the re¬ 
sistance of a surface to abrasion, a standard 
test has been developed by which plastics may 
be compared. The common tests for hardness, 
such as the Brinell test or the Rockwell test, 
do not suffice, since they measure body hard¬ 
ness. The Mohs mineral test is not sufficiently 
refined for the present purpose. 

The method of testing which was finally de¬ 
veloped consists in drawing back and forth 
over the surface a pad impregnated with abra¬ 
sive and in measuring the resulting wear by 
means of light scattered from the surface. Two 
devices were developed for this purpose, an 
abrader and a reflectometer. 

The Abrader. Figure 17 shows the abrader 
in schematic form. It consists of a beam assem¬ 
bly 40 cm long, to one end of which (at F) is 
fastened the sample plastic to be abraded. 
Weights may be hung at D to press the sample 
against the pad holder K with any desired 
force. A stage G sliding in two runners parallel 
to the beam is oscillated back and forth by a 
motor-driven eccentric N. Mounted on the 
stage is a motor J with vertical shaft which 
rotates the pad holder at 60 rpm. The stage 
oscillates with an amplitude of V 2 -in. approxi¬ 
mately 25 times per minute. A counter is at¬ 
tached to the motor which drives the stage, so 
that the number of strokes of the latter may be 
recorded. Samples varying in width from % to 
l!/ 2 -in., in length from %-in. to several inches, 
and in thickness up to % 6 -in., may be clamped 
in the carrier F and abraded. 

Two pads are used for the testing. One, 
called the 20-45 pad, is compounded of 40 per 
cent rubber, 45 per cent fine silicon (20 to 45 
/x diameter), 10 per cent whiting, and small 



372 


OPTICAL PLASTICS 


percentages of accelerator, stabilizer, and 
stearic acid. The other pad, called the CS-15 
pad, is cut from a rubber abrasive disk labeled 
“CS 15” which is used in the Taber abraser. c 
Before each test the pad must be dressed by 
allowing it to run for 50 to 100 c over silicon 
carbide paper. The choice of pad to be used 
depends on the hardness of the sample to be 
tested. 

In the operation of the device, the sample is 
placed in clamp F, the height of pad is ad¬ 
justed so that a level shows the sample to be 
horizontal, and the counterweight E is ad¬ 
justed to balance the beam. Then a 50-g weight 
is added at D, the motors are turned on and 
the stage allowed to make 100 c. For samples 
much softer than Lucite, fewer cycles and less 
weight are used. After the abrasion, the sample 


nected in series with the cell and associated 
resistors, is used for the measurement. 

The lens, mirror, and shields are adjusted 
until an image % in. wide falls upon the 
sample. Then a slide of Lucite, % in. thick, is 
placed over the aperture and the knife edge is 
adjusted until the beam reflected from the 
upper surface of the slide is intercepted. Thus 
for samples thicker than Vs in- only light re¬ 
flected from the lower surface enters the photo¬ 
cell. The area of sample measured is about 
Ys in. square. Once the optical system was ad¬ 
justed it was never changed. With this restric¬ 
tion, fluctuations due to area of illuminated 
spot, intensity of the light, etc., were consid¬ 
ered to be negligible. When a treated plastic 
was tested, all measures were referred to an 
untreated and unmarred sample of that plastic, 



is washed to remove any rubber and abrasive. 
The mark made by the pad is % in. long and 
Y 4 > in. wide, and is composed of numerous faint 
scratches, roughly circular at the ends but hav¬ 
ing a uniform diagonal pattern in the center. 
The abrasion is measured in this uniform mid¬ 
dle area. 

The Reflectometer. Figure 18 shows the opti¬ 
cal system of the reflectometer used to measure 
the abrasion of a sample. Light from the 100-w 
Mazda projection lamp £ is focused on the 
abraded area P of the sample at an angle of 45 
degrees. A knife edge E cuts off unwanted light 
reflected from the upper surface of the plate. 
Light reflected from the lower face of the 
sample is collected by the Weston photronic cell 
at C. A high-sensitivity galvanometer, con- 

c Manufactured by the Taber Abraser Corporation, 
North Tonawanda, New York. 


that is, the reflectometer was set to read zero 
by proper adjustment of a balancing resistance 
in the photocell circuit when the untreated 
plastic was placed over the aperture prior to 
each measurement of a treated plastic. 

In order to measure abrasion, the sample is 
placed over the aperture so that light falls on 
an unabraded area and a galvanometer reading 
is taken. Then an area abraded on one side is 
measured. The difference in galvanometer 
reading between measures on abraded area and 
average unabraded area constitutes the reflec¬ 
tometer measure. These usually range from 0 
to 5 for the abrasions customarily measured. 
The results are precise to about 0.1 unit on this 
scale, unless the surface is unusually irregular. 

If the sample is so warped that it will not lie 
flat on the stage of the reflectometer, or if it 
has a mottled or flawed surface, no reliable 













SURFACE-HARDENING COATINGS FOR OPTICAL PLASTICS 


373 


measures can be made with the instrument. At 
times, when a plastic is coated with a relatively 
thick film, there is an anomalous reflection 
effect which invalidates the measures. In these 
instances the abrasion is determined by com¬ 
paring the abraded area, obliquely illuminated 
under a 50X microscope, with an area of 
known amount of abrasion. 

Measures repeated for several months on 
Lucite with the 20-45 pad, weighted at 50 g and 
used at 100 c, yielded reflectometer readings 
with a range 3.0 to 3.7. Since these figures in¬ 
clude fluctuations of hardness in the samples 
as well as fluctuations in the abrasive power of 
the pad, they furnish an overall measure of 
the reliability of the test. Similar runs with the 
CS-15 pad on Columbia Resin 39 gave a con¬ 
stant abrasion of 0.7 to 0.8. 

A scale of surface hardness based on the re¬ 
flectometer measures uses the hardness of Lu¬ 
cite as a unit. The term abrasion number [AN] 
designates the amount of abrasion referred to 
Lucite (AN = 1). The hardness of Columbia 
Resin 39 is AN 15. The abrasive powers of the 
coarse and fine pads were determined em- 


Table 8 . Surface hardness of plastics, determined 
by abrasion test. 



Abrader 

Number 

Abrasion number 

Plastic 

pad 

of cycles 

untreated 

treated 

Lucite 

20-45 

100 

1.0 

15-20 

CHM 

20-45 

50 

0.7 

10-14 

Styrene 

20-45 

10 

0.1 

1-3 

Bakelite 

20-45 

50 

0.9 

... 

CR 39 

Du Pont 

CS 15 

100 

15 


hardened 

Lucite 

CS 15 

100 

16-20 


EDM-CHM 


... 

6-9 

... 

EDM 

CS 15 


20 



pirically by noting the abrasion each produced 
on a surface of AN 6. Under similar conditions, 
the coarse pad produces 3.0 to 3.5 times as 
much abrasion as the fine pad. Formulas for 
the calculation of AN have been developed for 
Lucite; other plastics are then referred to this 
standard. 7 " 1 

The results of abrasion tests on samples of 
Lucite and other plastics are shown in Table 8. 

Other Methods for Testing. Other tests 7b 
were used for measuring the correlation be¬ 


tween actual wear on a plastic surface, due to 
rubbing with cloth and, to weathering, and the 
abrasion number of the plastic. Another unit of 
measure, the scatter number [SN], was de¬ 
vised to measiire the wear effect. A wear ratio , 
defined as the ratio of the number of wipings 
a treated slide must receive to the number an 
untreated slide must receive so that both slides 
give the same SN number, gives a practical 
measure of the superior hardness of a treated 
surface. 

In the case of untreated CHM and styrene, 
the wear ratio is approximately the same as 
the AN (compared to Lucite). Hence the abra¬ 
sion test is a close measure of the resistance to 
actual wear. For plastics with hardened sur¬ 
faces, the wear ratio was always greater than 
the hardness given by AN; the abrasion test 
in these instances gives a conservative measure 
of the resistance to wear. The SN might be 
used as a criterion in deciding whether a sur¬ 
face is sufficiently free from blemishes to be 
useful in optical work. Unmarred, untreated 
Lucite has SN equal to 7. For an excessively 
marred surface SN exceeds 16. A treated plas¬ 
tic, to be of value, should have an SN of about 
11 or less. 

Crazing due to temperature fluctuations is a 
serious defect in hardened surfaces on plastics, 
and a method for measuring this effect has 
been developed. When applied to Lucite, it was 
found that craze lines did not develop on the 
untreated plastic. For films with a hardness 
less than 12 AN, crazing usually did not take 
place. For hardness greater than this value, 
the crazing increased with the hardness of the 
film. 

Method of Obtaining Hard Films with 
Silicon Tetrachloride 

Research in the production of hard surface 
coatings on plastics was directed toward the 
application of films of silicon compounds, mag¬ 
nesium fluoride, and aluminum oxide. Wher¬ 
ever possible, both liquid and vapor phase 
treatments have been attempted. Results of ex¬ 
periments with aluminum oxide films were not 
successful. The surfaces were not hardened ap¬ 
preciably and in some cases were attacked by 
the liquids or vapors used. It was possible to 







374 


OPTICAL PLASTICS 


obtain a film of magnesium fluoride on Lucite, 
but the adherence to the surface of the plastic 
was poor and no increase of hardness was 
observed. Furthermore the Lucite softens and 
becomes flexible in an atmosphere of HF 2 and 
can be deformed easily. After standing in air 
for several hours, it becomes rigid but retains 
its deformed shape. Details of these experi¬ 
ments are given elsewhere. 70 

Considerable success was achieved, however, 
with silicon tetrachloride vapor combined with 
either ozone or nitrogen dioxide. The former 
combination should lead to a suitable industrial 



Figure 19. Schematic diagram of the ozone train 
used in the SiCl 4 —ozone process. 


process for hardening plastics. The method has 
been applied successfully to Lucite and with 
moderate success to CHM and styrene. 

Silicon-Tetrachloride—Ozone Process. To re¬ 
move adhesive or grease, the plastic to be 
coated is cleaned in an organic solvent, such as 
95 per cent ethyl alcohol. Gentle rubbing with 
absorbent cotton is permitted, and excess sol¬ 
vent is blotted off with clean cotton towels. 

The surface is not wet by water, so it is 
washed in a thick lather made of Dreft and 
water which does wet the surface. It is finally 
rinsed with distilled water until the Dreft is 
completely removed. 

The clean sample is next suspended by glass 
hooks or Lucite strips in a specially constructed 
humidifier, 7d where it is exposed to air of con¬ 
trolled humidity for a length of time dependent 
upon the relative humidity, in order to condi¬ 
tion it for treatment. If the relative humidity 
[RH] is 75 per cent, the conditioning is con¬ 
tinued for at least 30 hr; at RH of 50 to 60 per 
cent the conditioning requires at least 65 hr. 
Experiments have shown that if RH is less 
than 50 per cent, the plastic will not be hard¬ 


ened appreciably; if RH is above 85 per cent 
the films have a tendency to chip off or to be 
cloudy. For small objects, it is desirable to 
maintain RH near the lower end of the range 
to insure an even film thickness. The recom¬ 
mended RH for large objects is also between 
50 and 60 per cent, although higher values 
yield equally good films. 

After the humidifier conditioning, the 
sample is transferred as quickly as possible to 
a treatment vessel, and the latter is evacuated 
to a pressure of 25 to 35 mm of Hg. These 
treatment vessels are either standard three- 
neck taper flasks or, for the larger samples, a 
10-in. vacuum desiccator. It is preferable to 
use a water aspirator rather than a vacuum 
pump because of the injurious action of silicon 
tetrachloride (SiCl 4 ) vapor on the latter. 

Recently distilled SiCl 4 , in a concentration 
0.8 ml per liter, is admitted to the treatment 
vessel through an evaporator, and the vessel 
is connected to a standard Siemens laboratory 
ozonizer, activated by a 15,000 v, 30 ma trans¬ 
former. A diagram of the ozone train is shown 
in Figure 19. After running for 20 min, the 
ozonizer will yield 3.5 to 5 per cent ozone at a 
rate of 0.2 1 per min. The ozone-oxygen mixture 
is allowed to flow into the treatment vessel at 
this rate until the pressure is atmospheric. The 
sample is then exposed to the ozone—SiCl 4 
mixture for about 2 hr. 

Neither the concentration of SiCl 4 nor that 
of ozone is critical. It has been found, however, 
that concentrations of SiCl 4 less than 0.5 ml 
per liter produce no hardening of the plastic 
surface. 

At the end of the exposure period, the vessel 
is partially evacuated (3 to 5 min with a water 
aspirator) and air, which has been dried by 
passage through a large column of calcium 
chloride, is admitted through the evaporator. 
Then the sample is transferred to the humidi¬ 
fier and conditioned for 2 hr more. It is not 
necessary that this second conditioning be at 
the same RH as the first, but for large samples, 
it is recommended that the same RH (50 to 60 
per cent) be used in both instances. For small 
samples, the second conditioning may be car¬ 
ried on at a higher RH than the first, say at 
75 per cent. 






























OPTICAL QUALITY OF PLASTIC ELEMENTS 


375 


Again, the sample is transferred to the treat¬ 
ment vessel and exposed to SiCl 4 and ozone for 
a period of i/2 to 2 hr. A full 2-hr period is 
recommended if the humidification has been 
done at low RH. If the conditioning has been 
done at high RH, the time of second exposure 
to SiCl 4 -ozone will determine the hardness to 
some extent. No increase in hardness of sur¬ 
face, however, will result from exposures over 
2 hr in length. Concentrations of SiCl 4 and 
ozone are the same as in the first exposure 
period. 

During this second exposure, bright inter¬ 
ference colors become apparent on the surface 
of the plastic. Variations in the thickness of the 
hardening film are indicated by the spotty or 
uneven character of the interference patterns. 
It has been found that the ozone must be pres¬ 
ent to insure an even deposit of the hard ma¬ 
terial on the plastic surface. A fully developed 
interference bloom on the surface indicates 
that the exposure to the vapor mixture is com¬ 
plete. 

A final conditioning in the humidifier at the 
RH used for the second conditioning for an 
hour concludes the treatment. It has been 
found desirable to allow the larger samples to 
stand for a few days after treatment, so that 
the film may set. 

While the above description of treatment is 
for Lucite, a similar procedure has been tried 
on CHM and on styrene. In the case of CHM, 
however, the film tends to be a little cloudy. 
Initial conditioning at low RH (50 to 60 per 
cent) for the longer time intervals is recom¬ 
mended to minimize this effect. Original faults 
and blemishes on the surfaces of CHM samples 
are intensified more by the hardening treat¬ 
ment than in the case of Lucite. 

The treatment for Lucite, when applied to 
styrene, increases its hardness by a factor of 
5 to 15. A modification of the technique in a 
single experiment on styrene, however, has 
been found to increase this factor to 20 to 30. 
In this experiment, the preliminary condition¬ 
ing was at 75 per cent RH for 77 hr, exposures 
to SiCl 4 and ozone were for 1 hr each time, 
and intermediate conditionings were at 75 per 
cent RH for 20 hr each. The samples gained 
in surface hardness by a factor of nearly 30. 


Tabulation of Results. The increased hard¬ 
ness of surface produced by application of the 
SiCl 4 -ozone process to various plastics is ex¬ 
hibited in thq last column of Table 8. Hard¬ 
nesses are also given for the newer plastic 
material, EDM, developed by Polaroid. 

It is evident from these preliminary experi¬ 
ments that further investigation to establish 
details for the processing of CHM and styrene 
is desirable. For an industrial process, a flow 
treatment would certainly be advantageous. 
The outstanding merits of the SiCl 4 -ozone 
process are that the treatment is done by gases 
and is carried out at temperatures below the 
softening point of the plastic. Objects of any 
shape can be treated without danger of de¬ 
formation. 

Silicon Tetrachloride—Nitrogen Dioxide 
Treatment. A second method of obtaining hard 
films has been tried sufficiently extensively to 
warrant further consideration. The procedure 
is similar to the SiCl 4 -ozone procedure, but the 
latter is replaced by nitrogen dioxide (NO). 
Alternate exposures to air of controlled RH and 
to a mixture of SiCl 4 and NO are made. The 
concentration of SiCl 4 is again 0.8 ml per liter 
and NO is used at partial pressures of 10 to 100 
mm of mercury. Two exposures to the vapor 
mixture have been found sufficient to harden 
Lucite to values between AN 11 and AN 20. 
The hardened surface is not as uniform, how¬ 
ever, as in the ozone process. Orange-peel 
irregularities are evident in reflected light, ap¬ 
plication of the process in its early stages to 
CHM was not successful, and the surface in¬ 
variably became cloudy and rough. By careful 
control of the humidity, however, this method 
might be developed usefully for CHM and other 
plastics. 


87 OPTICAL QUALITY OF PLASTIC 
LENSES, PRISMS, AND FLATS 

The quantitative measurement of surface 
irregularities and of internal variations in re¬ 
fractive index of CHM and styrene optical ele¬ 
ments has been carried out at the Massachu¬ 
setts Institute of Technology. 8 An attempt to 
evaluate the optical quality of plastic elements 



376 


OPTICAL PLASTICS 


as compared to glass and to give illustrations 
of their optical behavior forms the subject of 
this section. 

In general, the optical tests have consisted 
of: 

1. A study of the image formation by plastic 
lenses. 

2. A comparison of the surfaces of plastic 
lenses with test glasses of known curvature. 

3. A study by interference methods of the 
surfaces and internal structure of plastic 
prisms and flats. 


8.7.i Image Formation by Lenses 

Three of the T-108 telescope objectives (see 
Figure 11 for photograph) have been com¬ 
pared with a production T-116 glass objective 


LENS CLARK 
UNDER OBJ 


SLIT SLIT 

rf 


■ - —- 

1 MICRO 1 ^ 

OBJ 

Mi 

— 


LENS 

UNDER 

TEST 



CLARK 

OBJ 


s 


SCANNING 

SLIT 




PHOTOTUBE 


f CLARK OBJ = 72 

Figure 20. Optical system for scanning lens 
images. 


lens element furnished by the Frankford 
Arsenal. The T-116 telescope is similar to the 
T-108 but has glass optical elements through¬ 
out. For comparison, measures have also been 
made on a high-quality achromatic objective 
having a focal length of 30 in. 

The T-108 objective has a focal length of 
15.08 in. and the elements are made of CHM 
and styrene. All the plastic elements showed 
some astigmatism and one of them had “pull- 
away” faults and irregular local refraction. 
The measures were made in such a way as to 
minimize the influence of astigmatism. A mer¬ 
cury vapor source was used to eliminate chro¬ 
matic aberration as far as possible. 

Three methods of examining the images 
formed by these lenses were used. 

Method I. In the first method the lens to be 


studied was used to collimate the image of a 
slit. The parallel rays from this lens were then 
refocused by an excellent Clark astronomical 
objective, 6 ft in focal length, to form an image 
which could be scanned by a second movable 
slit, behind which was placed a photocell and 
amplifier system. Figure 20 shows the optical 
arrangement used for these tests. 

Since the ruling of a uniform slit narrower 
than 0.005 mm and of adequate length was a 


1 

V 

ui 




T 




YA 

4- 

Ja\ 

\\\ 

\\\ 

b-VA 

--ZERO WIC 

SOURCE< 
SCANNINC 

-BOTH SLI" 

-ACTUAL C 

ITH OF BOTH SL 

SLIT ZERO WIDTI 
> SLIT 3.6 MICRC 

rs 3.6 MICRONS 

)ATA FOR A GOOI 

ITS 

H 

)NS WIDTH 

WIDTH 

D LENS 

X \'> 

\ V 

\\ \ 

\\ 

C 

\ 



\\ 

c4\ 

\\ 

\ v 

, ».010MM-*‘ Ss '-. 


WIDTH OF SLI1 

0 


Z=2TrRr/f\ 


Figure 21. Diffraction patterns due to finite slit 
width. 


difficult task, the slit actually used for the 
source was the optical image of the physical 
slit produced by a microscope objective of 48 
mm focal length, so that the effective source 
was about 0.003 mm wide. The slit illumination 
was provided by a mercury lamp (AH-4 GE). 

The image of the source produced by the 
Clark objective was nearly 0.04 mm between 
secondary diffraction maxima. With a scanning 
slit 0.005 mm wide, adequately fine coverage of 
sections of the image was obtained. The scan¬ 
ning slit was attached to a micrometer screw 
so that lateral displacements could be meas- 



































OPTICAL QUALITY OF PLASTIC ELEMENTS 


377 


ured to 0.001 mm. Readings at different posi¬ 
tions across the image could be taken as de¬ 
sired. An RCA 931 photomultiplier tube with 
associated amplifier was used, the output of 
which could be read on a microammeter. The 
circuit diagram of this recorder is published in 
an MIT report. 8a 

It was found by experiment that the scan¬ 
ning slit could be placed satisfactorily in the 



Figure 22. Image profiles for plastic and for 
glass lenses. 


focal plane of the Clark objective by visual in¬ 
spection. This was checked in several instances 
by scanning the image at a few points in the 
neighborhood of the focal plane and picking 
from the response curves the position giving 
maximum image intensity. 

In order to judge the merits of the respective 
lenses whose image patterns were scanned with 
this apparatus, one must bear in mind the re¬ 
sults to be expected from a more or less ideal 
test. Effects of finite source slit and finite scan¬ 
ning slit must be considered. To this end, data 
were taken on the best astronomical objectives 
at hand and compared with the theoretical 
curves computed on the basis of diffraction 


theory. The results are exhibited by the curve 
of Figure 21. The necessary data for comput¬ 
ing the theoretical curves have been given by 
Selwyn. 9 

Curve a shows the pattern to be expected 
from an infinitely narrow slit, b shows the pat¬ 
tern to be expected from the scanning of a by a 
slit of finite width, and c shows the pattern to be 
expected in scanning the image of a finite slit 
with another finite slit. The slit widths used in 
calculating curves b and c were those currently 
in use in the optical system shown in Figure 20. 
The abscissa scale is in units of the scale used 
by Selwyn, Z = ^Rr/fx , in which r is the 
distance from the pattern center, R the lens 
radius, and / its focal length. The equivalent 
value of scanning slit width is also indicated. 
In addition to these three curves, one complete 
half-curve of type c is shown on a scale to make 
it comparable with the curve formed from 
scanning data on the perfect lens assumed in 
the computations. The degree of approach of 
the measured characteristic to the theoretical 
is evident from the figure. 

The profiles of the slit images obtained by 
scanning the plastic lenses, and also those from 
the T-116 glass objective, are shown in Figure 
22. The two curves for the glass lens were 
taken with the lens rotated about its axis at 
positions 90 degrees apart. The profile for the 
30-in. glass objective indicates an asymmetry 
which is probably due to the source slit. It is 
apparent from the figure that none of the 
plastic lenses match the comparable glass ob¬ 
jective T-116 in concentration of light. This, 
of course, is a measure of resolving power. 

A second indication of the relative resolving 
power of the lenses is furnished by the tracings 
shown in Figure 23. The single source slit has 
been replaced here by a group of five parallel 
slits. The spacing between centers is 0.015, 
0.025, 0.038, and 0.050 mm. The widths of the 
slits are indicated at the bottom of the figure. 
Clearly, the scanning method separates the slit 
images which are 0.025 mm apart. This would 
indicate a resolving power of 40 lines per mm, 
but one must bear in mind the limitations im¬ 
posed by the scanning method. By comparing 
the theoretical curves of Figure 21 with the 
corresponding data curve for the perfect lens, 





















378 


OPTICAL PLASTICS 


one sees immediately that the scanning method 
underrates the visual resolving power of the 
lens. A further visual test confirms this. When 
a low-power microscope is used to examine the 
images of the usual black and white Bureau 
of Standards test chart 10 formed by the three 
plastic lenses, all show a resolving power of 
100 lines per mm. 

It is a common observation of those using 
small telescopes with plastic optical systems 





LENS C 

^ ^- 








LENS B 



Aj\ 

LENS A 


^ nn n 

M 

_n_ cl 

^GLASS T-116 


O 0.10 0.20 0.30 0.40 

MM 

Figure 23. Image profiles due to multiple slit. 


that scattered or diffused light is a particularly 
objectionable feature. This has been brought 
out elsewhere 11 in some detail. The cause may 
lie in abnormal lens zones which divert per¬ 
ceptible amounts of light from the proper 
image location. The definition of the lens may 
not be seriously impaired, but the contrast is 
significantly reduced when dark objects are 
viewed against a bright background. 

An indication of the amount of light scat¬ 
tered into the image of a dark line when the 
surrounding field is bright is shown by Figure 
24. Dark lines, made by ruling and then photo¬ 
graphing on high-resolution plates, were 


imaged by the four lenses (three plastic, one 
glass) under test and scanned in the usual 
manner. Widths of these source lines varied 
from 0.005 to 0.020 mm. Profiles of the images 
constitute Figure 24. Before the profiles were 
taken, the lens under test was mounted in the 
position which gave the best visual perception 
of the finer lines. The amount of light scattered 
into the image of a line by the background is 
indicated by the extent to which the intensity 
falls below that of the background for the finer 
lines as compared to the broader lines. One 



Figure 24. Image profiles of opaque rulings. 


cannot draw quantitative conclusions regard¬ 
ing visual performance from these curves be¬ 
cause the method is not directly comparable. 
The method does, however, provide a valuable 
objective comparison between different lenses 
as regards scattered light. 

Method II. The second method of obtaining 
a record of the comparative image-forming 
characteristics of plastic and glass lenses con¬ 
sisted in photographing the monochromatic 
image of a pinhole source. The lens under test 
was used to collimate light from a round pin¬ 
hole about 0.015 mm in diameter. Mercury 
green radiation was used for illumination. The 
image of the source, formed by the Clark ob¬ 
jective, was magnified by a 32-mm microscope 
objective and photographed on 35-mm film. 
Pan-X emulsion was used to photograph the 
images formed by the T-108 and T-116 objec- 


































OPTICAL QUALITY OF PLASTIC ELEMENTS 


379 


tives. For the 30-in. glass objective, Microfile 
film was used in order to record the detail of 
the high quality image. 

Figure 25 displays the character of the 
image as a function of parfocal position for 
the lenses. In the case of the T-108 objective, 
the pictures correspond to intervals of 0.043 
mm in image space. The corresponding figure 


the distribution of light in an image has been 
described in detail by L. A. Jones and R. N. 
Wolfe. 12 Their method consists in photograph¬ 
ing the image of a line source, such as an in¬ 
candescent filament, through a wedge of gela¬ 
tin in which colloidal carbon has been sus¬ 
pended. If the wedge has a constant angle, its 
density gradient will be nearly constant. In the 



CHM LENSES 
T-108 X 32 


T-108 X32 


T-108 X32 


GLASS 

T-IIS X32 


HIGH QUALITY 
30“ GLASS 
OBJECTIVE x60 


Figure 25. Parfocal images of a pinhole source. 


for the T-116 is 0.048 mm. Overall magnifica¬ 
tion between image and print is about 32X for 
all except the 30-in. objective for which it is 
about 50 diameters. It is well to note the rela¬ 
tively more diffuse and complex character of 
the images formed by the plastic lenses as com¬ 
pared to those of the T-116 glass objective. The 
30-in. objective image is shown only as a more 
or less ideal type. It should not be compared 
directly with the others. 

Method III. A third method of examining 


present instance a gradient of 1.34 per cm was 
used. 

When the image of a uniformly bright line 
source is photographed through the wedge, its 
gradient being parallel to the line, the limits of 
the developed image represent a logarithmic 
plot of intensity distribution in the image. 

Images similar to those scanned photoelec- 
trically were magnified 24.5X by a microscope 
objective and photographed with the gelatin 
wedge directly in front of the photographic 





























380 


OPTICAL PLASTICS 


i i i i J i 4 i i 


CHM STYRENE 
ACHROMAT 
Hg 5461 LIGHT 
T-108 


B 


k k l k k k k k 


CHM STYRENE 
ACHROMAT 
Hg 5461 LIGHT 
T-108 


A 1 A A A A A A A 


CHM STYRENE 
ACHROMAT 
Hg 5461 LIGHT 
T-108 


111111 til 


GLASS 

ACHROMAT (FA) 
T-II6 


E 


F 


i 1 

i i 


1 

I 

l 

) i 

MM! 

1 i 

1 A 1 A 


HIGH QUALITY 
30“ TELESCOPE 
ACHROMAT 
Hg 5461 LIGHT 


HIGH QUALITY 
30” TELESCOPE 
ACHROMAT 
WHITE LIGHT 


Figure 26 . Intensity patterns of image of line source produced by plastic and glass lenses. 









OPTICAL QUALITY OF PLASTIC ELEMENTS 


381 


plate. A series of exposures was made at dif¬ 
ferent positions near focus by moving the 
microscope and camera in steps of 1.0 or 0.5 
mm. The resulting wedge pictures are shown 
in Figure 26. Mercury green light was used in 
all cases except one; that was for a glass lens, 
in which a 1-w pyrometer filament lamp was 
substituted for the original slit. When inter- 



FOUR CHM LENS EIGHT STYRENE LENS 

SURFACES SURFACES 


Figure 27. Interference patterns showing local 
accuracy of CHM and styrene lens surfaces. 

preting these pictures qualitatively, it should 
be borne in mind that a perfectly sharply de¬ 
fined image of any width would record as a 
shaded rectangle. The presence of a greater 
concentration of light in the center than at the 
edge of an image results in the peaked pattern. 
The more diffuse and spread out the image, the 
less sharp will be the wedge picture. These 
photographs confirm the results of the pinhole 
tests in showing superior imagery on the part 
of the T-116 glass lens as compared to the plas¬ 


tic T-108. No quantitative measures of light 
intensity were made/ in this examination of 
plastic lenses. 


8 72 Surface Curvatures of Plastic Lenses 

In the study of plastic lens performance, the 
figure of the surfaces of twelve styrene and 
four CHM lenses, cast in molds for the positive 
components of the T-108 objective, has been 
investigated. The first aim was to study the 
local irregularities of the cast plastic surfaces. 
Therefore, two test surfaces of Pyrex glass 
were used, one figured to match each face of 
the finished lens component. Since the radius 
of the test glass designed to test the shorter 
radius of the lens was longer than any of the 
radii found in the samples, the measurements 
were made at a somewhat elevated temperature 
(about 38 C). Otherwise, an excessive number 
of interference fringes would have been ob¬ 
served between test surface and sample. 

Photographs of the interference patterns 
formed between the test glass and the shorter 
radius surface of the plastic lens were made in 
sodium light. Some of these are shown in Figure 
27. In all cases the plastic was allowed to reach 
temperature equilibrium with its surround¬ 
ings. Since the room temperature was not the 
same for all the pictures of Figure 27, no at¬ 
tempt should be made to compare the surfaces 
quantitatively with the test glass by counting 
these interference fringes. 

Five of the most regular surfaces, as shown 
by the interference patterns, and one other 
were selected to determine quantitatively the 
departure of the surfaces from spherical form. 
This was done in the usual way by measuring 
the diameters of the interference rings of suc¬ 
cessive orders and comparing the run of di¬ 
ameters with that to be expected if the surfaces 
were perfectly spherical. In obtaining a for¬ 
mula for the relation between a given ring 
diameter and the phase retardation in wave¬ 
lengths between reflected rays at the two sur¬ 
faces of the interface, three corrections must 
be considered. First, the interfering rays are 
not normal to the lens surface in the interfer¬ 
ence interspace; second, the normal is not 



382 


OPTICAL PLASTICS 


parallel to the sagittal intercept on the lens 
axis; and, third, the curvatures are so great 
that the sagitta exceeds that given by the 
simple formula, h 2 /2R, where h is the radius 
of the zone of the interspace considered, and R 
is the radius of curvature of the surface. 



FRINGE NUMBER 


Figure 28. Departures of surfaces of plastic 
lenses from sphericity. 


These corrections are readily expressed in 
mathematical form, with the result that we 
obtain the basic interference equation for the 
mth ring: 


(m + e) 



+ 2 


1A 

2 V/ 


R 



d 2 R t - R 
8 R 2 


where /, R, p, e are the focal length, radius of 
curvature of interference face, distance from 
the face to the camera lens, and a fraction, re¬ 
spectively; R t is the radius of curvature of the 
test glass; and d is the diameter of the mth in¬ 
terference ring. If the surface were a sphere 


of so long a radius that the corrections in the 
brackets could be neglected, the integral orders 
of rings plotted against d 2 would fall on a 
straight line, whose intercept at d 2 = 0 indi¬ 
cates the value of e. If the surfaces are steep 
and the rays oblique so that the correction is 
necessary, it may be computed for each d 2 and 
the left-hand side of the equation will then be 
proportional to d 2 . 

Measurements of four surfaces, corrected and 
plotted, are shown in Figure 28. Each surface 
pattern was measured on two diameters normal 
to each other. The departures of these surfaces 
from sphericity, evident from the figure, are 
not serious. 

Probably the least important deviation of a 
lens from a standard dimension is in the radius 
of curvature (or in the curvature defined as the 
reciprocal of the radius of curvature). However, 
one of the purposes in testing the surface was 
to note how the radii of the lens specimens tested 
varied in their relationship to the radius of the 
test glass. The measures were made in a room at 
a controlled temperature sufficiently high (about 
38 C) to insure a reasonable fit between the two 
surfaces for all the specimens. 

The method of measurement was simple. The 
number of fringes of sodium light within a circle 
of 2-cm radius was counted for each surface. 
This number is directly proportional to the 
sagittal difference between the spherical sur¬ 
faces; each fringe observed indicates a differ¬ 
ence of 1.5 X 10~ 6 mm -1 in the curvatures of 
test glass and lens surface. The results of these 
measures are shown in Table 9. Column 1 gives 
the number of the lens surface, columns 2 and 3 
give the deviation in fringes between test glass 
and each lens surface. If these numbers are 
added algebraically, multiplied by n -1 (0.50 for 
CHM and 0.59 for styrene), and multiplied by 
1.5 X 10 -6 we obtain the deviation in power, 
or reciprocal focal length, due to the surface 
departures. Column 4 shows this deviation. 
Since the corresponding focal power is 5,900 X 
10~ 6 for the CHM lens and 6,960 X 10 -6 for the 
styrene lens, the overall deviation of these 
lenses does not exceed 0.5 per cent. A positive 
entry in Table 9 means that the surface under 
test has a greater curvature than the standard. 
There is no evidence that departures of curva- 




















OPTICAL QUALITY OF PLASTIC ELEMENTS 


383 


ture in one surface tend to be compensated by 
departures in the other surface. The molds from 
which these lenses have been cast are desig¬ 
nated in column 5 of Table 9. 


Table 9. Deviations of lens surface curvatures 
from a standard test glass. 


Lens 

Curvature 
difference* 
fringes 
(^i) ( r 2 ) 

Difference 

1 

/ 

Molds 

(#i) ( r 2 ) 

CHM 611f 

—3 

10 

5 X 10-6 

7X 

225L 

633 

—1.5 

—6 

—6 

«< 

« 

753J 

—5 

5 

0 

u 

M 

857§ 

—3.5 

3 

0 

a 

« 

Styrene 446 

—5.5 

—2.5 

—7 X 10-6 

7X 

225L 

447f 

—5.5 

3 

—2 

338A 

L55 

458 

—6 

1 

—4.5 

7X 

it 

459 

—5.5 

—5.5 

—9.5 

it 

it 

471 

—4 

1 

—2.5 

338A 

225L 

472 

—7 

2 

—4.5 

7X 

L55 

502 

—10 

6 

—3.5 

“ 

225L 

576|| 

—5.5 

7 

1 

a 

u 

581H 

—2 

—7 

—8 

a 

u 

589 

—8.5 

1 

—6.5 

a 

u 

654 

—11 

—12 

—20 

u 

«< 

660f 

—5.5 

9 

3 

a 

u 


* Nominal radii: R± = 265.0 mm, R 2 — 124.7 mm 
t Both surfaces very irregular 
i Edge fault in R\ 

§ Ri queer 
|j i?i irregular 
H Ro irregular 


Interference Tests of Flats 

The optical behavior of the plastic materials, 
CHM and styrene, was examined further by 
obtaining interference patterns on the surfaces 
of nearly flat, thick slabs, 4 in. in diameter, and 
comparing these with transmission patterns ob¬ 
served with a Twyman-Green interferometer. 
In general, the plates were not perfectly flat, 
but had one concave and one convex surface of 
some 100 to 200 m mean radius. 

The surfaces of the plates were examined 
against a good glass optical flat. Photographs 
of several of these fringe patterns are shown in 
Figure 29, A and B. Some of these cover only 
the central three-fourths of the plate, since the 
mercury illuminating system had a restricted 
aperture. Others, made with sodium light, cover 
the full aperture. 

To investigate the departures of the surfaces 


from spherical form, the fringe patterns were 
measured along major and minor diameters. 
For successive fringes, tabulation was then 
made of the diameters squared. The first differ¬ 
ences of these squared diameters should be con¬ 
stant for a spherical surface if the angle sub¬ 
tended at the camera lens is not large. Obliquity 
corrections for larger subtended angles would 
not exceed % fringe and could safely be omit¬ 
ted even for the most highly curved surface. 

The departure of the diametral surface sec¬ 
tions from circular form was found to be very 
small. In Figure 80, the values of d 2 for suc¬ 
cessive fringes, as shown by CHM sample 377, 
are plotted. The departure of the abscissa of a 
plotted point from the straight plotted line, in 
fringes, indicates the departure of the corre¬ 
sponding zone from the sphere in half wave¬ 
lengths of the sodium light used. The lines 
drawn represent the best sphere with a diam¬ 
eter of 2.5 in. 

Figure 29 also shows the transmission inter¬ 
ference patterns photographed with the Twy¬ 
man-Green interferometer. A comparison of 
the surface fringe pattern with the transmis¬ 
sion pattern enables one to draw conclusions 
regarding the internal homogeneity of the 
plates. Each fringe of the surface pattern 
means an increment of surface departure of 
one-half wavelength. When a beam is trans¬ 
mitted once through an imperfect surface, there 
will result a local deviation of n — 1 half wave¬ 
lengths (n being the refractive index of the 
material) for each fringe observed by interfer¬ 
ence with a test flat. When both surfaces are 
not plane, the algebraic sum of their respective 
fringe-pattern counts must be used. 

In the interferometer the beam traverses the 
surface twice. Thus, for each surface fringe, 
there will be in the interferometer a retarda¬ 
tion of n — 1 whole wavelengths, each of which 
is indicated by 1 transmission fringe. For 
CHM, of index 1.50, every 2 surface fringes 
correspond to 1 transmission fringe if the ma¬ 
terial is optically homogeneous. For styrene of 
index 1.59, 1.7 surface fringes correspond to 
1 transmission fringe. 

The less complex patterns shown by the sty¬ 
rene specimens I and II, Figure 29B, allow a 
more satisfactory comparison of the surface 







384 


OPTICAL PLASTICS 








TOP 

FACE 



BOTTOM 

FACE 


TRANSMISSION 


Figure 29A. Interference fringe patterns for flat disks of CHM and styrene. 


























OPTICAL QUALITY OF PLASTIC ELEMENTS 


385 



BOTTOM 

FACE 


TRANSMISSION 


TWO STYRENE PLATES 


Figure 29B. Interference fringe patterns for flat disks of CHM and styrene. 




386 


OPTICAL PLASTICS 


and transmission characteristics than do the 
more irregular CHM patterns. Sample II has 
one surface that is so nearly flat except near 
one edge that the transmission pattern can be 
ascribed to the other surface alone. The simi¬ 
larity of the transmission pattern to that for 
the curved face, allowance being made for the 
1.7 ratio, shows that the internal optical struc¬ 
ture of the sample is remarkably uniform. 

In the case of sample I, a different procedure 
was used. From measures of diameter, the 



Figure 30. Departure of surfaces of CHM plate 
from spherical form. 


fringes were located relative to an estimated 
center and plotted as shown in Figure 81. Ordi¬ 
nates are the successive numbers of half wave¬ 
lengths. A curve was drawn for each surface 
pattern at corresponding diameters. At an 
abscissa point where these surface curves are 
1.7 fringes apart in ordinate, the first trans¬ 
mission fringe should be found. Where the sur¬ 
face curves are 3.4 fringes apart, the second 
transmission fringe should be found, and so 
forth. When analysis of styrene sample I is 
made in this way, the fringes of the transmis¬ 
sion pattern are found where they are pre¬ 
dicted, within the accuracy of measurement. 


The conclusion to be drawn from these pat¬ 
terns is that, for styrene, optical inhomogeneity 
is insignificant compared to the accuracy of 
figure of the molded surface. The patterns indi- 


10 -1-1-1-1-1-1-r 



01 2345678 


DISTANCE FROM EDGE IN CM 

Figure 31. Surface deviations of styrene plate for 
comparison with transmission fringe pattern. 

cate a constant refractive index in the styrene 
samples tested to within 4 parts in a million 
(or 0.000002 in the index itself). 

Corresponding analyses for the CHM sam- 








476/1 



\ 




/4<>j 

476 _ 

1 

W 1 j 

1 

/ 



6 4 2 0 2 4 6 

DISTANCE FROM CENTER IN CM 


Figure 32. Surface deviations of CHM plates cor¬ 
related with their transmission. 

pies are shown in Figure 32. They are not 
nearly as conclusive in this case because of the 
greater irregularity of pattern. There seems to 
be a tendency for excess optical density in the 






















































OPTICAL QUALITY OF PLASTIC ELEMENTS 


387 


outer zones of the plates tested. In Figure 32, 
the heavy long dashed lines are the transmis¬ 
sion pattern cross sections predicted from the 
surface curves; the thick broken lines repre¬ 
sent measures made on the transmission pat¬ 
terns. The discrepancy between predicted and 
observed patterns is several times greater with 
these CHM samples than with the styrene 
sample tested. 

It must be borne in mind, in evaluating these 
results, that the surface patterns are very 
susceptible to temperature fluctuations while 
the transmission patterns are not. All possible 
precautions were taken to minimize these 
effects. 


Interference Tests of Prisms 

Four 90-degree total reflecting prisms and 
six 60-degree prisms, all of styrene, were ex¬ 
amined for surface and internal optical quality. 
The surfaces of the 90-degree prisms were pho¬ 
tographed in mercury green illumination 
against an optical flat, with the results shown 
in Figure 33. It is evident that the faces are 
far from plane. No attempt was made to evalu¬ 
ate the departures quantitatively, because of 
their size. 

The observation of the transmission patterns 
in the interferometer required the use of a 
small pinhole source to resolve the fine-grained 
pattern. The 60-degree prisms were observed 
at minimum deviation, and the 90-degree 
prisms with the beam incident normally on the 
right-angle faces and with total reflection on 
the hypotenuse face. One of the elements 
(W-211) was also used as a 45-degree refract¬ 
ing prism. Figure 34 shows the interference 
patterns so obtained. 

All of the prisms showed evidence of pro¬ 
nounced internal strain, which appears as local 
zones of ill-defined interference. Attempts to 
record the patterns in polarized light, with 
polarizer and analyzer crossed, were unsuccess¬ 
ful because of the faint illumination resulting 
from the small pinhole. One prism (V-903), 
however, was examined satisfactorily with the 
polarizer and analyzer parallel. The photo¬ 
graph obtained is shown also in Figure 34. 


The irregularities shown by these patterns 
indicated the desirability of a direct check on 
the visual optical performance of the prisms. 
To this end, a Bureau of Standards chart 10 was 
set up at a distance of 70 ft, so that with a glass 
prism placed before a 12 X telescope of 30-cm 
focal length the finest line pattern was not re¬ 
solved but the next was easily resolved by two 
different observers. Then the best resolution 
with each of the 45-degree styrene prisms was 
obtained by setting it in turn in place of the 
glass prism. Refocusing was done each time to 
insure the best performance of each prism. The 
resolving power in lines per mm in the focal 
plane of the telescope is given in Table 10. All 
of the plastic prisms show considerably less re¬ 
solving power than the glass prism used as 
standard and most of them have appreciable 
astigmatism. 


Table 10. Resolving power of styrene prisms, 
lines per mm at focal plane of 30-cm telescope. 


Ob¬ 

Glass 

V 904 

V 905 

W 212 

W 211 

server 

vert. hor. 

vert. hor. 

vert. hor. 

vert. hor. 

vert. hor. 

P 

85 85 

67 

54 

70 70 

75 

48 

75 

65 

R 

95 95 

80 

60 

80 80 

62 

40 

85 

80 


87-5 Conclusions 

The evidence supplied by these examinations 
of an admittedly small number of plastic sam¬ 
ples seems to rank the material decidedly sec¬ 
ondary to glass, at least as the elements are 
fabricated at present. Most of the irregulari¬ 
ties, however, in the symmetrical elements such 
as lenses and flats, appear to be surface defects 
which may be overcome with added improve¬ 
ments in the fabrication technique. It is to be 
expected that the prism faces show the least 
satisfactory optical quality because of the 
greater asymmetry in the contracting stresses. 

Evidence from the interference patterns of 
CHM and styrene plates indicates that the 
latter is superior in optical homogeneity. The 
best styrene specimens show an index variation 
of about 2 X 10 -6 over a 3-in. diameter speci¬ 
men ; the CHM variation is about three or four 
times as much. The index fluctuation is zonal 
in character and astigmatic in the one plate 







388 


OPTICAL PLASTICS 


thus studied. For both materials, there is no 
evidence of flocculent or other moderately fine¬ 
grained variation of index. 

The presence of considerable scattered light 
in plastic optical elements, high thermal expan¬ 
sion properties, and surface irregularities are 
the features which for the time being limit the 
applicability of this material to other than low- 
power systems. A secondary disadvantage is 
their softness, which may be overcome by hard- 



PRISM FACES 


Figure 33. Surface deviations of four styrene 
prisms. 

ening techniques or by the future development 
of hard cross-linked polymers. 


88 RECOMMENDATIONS BY NDRC 

The internal homogeneity of present plastic 
elements is excellent, but further work is indi¬ 
cated to improve strain, surface accuracy, haze, 
and surface hardness. 

The development of cross-linked polymers 
appears to offer the most promising means for 
increasing surface hardness, but efforts should 


be continued to develop an improved method 
for depositing hard surface coatings by evapo¬ 
ration. 



TRANSMISSION PATTERNS 
FOR 10 PRISMS 


Figure 34. Transmission interference patterns of 
styrene prisms. The pattern V903-Pol. was taken 
in polarized light. 

An extensive chemical program is indicated 
to explore many different types of compounds 
which have not yet been investigated. This pro¬ 
gram should aim to discover compounds which 
have favorable characteristics from the point 
of view of optical design. It would also be very 
desirable to develop plastics which have smaller 
temperature coefficients of index of refraction 
than CHM and styrene. 











Chapter 9 

OPTICAL TECHNIQUES 

By James G. Baker and H. F. Weaker' 


i 


T his chapter describes several miscella¬ 
neous optical techniques. These include tech¬ 
niques for the grinding and polishing precision 
lenses and mirrors (Harvard University, Con¬ 
tract OEMsr-474), for grinding and polishing 
roof prisms (Mount Wilson Observatory, Con¬ 
tract OEMsr-101), the molding of glass lenses 
(Eastman Kodak Company, Contract OEMsr- 
421), the making of reticles (Edward Stern 
Company, OEMsr-293, and California Insti¬ 
tute of Technology, Contract OEMsr-389), and 
the deposition of low-reflection and high-effi¬ 
ciency films (California Institute of Technol¬ 
ogy, Contract NDCrc-118, Yard, Inc., Contract 
OEMsr-529, and the University of Rochester, 
Contract OEMsr-160). 

Some of these techniques were developed in 
the course of establishing shop groups capable 
of turning out quickly prototypes of optical 
fire-control instruments and cameras for the 
Armed Forces. Others were set up specifically 
to develop new techniques as an aid in the mass 
production of critical items. 


91 METHODS FOR GRINDING AND 
POLISHING LENSES AND MIRRORS 

The Harvard contract (OEMsr-474), devoted 
primarily to design and construction of equip¬ 
ment for aerial photography, required at the 
outset that fairly large optical and machine 
shops be set up to handle the difficult instru¬ 
mental problems encountered in the air. Al¬ 
though the ensuing shop work emphasized pre¬ 
cision of manufacture to a very large extent, 
rather than short-cut methods, a number of in¬ 
teresting procedures were developed. 

The optical industry on the whole is one long 
known for the close keeping of trade secrets. 

a Dr. Baker of Harvard College Observatory has com¬ 
piled the first section of this chapter relating to 
“Methods for Grinding and Polishing Lenses and 
Mirrors.” The other sections of this chapter have been 
compiled by Dr. Weaver of the University of California. 


To a very large extent, these secrets are pro¬ 
cedures that ought to occur to any worker but 
which in practice seem to come only after years 
of often misplaced effort. 

Outstanding opticians are most often men of 
inherent ingenuity and intelligence who think 
of their handicraft as an art rather than as a 
trade. These individuals professionally are 
often reluctant to reveal even to their fellow 
workers the ideas and methods that have solved 
their own problems. Consequently, very few 
textbooks exist that can aid a serious minded 
beginner. The future skilled optician most often 
learns through his own experiences. 

In 1940, the United States possessed very 
few men with the training required for unin¬ 
structed fabrication of optical parts of large 
size. The onset of World War II found these 
few workers almost submerged by the need to 
organize unfamiliar production line techniques, 
train new workers, and fabricate individual 
optical parts. The skill in fabrication necessi¬ 
tated years of experience and therefore could 
not be passed to others. Almost without excep¬ 
tion such workers in the optical industry re¬ 
sponded to their imposed responsibility, and, 
as a very small group, are directly to be cred¬ 
ited for a very major contribution to their 
country’s good. 

The years of intensive wartime application 
to production of a wide variety of optical goods 
produced large numbers of new workers of dis¬ 
tinction. It is no longer considered that a pre¬ 
cision optical worker must inherit his aptitudes 
beyond the possession of great patience, intelli¬ 
gence, and alertness. 

The Harvard contract in 1942 was not in a 
position to borrow or obtain opticians of requi¬ 
site training from established industries. It 
was necessary to train new men for the task 
by dint of many hours of trial and repetition. 
Fortunately, a group of amateur craftsmen 
existed locally which had years of experience 
at figuring large mirror surfaces to the highest 


389 



390 


OPTICAL TECHNIQUES 


optical standards. These men were drawn into 
the project and throughout World War II 
proved the value of their own training and 
hobby. Where others had found that amateur 
workers rarely appreciate the need for output, 
in this case the pressure of the war and the 
experimental character of the work eliminated 
such problems. 

The following pages will attempt a summary 
of optical procedures employed in the Harvard 
contract. It is hoped that these otherwise unre¬ 
ported comments will aid in further work 
under government contracts, and that the de¬ 
scription of self-evident techniques in the opti¬ 
cal trade will be taken as a serious effort by 
an organization starting from the sketchiest 
beginnings. In spite of the lack of the usual 
decades of industrial “know how,” it is evident 
from the favorable test results in Chapters 1 
and 2 and from the many successful aerial pho¬ 
tographs that high standards of quality were 
achieved. 


9,1,1 Manufacture of Lens Elements of 
Large Size 

In connection with the optical and engineer¬ 
ing design of a long-focus aerial lens system, it 
is vital that wisely assigned tolerances be met 
in the optical and mechanical shops. To this end 
arise a number of details demanding incessant 
care by the workmen. The final experimental 
optical instrument is a kind of integration of 
a common effort. Rarely is it that an accident 
in manufacture can benefit a properly designed 
instrument. The perfected optical instrument 
is so unaccidental a product that it lies at the 
very pinnacle of many individual efforts and 
thoughts. 

The following procedure for the fabrication 
of a large lens element became nearly standard 
in the Harvard Optical Shop. 

1. Diamond milling to approximate curve 
and thickness of the required lens was done. 
The quality of the milling was limited by the 
type of diamond machinery available, but in 
practice was within several thousandths of an 
inch of the desired geometrical shape. Proper 
excess on thickness of a milled blank depended 


partly on hardness and partly on the chemical 
nature of the glass. In general, a hard glass 
like BSC-2 could be finished satisfactorily from 
the milling stage if 0.020 in. of excess central 
thickness were permitted. A soft glass or an 
unstable glass like DBC-1 required about 0.040 
in. excess. 

2. Diamond milling of Pyrex tools followed. 
Pyrex tools were used instead of iron in order 
to minimize the time element in preparing 
radii, and in order to afford the optician a 
ready means for altering his radius by differen¬ 
tial amounts during the fine grinding without 
destroying the essential spherical surface. 

3. Hand-grinding on a rotating spindle with 
an overhead arm and pin was the next step. 
Tool or lens was often pitched to a chuck for 
easy handling. Preferably, this chuck should 
have a standard thread for attachment to 
threaded spindles. In practice a 3.5-degree 
taper was adopted for all interchangeable 
spindle work. 

4. Following diamond milling, grade No. 120 
Carborundum was used for grinding. There¬ 
after, the steeper of the two sides was carried 
on to No. 320 and brought to good spherical 
contact within this grade. The radius at this 
stage was generally on the long side if concave, 
and approximately right if convex. 

5. The lens was pitched to a special face 
plate attached to the centering machine. Tilting 
screws were adjusted until the lens as such was 
approximately centered relative to both faces 
at the same time that the No. 320 back face was 
running true, as measured by a dial gauge read¬ 
ing to 0.0001 in. In practice with the equipment 
at hand, the needle rarely fluctuated more than 
0.0002 in. when properly adjusted. 

6. The lens was edged to final diameter. Test 
for circularity and perpendicularity of edge to 
the reference smooth back was made by two 
dial gauges running on the respective surfaces. 
Usual demands required needle fluctuation to 
be less than 0.0002 in. at finish. This meant that 
the edge was sufficiently perpendicular to the 
back for all practical purposes. 

It is necessary that the edging be parallel to 
the lens axis in order that a conical edge be 
avoided. The usual edging machine has a back 
and forth sweep movement. This movement 



GRINDING AND POLISHING LENSES AND MIRRORS 


391 


should be long, and the lens should be allowed 
to grind itself out at the chosen diameter. Toler¬ 
ances on absolute diameter usually ran to 
within 0.001 in. 

The optician has a constant struggle with 
thermal effects on his gauges and standards, 
as well as effects of'coolant and grinding. It is 
important that the final stages of edging be 
done slowly if an accurate absolute diameter 
is to be achieved, and that all gauge measure¬ 
ments be referred to standards at the same 
temperature. In the Harvard shop the primary 
standards were Johannson gauge blocks. 

7. The edged lens was now put through No. 
320 Carborundum on the rougher side, and 
brought to radius also. During this process the 
centering of the lens was checked by a dial 
gauge. A stand was set up for the purpose with 
an adjustable set of three balls for support of 
the lens element in a horizontal position. The 
edge of the lens rested against two upright 
smooth metal posts. The five points now de¬ 
fined a fixed space reference for the lens ele¬ 
ment. Finally, a brass-tipped 0.0001 dial gauge 
pointer was adjusted against the surface with 
the point directly over one of the three refer¬ 
ence balls. 

The optician now made variation in thick¬ 
ness measurements at equally spaced intervals 
around the periphery. With spherical surfaces, 
the centering error was immediately determi¬ 
nable. The optician corrected the centering by 
uneven pressure as required on the side of the 
lens opposite the reference face. It was always 
important to center at each stage of fining to a 
fraction of the amount of glass to be removed 
during the next fining stage in order that 
spherical surfaces might be preserved. 

8. Fining, centering, and radii adjusting 
were carried on by the skill and attention of the 
optician until the polishing stage. From stages 
where the use of emery for grinding is begun, 
radii were checked by a radius measuring 
microscope arrangement. To aid in this de¬ 
termination, 5-min. temporary polishes were 
given the concave surface to be measured, 
whether lens or tool. Change of radius was 
accomplished by stroke and pressure. Fining 
was most often carried on by automatic ma¬ 
chine, followed in the last stage by careful 


handwork. The hand-centering procedures gen¬ 
erally added only an hour or two to the total 
lens work. 

9. Polishing was accomplished by hand and 
machine, as required. Polishing time depended 
on excellence of the fining, weight, speed of 
stroke, and on the polishing material. During 
the last half of World War II, Barnesite was 
used almost exclusively on all types of optical 
glass polishing. Figuring was accomplished 
mostly by hand on a slowly rotating spindle. 

10. A finished 10-in. diameter lens element 
usually had the following characteristics: Each 
surface would be'figured to better than one- 
quarter wave of green mercury light with em¬ 
phasis on smoothness of figure, especially radi¬ 
ally; each surface would have a radius within 
one part in two thousand or better; the lens 
would be circular and within 0.001 in. of an 
assigned diameter on an absolute basis; center¬ 
ing would show less than 0.0002 in. variation in 
thickness around the edge of the lens; central 
thickness would be within 0.005 in., since better 
accuracy rarely mattered in large optical sys¬ 
tems. 

Quality of polish obtained during the latter 
stages of World War II was uniformly excel¬ 
lent. Barnesite was the greatest single factor 
in this success. Earlier, it was found that glass 
types differed widely in their responses to figur¬ 
ing techniques which varied widely also among 
workers. From all around experience it would 
seem that the polishing agent was much more 
important to final polishing quality than was 
dust and emery contamination on laps. Indeed, 
it is very improbable that floating emery ever 
failed to get between the polishing surfaces. 
Quality of polish was most often tested by 
placing the lens in a strong beam of directed 
light, viewed against a black background. Bet¬ 
ter polish than shown by this test for aerial 
purposes was unnecessary. Such a test will 
show fine sleeks very easily. 

Many times during the project, concave lens 
surfaces were mounted by means of a flat ledge 
around the edge of the lens. This flat ledge was 
hand-ground with emery on a flat iron plate, 
and was as carefully centered as were the sur¬ 
faces. The machinist found flat ledge references 
necessary for accurate work. The optician 






392 


OPTICAL TECHNIQUES 


minded the extra work very little. The flat 
ledge usually served as a definitive seat for the 
lens with known sagitta. 

Ever present in all figuring work are tem¬ 
perature effects. The optician is obliged to wait 
until a lens in contact with a test plate has 
reached thermal equilibrium before judging the 
results of his previous efforts. With large work, 
it proved necessary to enclose both pieces in a 
glass-covered box before reliable tests could be 
achieved. The skilled optician occasionally made 
thermal effects work in his favor by proper 
handling of lens and pitch lap. Almost all the 
skilled opticians found it possible to figure ac¬ 
curately, in spite of holding the lens element in 
the hands. 

Thin lenses must be rested on edge before a 
reliable test of figure can be made. The disk is 
far more stable with edge support than with 
back support. For this reason all concave sur¬ 
face testing was carried out with the tool or 
lens on edge. 

Beveling of lenses was accomplished most 
generally by means of fine emery and grinding 
in a concave diopter tool of adequate size. It 
was often deemed important to follow up the 
beveling by a slight bevel polish with felt and 
rouge or Barnesite. With soft glass types there 
seemed to be less tendency thereafter for fine 
edge particles to break off during the last stages 
of fine grinding and to cause time consuming 
scratches. It was of advantage to have a small 
hole in the bottom of the diopter beveling tool 
in order to prevent an air seal and difficulty of 
removing the element. 

Thick lenses should not be edged in practice 
to a right circular cylinder, but instead should 
be slightly convexed. Ideally, the edge should 
have a spherical curve of diameter equal to the 
diameter of the lens, following the principle 
of a ball inside of a cylinder. In practice, the 
careful assembler rarely will tilt a lens more 
than a few degrees. Jamming of lens elements 
in their cells and edge chipping will almost 
never happen if the edges are slightly convexed. 
For aerial lenses, it is far more important for 
the reduction of vibration to achieve a tight fit 
in the cell by such means than to increase 
tolerances on diameters. As a fine point, the 
crown of the convexed edge should, if possible, 


lie in line with a plane through solid glass. In 
other words, a meniscus lens should be sup¬ 
ported as nearly as possible by a plane through 
glass, rather than by a plane passing into the 
air space of the concavity. If this method is 
followed, a tight fit of the lens in the lens cell 
will not bend the lens appreciably. 

In practice, it is rare that the quality of the 
instrument work matches the accuracy achieved 
by the optician. If instrument makers were 
permitted the same, slow, time-consuming lap¬ 
ping methods, no doubt their final accuracy 
would be as good. For the very best work it is 
desirable that cylindrical grinding be used ex¬ 
tensively for the manufacture of accurate lens 
cells. In all such work with aerial lenses, the 
cell wall is likely to be too thin to support the 
strain of machine work within the close toler¬ 
ances allowable. It is of the highest importance, 
therefore, that the instrument maker, or his 
foreman, design machine jigs to prevent any 
unnecessary stress or strain on the part in the 
course of machining, and to follow the machine 
work with a careful check of final accuracy. 

The most frequent fault of the instrument 
maker is his failure to watch temperature ef¬ 
fects on approaching final dimensions. Every 
gauge and standard requires periodic inspec¬ 
tion and calibration. Inside measurements are 
much harder than outside measurements, pri¬ 
marily because the tip of the gauge must follow 
a saddle-shape contour in space. Care must be 
used to refer the inside measurements to a 
plane perpendicular to the optical axis. For the 
most careful work, plug gauges should be used. 
Such gauges should be as carefully standard¬ 
ized as the micrometers themselves, and should 
have slightly convexed edges to insure accuracy 
and freedom from jamming. 

A most convenient way to check diameters 
objectively against standards is to use a dial 
gauge of the 0.0001-in. series. The spring pres¬ 
sure of the dial gauge insures that each worker 
will attain the same result, provided that care¬ 
ful attention is given to cleanliness, burrs, 
gauge lag, and to thermal effects. The dial gauge 
and accompanying stand also afford a con¬ 
venient way for checking conical edges and 
ellipticity of supposed circular objects. Any 
handling of parts should always be accompa- 



GRINDING AND POLISHING LENSES AND MIRRORS 


393 


nied by alertness for thermal troubles caused 
thereby. 

The subject of advanced cell and retainer 
ring designs is too lengthy for presentation 
here. Under the Harvard contract a prolonged 
attempt was made to improve on the methods 
of mounting and handling large lenses. Special 
attention was devoted to stabilizing and rust¬ 
proofing the steel used in lens barrels and cells, 
in order to prevent change of shape or condi¬ 
tion with time. The deep-freezing of steel was 
thought to be an absolute essential for success 
in prolonged service. It is believed that future 
efforts should be in the direction of stabilized, 
chemically blackened, stainless steel cells. The 
most promising alloy to date seems to be free- 
machining stainless 303. 

In practice, it was unwise to trust the finished 
article of any worker unless check measure¬ 
ments could no longer be made. The completed 
lens system should be known to the assembler 
down to the last detail, if his job is to be car¬ 
ried out with assurance. All errors in diameter, 
parallelism of faces, ellipticity of parts, conical 
walls, and centering should be evaluated, and 
allowance made for them. Any jigs made up to 
achieve such accurate measurements are a sav¬ 
ing of overall time and trouble. 

By mechanical measurement alone it is pos¬ 
sible for a skilled assembler to shim lenses and 
to handscrape metal faces until the assembled 
lens shows no faults of centering. The assem¬ 
bly process must be a cooperative one between 
instrument maker, opticians, assembly expert, 
and the optical designer. If one man can handle 
the entire job with meticulous care, so much 
the better. 

The engineer should design the lens mount¬ 
ing, if possible, to permit adjustable but defin¬ 
itive axial positioning of lens elements. The 
best solution of this kind seems to be the spacer 
ring. Such a ring can be lapped and checked 
separately to the smallest tolerance required. 
These rings should also have slightly convex 
edges to prevent jamming. It is helpful to the 
assembler if small spanner holes are provided 
for extraction of separator rings in otherwise 
unreachable spots. 

Metal fits produced by machining alone al¬ 
ways seem tighter than the dial gauge or hand 


micrometer would indicate. The metal surface 
seems to contain myriads of small fibers that 
catch on one another in a close fit. For this 
reason convexing of close-fitting parts is essen¬ 
tial if the finest accuracy is to be achieved. The 
depth of such convexing need be only one or 
two thousandths of an inch. Preferably, such 
tight-fitting parts should be cylindrically 
ground. Glass-to-metal fits can always be to 
closer tolerances than metal-to-metal. 

Glass elements are most easily inserted in 
cells if handled by means of a vacuum chuck. 
It is important that the vacuum proceed either 
from a large vacuum reservoir, or from large 
pumping capacity. To protect coated surfaces, 
it is advisable to employ a single thickness of 
a soft material like Kleenex. The porosity of 
such a material makes a high pumping rate 
imperative for sake of safety. The rim of the 
vacuum chuck should be lined with a thin flex¬ 
ible ring like neoprene, and should be kept 
thoroughly clean. The crushing effect of the 
vacuum chuck often changes the dielectric prop¬ 
erties of the nonreflective coat, so that a dis¬ 
figuring layer of dust will tend to form on the 
ring where the chuck rim has rested. For this 
reason, the rim of the chuck should be very 
broad. 

Normal centering methods were used on 
lenses smaller in diameter than 6 in. However, 
to replace visual methods, dial gauge indication 
was used on both surfaces of the lens simultane¬ 
ously. After edging, the lens was not considered 
satisfactory until an accuracy of better than 
0.0001 in. had been achieved on either side. 

Among important methods developed under 
Contract OEMsr-474 were procedures for con¬ 
struction of precision concentric surfaces and 
very thin steep meniscus lenses. These pro¬ 
cedures are described at length in the comple¬ 
tion reports. 

The final report 7 under Contract OEMsr-474 
lists a number of points likely to benefit con¬ 
tinuation or initiation of a new laboratory. 
Among the more important points is a strong 
recommendation for more general use of dia¬ 
mond methods in optical manufacture. The use 
of diamond tools was widespread in the Amer¬ 
ican optical industry during World War II. It 
is believed, however, that such methods are 



394 


OPTICAL TECHNIQUES 


capable of considerable elaboration, especially 
for production of large size optical goods in 
quantity. There seems to be little reason why 
diamond milling cannot be employed on lenses 
up to 30 in. diameter with precision at least 
as good as obtained in standard practice on 
metal parts. 

The chief task of the optician is the fine fig¬ 
uring of optical surfaces. It is advisable, there¬ 
fore, to procure any labor-saving machinery 
that can permit him more time on the most 
important part of the work. It is entirely pos¬ 
sible that diamond milling methods could pro¬ 
duce a lens, already adequately centered and 
edged, and ready for the finest grinding. 

In keeping with this suggestion is a strong 
recommendation to work out laboratory meth¬ 
ods for quick manufacture of prototype lenses 
on an experimental basis. It would be wiser to 
construct a 7-element prototype lens in several 
days than to employ computers who tried for 
weeks to obtain comparable results by diffrac¬ 
tion analysis and skew ray-tracing. It is unwise 
to construct an experimental model until the 
designer is certain that he is in the differential 
neighborhood of the desired result. Once near 
the final answer, however, it is possible for the 
designer to study the results obtained with a 
precision-made prototype on the optical bench 
and thereafter to decide what differential cor¬ 
rections are needed to obtain the final desired 
product. No amount of ray-tracing in a com¬ 
parable time at comparable expense would give 
comparable results. It is no discredit to a de¬ 
signer if he resorts to such practical methods. 
It is more intelligent to use expedient methods 
than to emphasize one procedure at the expense 
of another. 

It is to be expected, of course, that improve¬ 
ment in theoretical approach and in availability 
of electronic calculators will enable the de¬ 
signer to obtain ever better, more complex 
fundamental designs. 

9,1,2 Quartz Monochromator 

In connection with Navy research a specially 
made quartz monochromator was cemented and 
mounted under Contract OEMsr-474. Figure 1 
shows a view of the final assembly. 


The quartz monochromator consists of six 
segments of crystal quartz, all edged cylindri- 
cally to the same diameter, but varying in thick¬ 
ness geometrically by a ratio of two. Unit 
length of the quartz was so taken that the 
narrow bands of maximum transmission in¬ 
cluded both calcium and hydrogen wavelengths 
at 3,933 and 6,563A, respectively. 



Figure 1 . The quartz monochromator. 

The NDRC portion of the work was primarily 
that of assembly. The quartz segments were 
received already finished to size. The monochro¬ 
mator requires a film of Polaroid to be ce¬ 
mented between each layer of quartz. Counting 
end segments of cover glass, there are seven 
films of Polaroid in all. Daily trips were made 
to the Polaroid Corporation for determination 
of the best orientation of each successive Polar¬ 
oid film for maximum transmission at the 
required wavelength. Such a procedure guaran¬ 
teed maximum efficiency of an otherwise rather 
inefficient kind of filter. The disadvantages for 
general purposes are more than made up by 
the 5-A wide filter band, ideally suited to study 
of the sun. 

The unit length of the quartz filter was chosen 
to be of the required value at a temperature of 
135 F. Slight heating in sunlight cannot there¬ 
fore disturb the filter wavelength. Control of 
final operating wavelength and fine adjustment 
was provided by incorporating an adjustable 
thermostat and heating system into the mount¬ 
ing shown in Figure 1. 

9,1,3 Paraboloidal Molds 

In connection with other NDRC projects Con¬ 
tract OEMsr-474 was requested to manufacture 



METHODS FOR MAKING ROOF PRISMS 


395 


in a matter of days a number of //0.5 para¬ 
boloidal molds of 4 in. aperture. The molds 
were made of Pyrex glass with polished backs, 
sides, and bevels. 

The parabolic curve required was transferred 
by draw filing and dial gauge measurement to 
a brass template. The average error of the 
gauge in aspheric depth was less than 0.0002 in. 
The molds were hand-finished to fit the gauge 
and tested under strong tangential illumination. 
Fine grinding was accomplished with hand¬ 
work and flexible strips of metal. Polishing was 
accomplished on an automatic machine by use 
of very small diameter polishing buttons of 
pitch. The fact that the parabolic surface re¬ 
quired had no inflection point made it possible 
with a full over-center stroke to achieve a very 
smooth overall result. Later tests at the Polar¬ 
oid Corporation showed that several of the 
fourteen molds constructed were far better 
than the required tolerance of 0.025-mm image 
size at full aperture. 


9 2 METHODS FOR MAKING ROOF PRISMS 

9,21 Introduction 

The production of optical parts in large quan¬ 
tities presents unique problems in many ways 
because of the high degree of precision required 
in the final product. In general, for the manu¬ 
facture of such parts highly skilled workmen 
and specialized machines are required. 

In June 1941, the Office of Scientific Research 
and Development, in anticipation of a possible 
future need for the rapid expansion of the 
facilities then existing for the production of 
roof prisms, initiated a Contract OEMsr-101 at 
the Mount Wilson Observatory of the Carnegie 
Institution of Washington for the investigation 
and development of new methods of roof prism 
manufacture. This program, carried out under 
Project AC-11, was to have as its primary goal 
the development of new production methods 
which did not require highly skilled workers, 
and which did not involve elaborate specially 
built equipment. Any new methods developed 
were to use, so far as possible, standard indus¬ 
trial machines. 


922 General Principles of the Method 
Developed 

The manufacturing procedure developed at 
the Observatory 8 ’ 9 permits a division of the 
manufacture of roof prisms into two distinct 
stages. The end product of the first of these 
stages is a shaped and fine ground, but unpol¬ 
ished, roof prism of which the dimensions and 
the angles (with the exception of the roof 
angle) are established well within the required 
tolerances. Those surfaces, which need not be 
polished, require no further attention after this 
stage. 

In the second stage of manufacture the prism 
faces are fine ground and polished, and the roof 
angle is corrected for each prism individually 
to bring it within the tolerance specified. 

The process developed 5 for shaping the un¬ 
polished blanks makes use of a Blanchard No. 11 
vertical-spindle grinder equipped with diamond 
grinding wheels and suitable jigs for holding 
the prism blanks. The grinder carries, for work 
on glass, a 10-in. cup wheel with a 1-in. face 
having a speed of 1,200 rpm. The table on which 
the work is placed is a magnetic chuck 16 in. 
in diameter which rotates at a rate of either 
15, 24, 41, or 64 rpm. The chuck is mounted on 
ways, and may be withdrawn from under the 
wheel to facilitate loading. The spindle carry¬ 
ing the wheel can readily be adjusted so that 
its axis is parallel to that of the chuck. Over 
the center of the chuck passes the center of the 
1-in. face of the grinding wheel. The motor 
drive is directly on the spindle, which may be 
raised or lowered rapidly by a torque motor, 
or adjusted accurately by hand. The downward 
feed is automatic, and takes place in steps, 
which occur 34 times per minute. The size of 
step is variable from 0.004 to 0.070 in., but the 
spacing in time cannot be changed. Coolant 0 is 
pumped from a tank in the base of the grinder 


b The process to be discussed here was developed 
specifically for the Amici prism No. B 173131, of which 
14,000 were milled at Mount Wilson for the Ordnance 
Department. It is clear that for the milling of other 
prisms slightly different jigs and slight changes in pro¬ 
cedure would probably be required. 

c The coolant consisted of a mixture of the solvent 
Quaker Cut No. 101 in water in the proportions of 1 
gal of Quaker Cut to 80 or 100 gal of water. 




396 


OPTICAL TECHNIQUES 


and circulates out through the center of the 
wheel during the grinding process. 

Several types of wheels for grinding (or 
milling) glass have been tested. Diamond wheels 
have proved to be the most satisfactory. Norton 
wheels of 100 grit and 180 grit have been used 




GLASS IN TRIPLE-V JIG 
FOR CUTTING END FACES 



GLASS IN TRIPLE-V JIG 
FOR CUTTING ROOF FACES 


Figure 2. Jigs used in milling roof prism blanks. 


regularly, and a wheel of 400 grit has been used 
occasionally. Of these the first is used for rapid 
cutting, the second with slower feed for finish¬ 
ing, and the third with very slow feed for pro¬ 
ducing a nearly polished surface. The bonding 
of the diamonds is metal for grits below 150, 
and Bakelite for grits above that number. 

The accurate jigs used in the procedure de¬ 
veloped represent one of the most important in¬ 
novations of the process. It is through their use 
that the angles of the unpolished prism blanks 
can be established with the required degree of 
accuracy. Three types of jigs, illustrated in 
Figure 2, have been employed. The end faces 
and roof faces of the prism were milled with 
the triple-V jig, sides and bases with the V-bar 
jig, and tips with the L-bar jig. (For the no¬ 
menclature of the various faces of a prism see 
Figure 3, in which the dimensions and toler¬ 
ances of prism B 173131 will also be found.) 


The two defining surfaces of the triple-V jig, 
which rest on the surface of the chuck, are ac¬ 
curately lapped and ground to a 90-degree 
angle, with the aid of an autocollimator and 
two optical flats as illustrated in Figure 4. This 
angle defines the angle between the milled end 
faces (see bottom left of Figure 2), and also 
the angle between the two roof faces of the 
prism (see bottom right of Figure 2). Accuracy 
of the angle between the roof and end faces 
(the “angle of twist”) is also important. This 
is controlled by the angle of the jig between 
the edges of the base on which the jig rests and 
the faces of the small V blocks in which the 
glass is fastened with wax during the milling 
process. The V blocks are made of hot-rolled 
steel. They are not hand-lapped as the bases 
are, but are ground accurately and checked 
with a precision square. They fit into a channel 
in the Meehanite base. The V blocks are shaped 
so that they give the maximum amount of sup¬ 
port to the edge of the cut made during the 
milling of the roof faces. 

The L-bar jigs are used for milling the prism 
tips. Each jig holds eleven prisms; six jigs are 
placed on the chuck at one time. The two legs 
are symmetrical, so that the setting of the ma¬ 
chine for milling the first tip is also correct for 
milling the second tip. 

Each of the V-bar jigs, used for milling the 
base and two sides of the prism, holds seven 
prism blanks placed in the V with roof faces 
down. The bottom of the V is generously re¬ 
lieved in order to avoid chipping the milled roof 
edge. The sides of the prisms are milled with 
the jigs lying flat on a special fixture shown in 
Figure 5. This is a circular magnetic plate 
which provides clearance for the glass which 
overhangs the sides of the jigs as indicated in 
Figure 6. 

A necessary piece of auxiliary equipment for 
warming the jigs and prisms prior to waxing, 
and for removing the waxed prisms from the 
jigs following milling, was a Detrex degreaser 
of the type shown in Figure 7. Boiling trichloro¬ 
ethylene in the left-hand tank maintains an 
atmosphere of hot vapor up to the level of the 
water jacket. Here condensation occurs, and the 
freshly distilled solvent returns to the tank at 
the right. Two shelves (one within the vapor 



METHODS FOR MAKING ROOF PRISMS 


397 


and one above it) are provided for wire baskets put in groups of 400 to 2,000 for the sake of 
in which jigs and prism blanks are placed. Any efficiency in handling and in order to avoid the 
object lowered into the vapor is rapidly warmed continual resetting of the adjustments of the 
and flushed by the condensation of the solvent machine. 



upon it. Air is drawn off from the lip of the tank 
through the flue shown in the upper part of the 
figure. This was added to reduce the amount of 
vapor that escaped from the tank when the 
basket was removed. 

9-2 3 The Milling Procedure 

The milling procedure itself is divided into 
four operations, through which the prisms are 


Preparation of the Glass 

Originally it was necessary to cut the rough 
blanks from large slabs of glass. A rapid method 
of accomplishing this with the aid of a diamond 
saw was devised. 8a 

In later work the glass was received in the 
form of pressings, known as Bureau of Stand¬ 
ards pressed blanks, which have, roughly, the 
shape of roof prisms. About Ys in. of glass must 
be removed from each roof face and about in. 














398 


OPTICAL TECHNIQUES 





Figure 6. Section through chuck, plate, and V- 
bars. 

Operation 1. Mill End Faces 

A set of triple-V jigs, together with the 
proper number of prism blanks, are placed in 
a metal basket and warmed in the degreaser 
for about 20 sec in preparation for waxing. The 
heated jigs are transferred to a stand which 


Figure 8. Triple-V jigs on chuck of grinder ready 
to cut end faces. 

bottom left) and slipped endwise until it is ap¬ 
proximately centered in the V block. After all 

d The most satisfactory wax mixture to hold the 
blanks in place was found to consist of three parts bees¬ 
wax, three parts rosin, and two parts Montan wax (by 
weight). This was much stronger than any other mix¬ 
ture tested. It melted easily, set quickly, and had a dark 
color. This latter property was an advantage during 
the waxing of a finished surface to the jigs or fixtures 
since it enabled the operator to detect readily any 
variation in thickness or contact by a change of color. 


holds them upright; the jig faces which come 
in contact with the glass pressings are wiped 
clean. Wax (1 is applied to the faces of the press- 


Figure 5. Circular magnetic plate used in milling 
sides. 

plished by rubbing the pressing across the sur¬ 
face of a rotating cast-iron plate covered with 
a coarse abrasive. 


Figure 7. Interior of degreaser. 

ings which touch the jigs. Each prism is set 
into the jig, roof side downward (Figure 2, 


of glass from the ends of these blanks. Irregu¬ 
larities or ridges on the pressing frequently 
prevent them from fitting into the jigs properly 

AUTOCOLLIMATOR 


OPTICAL 
FLAT 


Figure 4. Optical testing of jigs. 

for the first cut. They are therefore inspected 
and, if necessary, cleaned up before going to 
the milling room. This cleaning up is accom- 


OPTICAL 

FLAT 






















































METHODS FOR MAKING ROOF PRISMS 


399 


prisms are in place and the wax has set for a 
short period, the jigs are transferred to a cold 
table which hastens the completion of the 
setting. Since the pressings are rough, gen¬ 
erous amounts of wax are used in this opera¬ 
tion. 

Eight jigs containing blanks are placed on 
the magnetic chuck of the Blanchard grinder 
(Figure 8) and the first face is milled. When 
this operation has been completed, the jigs are 
rotated 90 degrees about their longitudinal axis 
and the milling process is repeated for the sec¬ 
ond face. In this operation of milling the faces, 
two cuts are made on each face, one a roughing 
and the other a finishing cut. 

The roughing cut is made with a 100-grit, 
100-concentration metal-bonded wheel. The rate 
of feed for this first cut is usually about 0.070 
in. per minute. At this speed there is some 
chipping, but this is not serious since most 
chips are cleaned up by the finishing cut, and 
all other faces of the prism still remain to be 
milled. About % in. of glass must be removed 
from each face during this operation. 

After the roughing cut has been made on 
both faces, the jigs are transferred to a second 
Blanchard grinder for a finishing cut with 
a 180-grit, 100-concentration resinoid-bonded 
wheel. The rate of feed for the finishing cut 
is from 0.016 to 0.028 in. per minute. About 
0.020 in. of glass is removed during this finish¬ 
ing process. 

The depth of cut is controlled by means of 
an indicator as illustrated in Figure 9. A small 
brass plate bridges the prisms, and the indi¬ 
cator is set by means of gauges calibrated to 
the various heights of the finished work above 
the chuck. These heights depend, of course, 
upon the dimensions of the jigs as well as on 
the prism specifications. The brass plate is 
placed under the gauge block and the indicator 
adjusted to read zero. As the cut progresses, 
the depth of glass remaining to be taken off is 
read directly on the indicator. Once the correct 
dimensions are reached, a zero point on the 
scale of the vertical feed can be established, 
and thereafter the grinding wheel can be 
stopped readily at the proper point without 
recourse to the indicator. The indicator dial 
method described need be used only occasion¬ 


ally, after the setting is once made, as a check 
on the wear of the grinding wheel and possible 
shifts in the adjustments. 

When the blanks have been milled to the 
proper dimensions, the jigs and blanks are re¬ 
moved, put in a wire basket together with the 



Figure 9. Indicator for establishing dimensions. 


next set of blanks to be milled, and placed in 
the vapor of the degreaser. In 15 to 20 sec the 
blanks become warm and slide off the jigs; 
within another 15 sec the wax is completely 
washed away. The basket is removed, the par¬ 
tially milled blanks are stored away, and the 
new pressings which were warmed in the de¬ 
greaser are waxed to the jigs in preparation 
for milling. 

Operation 2. Mill Roof Faces 

When the end faces have been milled for the 
entire group of prisms, the second operation, 
that of milling the roof faces, is begun. The 
triple-V jigs are used once more, the blanks 
being waxed in them with the newly milled sur¬ 
faces downward as illustrated in Figure 2, 
bottom right. Care must be exercised during 
this second milling operation because the roof 
edge and 45-degree end cuts are fragile and are 
easily chipped. A roughing and finishing cut are 










400 


OPTICAL TECHNIQUES 


generally taken as before, e although owing to 
the small amount of glass to be removed it would 
probably be equally efficient to make only one 
cut with the 180-grit wheel and thus avoid 
transferring the work from one machine to 
another. 

When the milling of the roof faces has been 
completed, the prisms are removed from the 
jigs as before and stored until operation (3). 

The roof cut determines the size of the 
prisms. After the milling of the roof faces, one 
or two prisms out of each group on the chuck 
are checked for dimensions from roof edge to 
base edge. In order to make this measurement, 
the prism is placed in a V block base down, and 
a smaller V block is placed over the roof. These 
are slipped under an indicator set for the proper 
dimension. 

Operation 3. Mill Tips 

The prism blanks, properly warmed in the 
degreaser, are next waxed to similarly warmed 
L-bar jigs, in preparation for milling of the 
prism tips. Six jigs, each carrying eleven prism 
blanks, are placed on the chuck. A cut having 
a depth of about % in. is taken in the roughing 
and finishing operations. Since the edges may 
become slightly chipped, the finishing cut is 
made about 0.040 in. deep in order to remove 
these small defects. 

Operation 4. Mill Base and Sides 

The blanks are next waxed into V-bar jigs, 
roof-edge down as indicated in Figure 6, seven 
blanks to a jig. Ten jigs are placed on end on 
the chuck and the bases of the prisms are milled 
to the proper dimension. 

A special magnetic plate of alternate steel 
and brass rings (see Figure 5) is then set on 
the magnetic chuck of the Blanchard grinder. 
Six V-bar jigs are placed on this plate (see 
Figure 6) and the sides of the prisms milled, 
42 prisms being handled at a time. After the 
completion of the first side, the bars are turned 
over and the second side milled. 

Since very little glass has to be removed from 
the sides, no roughing cut is made, and the 
entire operation is carried on with a 180-grit 

e The finishing cut in this second operation is usually 
rather deep in proportion to the roughing cut. 


wheel. This operation completes the milling of 
the prisms. They are removed from the jigs, 
cleared, inspected, and packed for shipping, or 
are sent to the fine grinding and polishing de¬ 
partment. 

Operational Data 

Routine milling of the type described in pre¬ 
ceding paragraphs was generally carried out 
by two women operators, one doing the waxing 
and one operating the machines. One week of 
training was found sufficient for learning the 
waxing technique, and satisfactory skill in the 
routine operation of the machines could be 
gained in about a month. A supervisor spent 
about one-third of his time in the milling shop, 
much of this time being devoted to the improve¬ 
ment of the milling process and in the develop¬ 
ment of special equipment. 

Table 1 gives data on 12 of the 16 runs dur¬ 
ing which 14,000 prisms were milled for the 
Army Ordnance Department. The first few 
runs show considerable fluctuation in the per¬ 
centage yield and in the shop time per prism. 
The relatively poor results of run No. 1 may 
be attributed to inexperience and to the diffi¬ 
culties involved in getting the process started. 
In run No. 3 a silicon carbide wheel was used. 
It was found to be rather too hard and caused 
chipping. In runs No. 14 and No. 15 a more 
rigid inspection for chips was instituted and at 
the same time an attempt was made to speed 
up the process. The losses were reduced to a 
more reasonable level in the last run. In aver¬ 
aging the data, runs No. 1 and No. 3 have been 
omitted since it is believed that they were not 
typical. 

The shop time per prism as given in the 
table, namely, 2.90 man-minutes, includes for 
each operation the time required for the entire 
process of waxing, milling, and removing the 
glass from the jigs. Those parts of the process 
which are not directly related to the use of 
machines are not included in the times given. 
Thus the hand-smoothing of blanks before the 
first waxing, the final inspection of the milled 
blanks, the packing for shipment (which re¬ 
quire approximately *4, %, and Vs man-minutes 
per prism respectively) are not included in the 
entries of Table 1. 




METHODS FOR MAKING ROOF PRISMS 


401 


9.2.4 Precision of the Milled Blanks 

A rather extended series of milling runs was 
made in order to test the accuracy that could 
be expected of the roof angles, and to compare 
certain details of milling technique. Eight 
triple-V jigs, picked at random, were used in 
all the tests. The milling was carried out with 
the jigs at definite positions marked on the 
chuck. Three runs, called Series I, were made 
with the jigs in arrangement on the chuck as 
shown in Figure 10A. Three more runs, Series 
II, were made with the jigs in the positions 
shown in Figure 10B. A third set of three runs, 


errors in the alignment of the chuck. In Series 
III, illustrated in Figure 11B, the jigs were 
turned end for end, a procedure which tends to 
accentuate any error in the chuck. 

In each run, the blanks were marked before 
removal from the jigs, so that the exact posi¬ 
tion of any blank during milling could there¬ 
after be identified. The roof angles were all 
measured with a sensitive airflow gauge 9a with 
which the roof angle could be determined read¬ 
ily and consistently with an accuracy of about 
2 sec of arc. Each roof angle was measured at 
three positions: near each end and at the cen¬ 
ter of each blank. The reading along the vertex 


Table 1. Operational data. 





Total 

Shop time per prism accepted (minutes) 

Run 

Blanks 

Per cent 

shop time 

sides and 

No. 

processed 

loss 

(hr) 

faces roof tips base total 


1 

1,633 

22.8 

131.5 





5.7 

2 

2,026 

9.0 

67. 

0.54 

0.67 

0.41 

0.57 

2.19 

3 

1,690 

16.0 







5 

551 

1.2 

26. 

0.72 

0.99 

0.39 

0.77 

2.87 

6 

578 

1.9 

25.25 

0.75 

0.78 

0.45 

0.69 

2.67 

7 

551 

2.3 

22.5 

0.69 

0.75 

0.35 

0.61 

2.40 

11 

551 

4.4 

24. 

0.77 

0.68 

0.48 

0.80 

2.73 

12 

536 

6.7 

25.75 

1.05 

0.75 

0.42 

0.87 

3.09 

13 

430 

2.8 

19.5 

0.90 

0.72 

0.36 

0.83 

2.81 

14 

719 

19.3 

26.75 

0.78 

0.86 

0.36 

0.78 

2.78 

15 

868 

15.5 

35. 

0.90 

0.78 

0.49 

0.70 

2.87 

16 

2,106 

3.1 

157.5 

1.37 

1.69 

0.76 

0.77 

4.59 

Avg. 

891 

6.7 

42.9 

0.85 

0.87 

0.44 

0.74 

2.90 


Series III, was made with the jigs in the same 
positions as in Series I, but, in preparation for 
cutting the second face in Series III, a method 




A SERIES I AND nr B SERIES n 

Figure 10. Arrangement of jigs on chuck. 


of turning the jigs that was different from the 
method followed in the other series was used. 
In Series I and II the jigs were turned 90 de¬ 
grees about the longitudinal axis as shown in 
Figure 11 A. This procedure tends to cancel any 


varied slightly. This variation was apparently 
caused primarily by a series of ridges or sur¬ 
face irregularities left by the grinding wheel. 
The ridges were approximately % in. apart, 
and appeared to be an inevitable result of the 
fact that the wheel moves approximately this 
distance across the work each time it revolves. 

In all, 648 roof-angle measurements were 
made which included three measurements of 
each prism, twenty-four prisms in each run 
(eight jigs each containing three prisms), three 
runs in each series, and three series. These 
constituted experiment A. 

Following the completion of experiment A, 
the spindle carrying the grinding wheel was 
realigned and the surface of the chuck was 
ground as nearly flat as possible. The three 
series of test runs were then repeated. These 
new series formed experiment B. 

In Figure 12, A and B, the distributions of 































402 


OPTICAL TECHNIQUES 


errors observed for the six series are exhibited. 
It is immediately evident that the errors in 
Series III are, on the average, considerably 
greater than those of Series I and II. This is 
not unexpected since it has already been pointed 
out that the method of turning the jigs em¬ 
ployed in Series III accentuates any errors 
caused by misalignment of the chuck. 

The departures of the roof angles from ex¬ 
actly 90 degrees may be traced to several sources 
of error, of which the most important are: 

1. Errors in the alignment of the chuck and 
wheel, and lack of flatness of the chuck. 

2. Errors in the jigs. 

3. Irregular placing of the jigs on the chuck. 

4. Lack of flatness of the milled surfaces. 

The dispersion or scatter of the errors in any 

series can be ascribed primarily to irregular 
placing of the jigs and lack of flatness of the 
milled blanks, sources of errors that produce 
discrepancies of random size and sign. The av¬ 
erage error of any series may be ascribed pri¬ 
marily to errors in the chuck and in the jigs. 
These are of the nature of constant errors. 
Assume, in the discussion, that errors in the 
jigs are independent of the state of the chuck 
and the method of turning the jigs, and that 
the error caused by the chuck is independent 
of the positions of the jigs on the chuck. 


Table 2. 
(sec). 

Observed 

average 

error for each 

jig 

Jig 

Experiment A 
Ser. Ser. 

Experiment B 
Ser. Ser. 

No. 

I, II 

III 

I, II 

III 

1 

15 

42 

1 

- 8 

2 

17 

31 

4 

0 

3 

11 

30 

— 3 

- 5 

4 

1 

27 

—12 

-25 

5 

15 

46 

— 1 

-10 

6 

14 

31 

0 

0 

7 

10 

24 

— 6 

-11 

8 

25 

45 

6 

-10 


In Table 2 are listed the average errors of all 
the prisms milled in each jig during the various 
series. f Each of the numbers listed is essentially 

f In this table, Series I and II in each experiment 
have been combined for convenience in discussing the 
data. It is clear from Figure 11 that these series are 
very similar, although the dispersion of Series I is, in 
both experiments, somewhat greater than that of Series 
II. 


the sum of the error of the chuck and the pe¬ 
culiar error of the jig, the random errors caused 
by the irregular placing of the jigs, and the 
lack of flatness of the prism faces tending to 
cancel out. We may represent the error of the 
chuck during experiment A, Series I and II, by 
Cxi, during experiment A, Series III, by Cx 2 , 
during experiment B, Series I and II, by C B i, 
and during experiment B, Series III, by C B 2 - 
The peculiar error of jig i may be represented 
by The values of Table 2 may be represented 

FIRST FACE SECOND FACE 



Figure 11. Methods of turning jigs. 


by combinations of these errors. For jig 1, for 
example, we may write that 

Cxi + J i = 15 J CX 2 J i = 42; 

Cfil + Jl = 1; c B2 j 1 = — 8. 

We can form from Table 2 thirty-two equations 
of the form shown above in twelve unknowns, 
four C”s and eight J’s. Unfortunately, the 
twelve normal equations derived from these by 
the methods of least squares are linearly de¬ 
pendent, and hence, being equivalent to only 
eleven independent equations, do not admit of 
a complete solution. In order to obtain numer¬ 
ical values for the C s and J’s, we make the 
assumption that when the jigs are turned end 
for end, as in Series III, the constant error 
caused by the chuck is greater by some definite 
but unknown factor n than when the jigs are 
turned about a longitudinal axis as in Series I 
and II. This assumption leads to the two equa¬ 
tions 

nCxi = Cx 2 and nC B i = C B2 

which, combined with the equations in the C ’s 













METHODS FOR MAKING ROOF PRISMS 


403 


and J’ s, give us thirteen equations in thirteen 
unknowns which can be evaluated. We find 
from a solution of these equations that: 

Cm = 11.1 sec Ji = 5.4 sec J 5 = 5.4 sec 

Ca 2 = 32.1 sec J 2 = 5.9 sec J 6 = 4.1 sec 

Cbi = - 3.8 sec J 3 = 1.1 sec Ji = - 2.9 sec 

Cb 2 = - 11.0 sec Ja = - 9.4 sec J 8 = 9.4 sec 

n = 2.89 Average J = J = 2.4 sec 
Average deviation of J’s = ±5.9 sec. 

The error of the chuck, it will be noted, was 
overcorrected by the adjustments which were 
made after experiment A was completed, and 
the necessity of further adjustment is indicated. 




Figure 12. Distribution of angular errors. Ex¬ 
periment A, series I, II, and III. Experiment B, 
series I, II, and III. 


The errors of the jigs are gratifyingly small. 
Before these experiments were carried out, the 
eight jigs tested and twenty-two others had 
been used in the milling of 17,000 prisms. Each 
jig had been through the waxing process about 
375 times, and had been put on and taken off 


the chuck about 1,500 times. Such usage ap¬ 
parently has little tendency to distort a jig or 
to effect its accuracy. 

The third source of error, irregularities in 
placing the jigs on the chuck, was investigated 
by forming for each jig the deviations of the 
individual runs from the average of the six 
runs of Series I and II. In Figure 13 the dis¬ 
tribution curve of such deviations is exhibited. 
Experiments A and B are both included in the 



-15 -10 -5 O 5 10 15 


DEVIATION OF RUN FROM AVERAGE FOR JIG IN SECONDS OF ARC 

Figure 13. Distribution curve of deviations among 
individual prisms for each jig, which arise from 
irregular jig positioning. 

results given, although the data from each ex¬ 
periment were treated separately in the calcu¬ 
lations. The curve as derived must be regarded 
as providing an estimate of the upper limit of 
the effects to be expected from this source of 
error, for the points of the curve have not been 
freed completely from the effects of the other 
three sources of error. The indications are that 
any irregular positioning of the jigs on the 
chuck is of very minor importance. 

The fourth source of error, nonuniformity of 
the milled surface itself, can be evaluated from 
a study of the scatter of the nine values of the 
angle determined for the prisms taken from 
one jig. The observed distribution curve for 
these errors is shown in Figure 14. The values 
on which the curve is based are independent 
of jig errors, chuck errors, and positioning er- 

























404 


OPTICAL TECHNIQUES 


rors, provided these do not cause a warping of 
the jig. There appears to be a slight indication 
of the presence of such a warping, but it cannot 
be distinguished clearly from other causes of 
variation in the readings on one jig. 

The dispersion in the errors caused by sur¬ 
face irregularities is large, and accounts for 
the greater part of the dispersion in the orig¬ 
inal distributions illustrated in Figure 12. The 



Figure 14. Distribution curve of deviations among 
individual readings for each jig, which arise from 
nonuniformity of the milled surface. 

irregularities in the surfaces are due, as has 
already been pointed out, to the fact that the 
wheel moves an appreciable distance (% to 
V 2 in.) during one of its revolutions. Since the 
periphery of the wheel can never be completely 
uniform, this motion results in a pattern of 
surface ridges on the face of the prism. Meas¬ 
urements indicate that these are from 10 to 
25 X 10 -6 in. deep. 

These surface irregularities can be decreased 
in magnitude in several ways. From a prac¬ 
tical point of view, however, their reduction is 
scarcely worth the effort required, for the meth¬ 
ods of fine grinding and polishing now in gen¬ 
eral usage cannot take advantage of milling 
more accurate than that already attained in the 


process described. During fine grinding and 
polishing by the conventional methods of block¬ 
ing the prisms in plaster, the milled angles are 
sometimes changed by as much as 30 sec of arc. 
In any finishing process in which errors of such 
magnitude are possible, it will always be nec¬ 
essary to correct each roof angle individually 
to keep it within the specified tolerance of 2 sec 
of arc. The dimensions and angles of the prism 
(aside from the roof angle) produced by the 
present milling process are of such a degree of 
accuracy that, in spite of errors introduced 
during blocking, no attention is required after 
the milling stage. All are well within the speci¬ 
fied tolerances when the blank passes through 
the last milling operation. 


Fine Grinding and Polishing 

Although the work done at Mount Wilson 
Observatory was not primarily concerned with 
the finishing of roof prisms, valuable experience 
was gained from the work that was carried on 
in this field. 

Preparation of the Prism Blanks 

The Blocking Process. On a fiat iron plate 
suitably coated with a thin layer of Kasson’s 
waterproof grease, the prisms are laid out in a 
predetermined pattern, and a row of protective 
glass fillers is arranged on the plate around 
them. A flat % 6 -in. thick brass ring is placed 
around the prisms and fillers, and on top of this 
is placed a steel reinforcing ring % in. thick 
and 2 in. high. The thin brass ring serves only 
as a spacer, and is later removed. Wax (a 40-60 
mixture of Montan wax and paraffin) is poured 
into the reinforcing ring until it forms a layer 
around the prisms Vs in* thick. When the wax 
has hardened, plaster is poured into the mold 
formed by the reinforcing ring until the prisms 
are covered and the ring is completely filled. 
When the plaster has set, the block thus formed 
is slid off the iron plate, the spacer is removed, 
and the wax “etched” back with xylene to ex¬ 
pose the faces of the prisms to be fine ground 
and polished. This grinding and polishing is 
carried out with the blocks face-down. 

It has been found most efficient from the 




METHODS FOR MAKING ROOF PRISMS 


405 


point of view of minimum operator fatigue, 
size and type of processing machine used, etc., 
to make the blocks about 10 in. in diameter. 
The arrangement of the prisms within the block 
(each block carries 60 prisms) has been found 
to be of importance in determining the strength 
of the block and hence its tendency to warp and 
cause errors. Blocks with circular arrays of 
prisms were the most successful; those made 
with the prisms arranged in straight rows w T ere 
unsatisfactory. For small prisms the practice of 
placing the prisms face to face in pairs was 
found to be particularly bad. 

In the construction of the blocks, experience 
has demonstrated that grease is a more satis¬ 
factory temporary adhesive than wax, which 
tends to cause errors in the final polished 
prisms. For forming the block, ordinary 
“thirty-minute setting plaster” was found to 
be more suitable than various longer setting 
plaster mixtures. Blocks made up of pure plas¬ 
ter frequently have a tendency to warp; this 
was overcome by use of the relatively heavy 
reinforcing ring. 

Fine Grinding. Fine grinding and polishing 
are carried out on identical machines. 9 Ade¬ 
quate control of the flatness of the prism sur¬ 
faces is maintained during the fine-grinding 
process by means of a very simple and rugged 
air jet spherometer, operating on the same prin¬ 
ciple as the angle measuring device mentioned 
previously. By use of this instrument, depar¬ 
tures from flatness or from a given spherical 
surface can be determined within two fringes. 

The surface of a typical prism blank shows 
milling marks which have a visual depth of 
about 0.002 in., but which are, in fact, much 
deeper. In order to insure the removal of these 
fissures, it is customary to employ fine grinding 
to remove a layer of glass 0.005 in. thick from 
each surface to be polished. The time required 
to accomplish this is divided equally among 
three grinding stages, successively finer grades 
of milled natural garnet being used as the abra¬ 
sive during the successive stages. 

Fissures readily visible in the milled blanks may be¬ 
come completely invisible in a newly polished surface. 
They will later reappear, however, if the surface comes 
in contact with plaster during subsequent blockings. 
They will often be seen after the prisms have been 
cleaned in strong alkali or other liquid. Evidently, the 


sides of the cracks go into optical contact during polish¬ 
ing and appear later owing to the etching action of the 
alkali. 

Polishing. Polishing and figuring are carried 
out with the block face down on polishing tools 
which are made of Swedish pitch loaded with 
ground walnut shell and tempered with pine 
tar, and which have a diameter about 20 per 
cent greater than that of the block being 
worked. Polishers made of such materials trim 
easily, do not scratch because of the soft sur¬ 
face layer which constantly works up from the 
body of the tool, and maintain their shape and 
serviceability for a very long period of time. 

When finished, prism surfaces are given a 
coating of bituminous enamel for protection 
during subsequent blocking. 

Roof Angle Corrections. When the two end 
faces and one roof face have been polished and 
figured, the roof angle is corrected. Two or 
three prisms are placed on each side of a con¬ 
tact blocks (or “fence”), properly aligned with 
the aid of a jig, and gently pressed into optical 
contact. 

In principle, the process of obtaining optical contact 
is very simple. Two perfectly clean, dry, and flat glass 
surfaces are gently pressed together; they will cling 
so firmly that they may be handled or worked as a 
solid piece. True optical contact is characterized by a 
complete lack of reflecting power at the contact surface. 

Surfaces to be placed in optical contact must be 
absolutely clean; the slightest film of oil or speck of 
dust will effectively prevent contact. The surface may 
be cleaned by light rubbing with a cotton swab mois¬ 
tened with acetone to which a few drops of ammonia 
have been added to assure alkalinity, which is an aid 
in obtaining contact. The swab should not be dipped 
directly into the cleaning liquid. The liquid should be 
applied to the s\^ab ; f rom a dropper in order to avoid 
contaminatiffg^he cleaner with oils from the cotton. 

Another and better method of cleaning glass surfaces 
in preparation for obtaining optical contact is to rub 
them with a dry chamois pad which has been lightly 
impregnated with rouge. The use of such a rouge pad 
requires caution since continued or violent rubbing can 
seriously deform the surfaces. With proper care, how¬ 
ever, a contact block can be cleaned fifty or more times 
and still remain serviceable. If the glass is sufficiently 
clean for contacting, moisture from the breath will 
condense in an even gray film over the surface. A 
streaked breath pattern indicates an oil film. 

The rubbing of the surface of the glass during the 

s These contact blocks have accurately plane surfaces 
which are parallel in the direction perpendicular to the 
edge of the block in contact with the polishing tool. 




406 


OPTICAL TECHNIQUES 


cleaning process generates a static charge which at¬ 
tracts particles of dust. The elimination of these par¬ 
ticles is simplified by keeping the humidity of the room 
high, or by allowing rays from a quartz mercury lamp 
to fall upon the surface. Any particles of fluff or rouge 
from the pad may be removed with a small “gilder’s 
tip” or camel’s hair striping brush. 

When the surfaces have been cleaned, the prism is 
placed gently on the contact block. If the surfaces have 
been properly prepared, the interference pattern first 
seen when the surfaces are brought together will settle 
down to a uniform gold color. In this condition the prism 
is not yet in optical contact; it can still be moved about 
slightly. The position of the prism on the contact block 
is corrected until the roof edge is in proper alignment 
with the edge of the block. When this adjustment has 
been made, a gentle pressure is applied to the prism 
until true optical contact is obtained. 

The assembly of contact block and prisms is 
mounted in a cage, as illustrated in Figure 15, 
and the process of fine grinding and polishing 
is started. During this process the roof angle 
is corrected by proper weighting of the cage 
around its periphery. Convenient movable 
weights are provided for this purpose. The 
greater part of the necessary correction is 
made during the fine grinding. 

For testing the roof angle, the cage is placed 
on three Stellite-faced spacers which define the 
plane of the roof faces being ground or pol¬ 
ished. An autocollimating testing device is used 
to compare the direction of a beam of light re¬ 
flected from the vertical face of the contact 
block (or “fence”) and that of a beam of light 
reflected from an adjustable plane mirror 
mounted beside the cage. From the adjustment 
of the movable plane mirror and the relative 
positions of the two beams of light as viewed 
through the testing device, it is possible to 
measure the roof angle quickly, easily, and with 
a high degree of precision. 

92 6 Conclusion 

As a result of the experience gained in the 
manufacture of more than 18,000 shaped but 
unpolished roof prisms, it can be concluded that 
the process of diamond milling of roof prisms 
as developed at the Mount Wilson Observatory 
is highly successful. It appears that one milling 
plant of not very great size could supply enough 
blanks to keep in production all the optical 
shops in the country. For such a plant it would 


be worth while to introduce into the method 
described here certain refinements which would 
reduce the labor cost appreciably, but no essen¬ 
tial changes in the process would be necessary. 
If the milling process were carried out on a 
commercial scale, it appears that the price for 
milling blanks would be approximately 50 cents 
per prism. 



Figure 15. Cage for correcting roof prism angle. 


In particular it should be pointed out that 
the accuracy of the blanks now produced by the 
milling process is as great as is warranted by 
the present fine-grinding and polishing tech¬ 
nique. Before further improvements in the mill¬ 
ing technique are attempted, improvements in 
the finishing technique should be made. If a 
finishing process which would maintain the ac¬ 
curacy of the milled blanks could be found (and 
such a development would be of fundamental 
importance to the optical industry) it seems 
very likely that roof prisms could be finished 
without being corrected individually for roof- 
angle errors, for it is very probable that im¬ 
provements could be made which would permit 
the milling of blanks within the prescribed tol¬ 
erances in angle. Primarily, these improvements 
would involve the generation of flatter surfaces. 
All other sources of error could very probably 
be kept within the necessary limit by close at¬ 
tention to details and without any fundamental 
changes in the milling technique which has 
been described. 

93 GLASS MOLDING 

931 Introduction 

In its general search for optical production 
methods that would not require skilled optical 











GLASS MOLDING 


407 


workers, the Office of Scientific Research and 
Development, under Project AC-11, entered into 
Contract OEMsr-421 with the Eastman Kodak 
Company in March 1942 for the investigation 
and perfection of methods of molding precision 
optical elements. 10 * 11 It was felt that such a 
molding process, if it could be perfected, would 
be of use not only as a method for producing 
ordinary spherical optical elements, but would 
also make possible the mass production of 
aspherical elements such as Schmidt correcting 
plates and lenses with parabolic surfaces. The 
possibility of producing such aspherical ele¬ 
ments in quantity would give the optical de¬ 
signer a new degree of freedom in his designs, 
and would enable him to improve the perform¬ 
ance of many types of optical instruments. 


9 ‘ 3 ' 2 Early Experimental Work: the Con¬ 
struction of the First Molding Press 

The ordinary process of molding glass ele¬ 
ments for use in roadside signs and similar 
devices involves the use of stainless steel molds. 
These are heated to a temperature of approxi¬ 
mately 400 C and then used to press a piece 
of heated, plastic glass of suitable size and shape 
into an element of the desired form. The tem¬ 
perature of the glass in the plastic state is con¬ 
siderably higher than that of the molds, and a 
common fault of elements molded in air in the 
manner just described is the descriptively 
named “orange-peel surface” which results 
from the sudden uneven chilling of the outer 
layer of hot glass as it comes into contact with 
the relatively cool mold. Such an orange-peel 
surface, of course, makes the element useless 
in a precision optical instrument; Newton's 
rings are rarely, if ever, shown when a test 
glass is applied to an ordinary molded glass 
surface. 

As a first step toward the elimination of 
this defect, the pressing must be carried out 
with the molds at a temperature approaching 
that of the plastic glass. At such temperatures, 
however, the glass sticks to the mold, and the 
mold rapidly oxidizes. A search was made, 
therefore, for metals to which glass would not 
stick when both glass and metal were very hot, 


and which, in addition, would not oxidize easily. 

As the experiments progressed, it became 
apparent that, in order to prevent rapid oxida¬ 
tion and deterioration of the molds when used 
at temperatures of 600 to 700 C, it would be 
necessary to carry out the heating and molding 
process in an oxygen-free atmosphere. A simple 
experimental press, which is illustrated in Fig¬ 
ure 16, was made up to determine the effects 
of some of the common inert gases on various 
metals and glasses heated to high temperatures. 
The operation of this press was very simple. 
The gas to be tested was admitted through the 
inlet at the bottom of the tube and it flowed 



Figure 16. Experimental molding press. 


through the outlet at the upper end of the tube. 
When the gas was flowing, glass in the cruci¬ 
ble, which was surrounded by a metal shell, 
was heated by a high-frequency induction cur¬ 
rent. As the glass approached the plastic state, 
the mold on the end of the quartz rod was 
pushed down into the high-frequency field, 
heated to a temperature approximately equal 
to that of the glass, and then pushed down 
farther, until an impression was made in the 
plastic glass. 

Commercial nitrogen, helium, and hydrogen, 
all freed from oxygen and water vapor by being 
passed over hot copper trimmings and calcium 
chloride, were tested in this device. Steel molds 
became dulled when heated to 600 C in nitrogen 
and helium. They remained bright, and showed 





















408 


OPTICAL TECHNIQUES 


no tendency to stick to the glass, when heated 
in hydrogen. 11 Unfortunately, however, three 
types of glass tested in this press developed 
dark surface films when heated in hydrogen, 
owing to the reduction of the metallic oxides 
in the glass by the hydrogen. This surface 
darkening was only slightly noticeable when 
the glasses were heated in nitrogen. 

The possibility was considered of construct¬ 
ing a press in which the glass, in pellets of 
approximately the correct size and shape, would 
be heated in an antechamber containing an at¬ 
mosphere of nitrogen, transferred to a hydro¬ 
gen-filled chamber where they would be pressed 
by the heated molds, and then be transferred 
to a second nitrogen-filled chamber for anneal¬ 
ing. Many difficulties were encountered in the 
design of such an apparatus, particularly in the 
design of the various gas traps through which 
the glass was to be transferred from one cham¬ 
ber to another. The simpler, single-chamber 
press illustrated in Figures 17 and 18 was 
therefore constructed. 

The essential details of this press are shown 
in Figure 18. Fundamentally, the apparatus 
consists of a pneumatic press having a 4-in. 
piston which moves the upper mold down upon 
the stationary lower mold. The downward travel 
of the piston is limited by an adjustable stop. 
Surrounding the molds is a gastight Pyrex 
glass envelope 42 mm in diameter and 250 mm 
long. Opposite the molds in this envelope is 
located a side port of 80 mm aperture extend¬ 
ing about 35 mm out from the main envelope. 
Hydrogen * 1 enters this envelope through two 
inlets and escapes through the side port where 
it is burned as it flows out into the open air. 
The flame very effectively prevents oxygen from 
entering the molding chamber. The molds, 
mounted on stainless steel tubes, are heated by 
a high-frequency induction current. 

The glass to be molded in the press is used 
in the form of rods approximately 12 mm in 
diameter. One end of the rod is heated in a gas- 

h The hydrogen was burned as it passed through the 
outlet into the open air. 

1 The hydrogen was purified by being passed through 
an iron tube filled with platinized asbestos maintained 

at a temperature of 700 C (which removed any oxygen 
present), and dried by being passed through a U tube 
filled with drierite (anhydrous CaS0 4 ). 


fired muffle. When the tip of the rod is in the 
proper plastic condition, it is withdrawn from 
the muffle, passed through the hydrogen flame 
and the side port, placed between the molds, 
and squeezed by the press. As soon as the 
glass and mold cool below the yield point of 



Figure 17. The first molding press. 


the glass (a period of 10 to 15 sec is required 
for them to reach this point) the pressure ex¬ 
erted by the press on the glass is released, and 
the molded element is removed by means of the 
glass rod to which it is still attached. 

Experience with this first working press has 
shown that: 

1. Pure hydrogen is the most satisfactory 








GLASS MOLDING 


409 


gas for use in the molding chamber in spite of 
its tendency to darken the surface of some 
types of glass through reduction of their ox¬ 
ides. Various mixtures of nitrogen and hydro¬ 
gen in the molding chamber have been tried 
in an effort to eliminate or reduce this surface 

MOVABLE ARM OF 



darkening, but the results have been unsatis¬ 
factory. When enough hydrogen is added to the 
nitrogen to make the mixture sufficiently re¬ 
ducing to maintain clean molds, and at the 
same time to make it sufficiently combustible 
to burn and form a flame seal at the side port, 
the mixture will also darken the glass, and so 
offers no advantages over pure hydrogen. Illu¬ 
minating gas has been tried as an inert 
atmosphere, but has proved to be unsatis¬ 
factory. 

2. It has been found that the pressure ap¬ 


plied to the glass during molding can be varied 
within the range 150 to 800 psi without caus¬ 
ing the glass to stick to the molds or affecting 
the properties of the finished element. 

3. A safe working temperature for the molds 
has been found to be about 600 C when lime- 
soda and borosilicate glasses are molded, 
and a temperature of about 650 C has been 
found to be satisfactory when Pyrex glass is 
molded. 

4. During the process of heating the glass in 
the gas-fired muffle in preparation for molding, 
the lime-soda and borosilicate crown glasses 
have a tendency to darken on the surface and 
thus to have their optical qualities impaired. 
This effect can be reduced by heating the glass 
in an electric oven. The darkening appears to 
be caused to a large extent by actual contact 
between glass and flame. Pyrex glass does not 
show this flame effect. 

5. Stellite has been found to be the most sat¬ 
isfactory material for molds. It is very durable, 
it takes a high polish, and optical elements of 
high quality can be pressed with it. In Figure 19 
is shown an example of the average quality of 
Newton’s rings obtained with a %-in. diameter 
pressing formed by a Stellite mold with a plane 
surface. The figure of the glass surface is quite 
symmetrical, but is concave. The majority of 



Figure 19. Newton’s rings from Stellite molded 
plane surface. 

the pressings made showed approximately the 
same concavity. In production, this systematic 
difference between pressing and mold could be 
corrected by the introduction of compensating 












































410 


OPTICAL TECHNIQUES 


curvature in the mold, equal and opposite to 
that observed in the glass. 

Stellite had the disadvantage that it was gen¬ 
erally available in sheets at most Yq in. thick. 
It was finally supplied by the Haynes Stellite 
Company in the form of cast Stellite to which 
was welded a layer of rolled Stellite No. 6B. In 
making molds from these pieces, the molding 
surface was formed on the rolled Stellite. 

.Some stainless steels have also been used in 
the construction of satisfactory molds. Very 
generally, the stainless steels take a high polish; 
their durability, however, is critically depend¬ 
ent upon their exact composition. Experiments 
(which were carried on some time later than 
those on which conclusions 1 to 4 listed above 
were based) have shown that Stainless Steel 
No. 155 (machining type) is, next to Stellite, 
the most satisfactory material for molds that 
has been found. 

From the point of view of durability, Armco 
ingot iron ranks very high among the mold 
materials tested. Unfortunately, it does not 
take a very high polish. 

The question of suitable materials for molds was ex¬ 
amined very carefully. A number of materials (chosen 
on the basis of a survey of 264 heat-resistant materials 
and a number of experiments) in addition to those 
already mentioned were tested. 

Ordinary cold rolled steel rapidly deteriorated with 
use. 

Platinum iridium (80 per cent Pt, 20 per cent Ir) 
was a satisfactory material, but was too expensive for 
general use. 

No. 430 stainless steel took a good polish, but im¬ 
perfections soon appeared on its surface and the mold 
rapidly deteriorated. 

Monel metal No. 650 stainless took a good polish, but 
during the pressing copper was freed from the metal 
and the mold deteriorated. 

Chromium plated steel (thickness of chromium 
0.00075 in.) showed no tendency to stick to the glass, 
but after a few pressings had been made with it holes 
began to appear in the chromium and the mold became 
unusable. 

Hawk-Eye stainless steel , used very successfully in 
the pressing of roadside sign lenses at temperatures 
of 400 C in air, deteriorated rapidly in pressing opera¬ 
tions carried on at 600 C. 

Nickel was too soft, and rapidly became flaky and 
crystalline, and produced a poor surface. 

High-speed tool steel rapidly crinkled and completely 
lost its figure. 

Tantulum tungsten carbide could not be polished 
satisfactorily. 


The First Trial Production Test of the 
Molding Procedure 

As a test of the press and the quality of 
optical element which could be produced with 
it, a trial production run was made on small 
reflectors for a Schmidt-type autocollimator 
designed at the University of Rochester and 
illustrated in Figure 20. Owing to the size of 
this element, it was necessary to enlarge the 
molding chamber and side port of the press. 



Figure 20. Schmidt reflector. 

The side port in the revised model was provided 
with a stepped floor to eliminate back draft, 
and a fishtail air jet was placed below the port 
in order to produce a flat sheet of flame that 
would provide an efficient seal against the entry 
of oxygen into the molding chamber. These 
changes are illustrated in Figure 21. A new 
hydrogen purification system of large capacity 
was also provided for this enlarged chamber. 

Nitrogen was made available for the purpose of 
flushing air out of the system before introducing the 
hydrogen. Hydrogen was then mixed with the nitrogen 
until the mixture would burn at the port. In this way 
the explosive backfires liable to occur, if pure hydrogen 
were ignited at the port, were prevented. After the 
flame was established, the nitrogen was turned off 
slowly until hydrogen alone was admitted to the 
chamber at the rate of 15 cu ft per hr. The molds were 
heated only while the hydrogen was passing through the 
chamber. After the molds had been allowed to cool, 
the flame was extinguished by admitting nitrogen to 
dilute the mixture. 

Molds for the Schmidt reflector were made of 
Armco ingot iron according to the specifications 
shown in Figure 22. The small convex section 
of the plane mold shown in the upper part of 


















GLASS MOLDING 


411 


the figure was provided in the form of an ad¬ 
justable insert so that by trial and error adjust¬ 
ment the press operator could correct any sys¬ 
tematic distortion in the thickness of the press¬ 
ing caused by differential effects between the 
hot mold and the glass. 

Approximately 200 Schmidt reflectors were 
molded from Pyrex glass No. 774. They showed 



Figure 21. Section of glass-enclosed molding 

chamber. 

very regular patterns of Newton’s rings, but 
pits were always found in the molded surfaces. 
Since many of these pits occurred in fixed pat¬ 
terns, it is clear that they were caused by im¬ 
perfections in the molds. The flat surfaces were 
less satisfactorily formed than the spherical 
surfaces. Shrinkage in the region of the small, 
indented spherical surface caused the flat por¬ 
tion of the reflector to be slightly depressed in 
the region surrounding the indentation. This 
depressed area could readily be corrected, how¬ 
ever, by grinding and polishing the flat surface 
by rapid production methods, a procedure 
which in this particular case, would not be im¬ 
practical. 

There was a large variation in the thickness 
(the separation between the two spherical sur¬ 


faces marked T in Figure 20) of the molded 
elements. Measurements of T for twenty re¬ 
flectors (pressed consecutively) gave the dis¬ 
tribution curve exhibited in Figure 23. The 
spread in T, from 0.415 in. to 0.427 in., is much 
larger than could be tolerated in elements of 
this type to be used in production. 

Two reflectors of the correct thickness were 
selected and edged, and the optical quality of 
their spherical reflecting surfaces was tested. 
The performance of these surfaces, in spite of 
the pits which were present, were found to be 
satisfactory when compared with a standard 
instrument. 


93 3 The Construction of the Improved 
Model II Molding Press 

The variations in the thickness of the 
Schmidt reflectors appeared to be traceable, at 
least in part, to flexure in the frame and arms 



Figure 22. Section of molds for Schmidt reflector. 

of the press. The actual amount of flexure of 
the press during any particular pressing opera¬ 
tion was thought to be related to the volume of 
glass between the molds at that time. In order 
to eliminate this flexure in the frame and arms, 

































412 


OPTICAL TECHNIQUES 


a new press, operating under the same general 
principles as the first press (Model I), but 
much more sturdily built than the first, was 
constructed as illustrated in Figure 24. 

Molding chambers of several designs were 
tested during the construction of the Model II 



THICKNESS T IN INCHES 

Figure 23. Distribution curve of thickness of 

Schmidt reflectors. 

press. The chamber finally used was formed 
from a standard 3-in. Pyrex T-section pipe j 
with ^4-in. thick walls. The chamber, fitted and 
mounted as shown in Figure 25, proved to be 
very satisfactory. It allowed good visibility of 
the glass being molded, it had ample strength 
to withstand any stresses to which it might be 
subjected, and it was large enough to allow ele¬ 
ments up to 35 mm to be molded. 

Because of the increased size of the molding 
chamber of Model II, high-frequency induction 
coils could not be placed sufficiently near to the 
molds to produce satisfactory heating. The 
molds were therefore heated by spirals of 
Nichrome V ribbon k placed just in back of them 
as shown in Figure 26. With a current of 40 
amp at a potential of 6 v through these spirals, 
it was possible to heat the molds to a tempera¬ 
ture of 600 C within a period of 10 to 15 min. 

Hydrogen was admitted to the molding 
chamber of the Model II press through the 
stainless steel tubes carrying the molds and 
Nichrome heaters rather than through the 

3 The standard side arm of 5-in. length was reduced to 
4-in. length in the construction of the chamber. 

k The spirals were made up of 27-in. lengths of 
ribbon of dimensions 0.125x0.032 in. 


walls of the chamber itself as in Model I. This 
new procedure has two advantages over the 
old: (1) It maintains a hydrogen atmosphere 
around the heating coils which therefore suffer 
very little deterioration. (2) The hydrogen, in 
passing through the tube, comes in contact 
with hot steel, and thus has removed from it, 
just before it comes in contact with the molds 
any last trace of oxygen remaining after the 
regular purification treatment. 



Figure 24. The Model II press. 


The insertion of hot glass between the molds 
was found to increase their temperature to 
such a point that the glass had a tendency to 




























GLASS MOLDING 


413 


stick to them unless the heating element was 
turned off during the actual pressing process. 
In order to prevent this sticking, a tempera¬ 
ture regulation system, operating through a 
thermocouple of Chromel-Alumel mounted in 



Figure 25. Section of molding chamber for Model 

II press. 

the base of the lower fixed mold as shown in 
Figure 25, was installed. The regulator main¬ 
tained the temperature of the molds to within 
± 4 C when the press was idle. Within 2 to 5 
sec after the insertion of hot glass between the 
molds the regulator would cut off the heater 
current which then remained off for a period 5 
to 10 sec longer than the glass remained be¬ 
tween the molds. 


934 Experiments Performed With the 
Model II Press 

Schmidt Reflectors 

As a trial production run to test the method 
and apparatus, several hundred Schmidt re¬ 


flectors (See Figure 28 E) were pressed from 
Pyrex glass No. 774 with molds made of No. 
155 stainless steel. Two pressings, chosen at 
random as samples, arid carefully inspected for 
optical quality, exhibited highly satisfactory 
spherical surfaces. They had no zones which 
departed from true figure by more than one- 
quarter wavelength. There were, however, a 
few scattered pits on the surfaces. These pits, 
viewed during the testing procedure as black 
dots on the brightly illuminated spherical sur¬ 
faces, covered only a very small fraction of the 
total reflecting area. 

About 150 Schmidt reflectors were pressed 
with Stellite molds made from the special 
welded blanks supplied by the Haynes Stellite 
Company. These Stellite molds produced sur- 



Figure 26. Section of resistance heater in tube. 


faces of higher quality with fewer indentations 
than those pressed with the No. 155 stainless 
steel molds and showed less deterioration. 

Plane Plates 

Two series of trial production runs on plane- 
parallel plates pressed from blue cane (calcium- 
































































































414 


OPTICAL TECHNIQUES 


sodium silicate) glass were made in order to 
test the quality and figure of molded flat sur¬ 
faces. The first run, made with the molds illus¬ 
trated in Figure 27, was unsatisfactory because 
of poor mold design. The thin rim of the lower 
mold cooled very quickly and caused a localized 
cooling and hardening of the glass which filled 
the lower mold. Thus, while the glass could be 
pressed into satisfactory contact with the rela- 



Figure 27. Section of plane molds. 


tively hot upper mold, it could not, at the same 
time, be pressed into contact with the relatively 
cool lower mold, and the pressings which re¬ 
sulted were of poor quality. 

It is probable that another somewhat similar effect 
is also operative here. As the hot glass is introduced 
into the molds by hand, there is a strong tendency to 
allow the glass to come in contact with the lower mold 
an instant before the upper mold is pressed down upon 
it. The lower portion of the glass is thus generally 
cooled more rapidly than the upper portion. The con¬ 
sequences of this situation (which probably exists for 
all molds) are similar to those just described for the 
thin rim alone. Probably both effects combined to cause 
the failure of the molds with the thin rim. 

A second set of molds with a heavier rim was 
made from No. 155 stainless steel. These 
proved to be much more satisfactory than the 
thin rim set, although the glass did tend oc¬ 
casionally to grip the heavy rim. The two 
sides of a plate (illustrated in Figure 28) 
pressed with these molds tended to show dif¬ 
ferent figures although the figures of both 
molding surfaces were the same. The figure of 
one side of the plate would not depart generally 
from the figure of the mold with which it was 
produced by more than one or two wave¬ 
lengths ; the figure of the other side of the plate 
would depart from the figure of its mold, how¬ 


ever, by fifteen to thirty wavelengths. This 
effect, like the others which have been men¬ 
tioned, undoubtedly arises from some phe¬ 
nomenon of unequal cooling. 

A few pits were present in the surfaces of 
the majority of these flat molded plates. 

Molds of Unusual Shape 

In an effort to determine whether or not glass 
elements of unusual shape could be molded suc¬ 
cessfully, a mold 28 mm in diameter consisting 
of a flat plate with grooves of semicircular 
cross section across its face was constructed 
and tested. The glass (see Figure 28B) pressed 
with such a mold faithfully reproduced all the 
intricacies of the mold as nearly as could be de¬ 
termined. 

Thickness Control 

Measurements of the thickness of consecu¬ 
tive pressings made during the runs on the 
Schmidt reflectors and flat plates mentioned 
previously showed variations far greater than 
could be tolerated in production. The range of 
variation was similar in both series of measure¬ 
ments, and was approximately 0.04 in. The 
principal cause of these variations is the lack 
of control over the volume, and to a lesser ex¬ 
tent, the shape of the mass of plastic glass in¬ 
serted between the molds. 

The Molding of Various Types of Glass 

Pressings of some barium crown and boro- 
silicate glasses (which were cast in the form of 
bars for use in the press) have been obtained 
with clear surfaces of reasonably good texture. 
Pressings of DBC-3 glass were very unsatisfac¬ 
tory because of the chemical reduction of the 
constituents of the glass which took place dur¬ 
ing the molding process. Tests with the East¬ 
man rare-earth glasses were also very unsuc¬ 
cessful owing to the low viscosity of the glass 
at the softening point. It was impossible to 
soften the interior of a rod of rare-earth glass 
before the outer surface of the rod dripped off. 

Production Tests 

A production test of the Model II press was 
made at the Hawk-Eye Works of the Eastman 
Kodak Company. It was used to produce aspher- 







GLASS MOLDING 


415 


ical lenses of 20 mm diameter and 41.6 mm 
focal length for use in the Mark 14 illuminated 
sight (the “Fly’s-Eye sight”), which was de¬ 
veloped by the Eastman Kodak Company under 
Division 7 of the NDRC. 12 

The mold for this aspherical lens was made 
of Stellite 6B, and was brought to the final 
proper figure by a series of zonal corrections. 
A number of pressings were made with the 
mold at each stage of its figuring. The pressed 
and finished lenses were measured for back 
focal length in a number of zones, and from 
these measurements the zonal corrections nec¬ 
essary to obtain a lens of the desired figure 
were computed and applied to the mold. By 
means of a series of such approximations a 
mold was finally produced which would form 



Figure 28. Molded elements. 


aspherical lenses having the same back focal 
lengths for all zones. 

The temperatures of the molds were auto¬ 
matically regulated. The upper (concave, 
aspherical) mold was maintained at a tempera¬ 
ture of 500 C, the lower (flat) mold at a tem¬ 
perature of 400 C. 1 Blue cane glass was used 
for making the elements. 

No effort to control the precise thickness or 
quality of the plane surface of the lens was 
made. These were taken care of by grinding 
and polishing the plane surface by conventional 
methods after the molding had been completed. 

1 The practice of maintaining the molds at different 
temperatures represents a slight departure from the 
molding techniques used in the experimental work. 


The press was thus used to produce only the 
aspherical surface of the lens. 

The quality of the lenses that have been pro¬ 
duced by this method Is very satisfactory; the 
figures of the aspherical surfaces varied from 
one lens to another by only one or two wave¬ 
lengths. The yield of lenses judged on the cri¬ 
terion of definition alone has been approxi¬ 
mately 90 per cent. The overall yield, however, 
in which rejections for material and surface 
imperfections are included, has been about 75 
per cent. 


935 A Model III Molding Press for 
Larger Elements 

The limiting diameter of optical elements 
that could be produced in the Model II press 
was approximately 35 mm. In order to explore 
the problems involved in the molding of larger 
elements up to 50 mm in diameter, a new press 
designed generally after the pattern of the 
Model II press was constructed. 

A photograph of the Model III press and its 
accessories is shown in Figure 29. The molding 
chamber was constructed from a standard 4-in. 
Pyrex T-section. 

The primary point of difference between the 
Model II and Model III presses is the method 
of heating the molds. In the Model III press the 
heating of the molds is accomplished by high- 
frequency induction coils within the chamber 
and immediately surrounding the molds. The 
method of supporting the coils is shown in Fig¬ 
ure 29. The extended tubes serve both as elec¬ 
trical leads and leads for water to cool the 
coils. Induction heating has proved to be very 
satisfactory; the molds are heated much more 
uniformly by high-frequency induction currents 
than by resistance heaters. 

Tests of the Model III Press 

With the Model III press, a number of lenses 
(of blue cane glass) were made up from a 
plano-concave mold™ having a diameter of 50 
mm. From among the pressings produced (see 
Figure 28F, G), elements of high optical 

m The concave mold has a radius of curvature of 50 
mm. The molds were made of No. 155 stainless steel. 






416 


OPTICAL TECHNIQUES 


quality could be chosen. The general run of the 
lenses produced with the 50-mm molds, how¬ 
ever, were very variable in quality, owing pri¬ 
marily to the lack of control of the amount, 
shape, and temperature of the glass inserted 
between the molds during the pressing. 

There was no evidence of orange-peel surface 
on any of the elements. Depressions and irregu¬ 
larities in the surfaces of many of them were 
neither larger nor more numerous than those in 
the surfaces of the smaller elements. 

The figures of the lens surfaces were regular, 
but some difference between the curvature of 


the lens surface, and the curvature of the mold 
which produced it, was usually observed for 
these larger elements. In production, this cur¬ 
vature effect could readily be compensated by a 
slight change in the curvature of the mold. 

Two trial production runs were also made 
with plane molds of 50 mm diameter. Both 
Pyrex and blue cane glass were used for the 
pressings. The pressings made with the cane 
glass frequently fractured (because of their 
thinness) before they could be removed from 
the press. The general conclusions drawn from 
the run with Pyrex glass confirmed the conclu¬ 
sions drawn from the run made with the plano¬ 
concave mold. 


9 ' 3 ' 6 Conclusions 

A high yield of small lenses of satisfactory 
quality can, under certain limitations, be pro¬ 
duced by the molding process which has been 
described. One of the central features of this 
process is the manual manipulation of a heated 
glass rod which is fed into the press. As the 
size of the element to be molded increases be¬ 
yond 35 mm in diameter, the many uncon¬ 
trolled variables involved in this manipulation 
begin to play increasingly important parts in 
the molding process. As these uncontrolled 


variables, such as the amount and shape of the 
glass inserted between the molds, the tempera¬ 
ture of the glass at the instant of pressing, and 
the duration of pressing, begin to increase in 
importance, the operator begins to lose control 
of the molding process, and the yield of accept¬ 
able lenses begins to fall rapidly. This is par¬ 
ticularly true when optical glass is used. 

The problem of exercising sufficient control 
over the various factors involved in the mold¬ 
ing process can be met with the introduction 
of various mechanical devices into the process. 
The perfected molding press must be con¬ 
structed to perform in automatic sequence the 
following actions, possibly in different inert 



Figure 29. Molding chamber, right side, Model III press. 









METHODS FOR MAKING RETICLES 


417 


atmospheres: (1) Separate from the source of 
supply a stated amount of glass heated to the 
plastic state at the proper temperature either 
before or after separation; (2) Premold the 
stated amount of glass removed in the first 
operation so that the press will not be required 
to introduce a large change of shape in the 
plastic glass; (3) Insert the plastic preshaped 
slug between the molds, avoiding contact be¬ 
tween the glass and the molds until the press 
acts to cause them to strike the glass simul¬ 
taneously; (4) Release the molds at a stated 
temperature of the molded piece. 

In the absence of such a completely auto¬ 
matic press, the process of molding optical ele¬ 
ments, as far as its development has been de¬ 
scribed, is limited in its application by the fol¬ 
lowing considerations: 

1. The choice of glasses is restricted to types 
represented by blue and yellow cane glasses 
and Pyrex. 

2. No precise control of the thickness of the 
molded piece is obtainable. 

3. Only one satisfactory surface can be pro¬ 
duced consistently. 

4. A high yield of acceptable elements is ob¬ 
tainable for pressings up to 35 mm in diameter, 
and a progressively lower yield is obtained as 
the diameter increases beyond 35 mm. (The 
shape of the surface to be molded, however, is 
subject to wide variation without affecting 
the yield of acceptable elements.) 

5. A small number of minute depressions 
and irregularities may be expected in the sur¬ 
faces of the larger elements, but on the basis 
of the experience gained from the production of 
the Fly’s-Eye aspherical lens, it is possible to 
eliminate these defects entirely in small lenses 
through those controls which can be exercised. 

From the experimental evidence now avail¬ 
able, it seems possible that a fully automatic 
machine which would employ the principle of 
molding elements in an inert atmosphere could, 
in time, be devised to perform the four auto¬ 
matic operations listed above on optical glass. 
With the perfection of such a machine, it would 
be possible to produce a molded element opti¬ 
cally finished on both sides with either spherical 
or aspherical surfaces, and with thickness con¬ 
trolled within the usual optical tolerances. 


9 4 METHODS FOR MAKING RETICLES 

/ 

Introduction 

During the period preceding America’s en¬ 
trance into World War II and for some time 
thereafter, the reticle situation in this country 
was very critical. The sudden enormous de¬ 
mand for such equipment made it appear doubt¬ 
ful whether or not facilities for reticle produc¬ 
tion by the conventional engraving-etching 
techniques could be developed rapidly enough 
to supply all needs. Moreover, the increasingly 
popular reflector-sight instruments (see Chap¬ 
ter 12) required at that time a new type of 
reticle for which satisfactory methods of pro¬ 
duction had not yet been worked out. 


9 42 Photographic Methods of Making 
Reticles 

One possible means of alleviating this critical 
situation, at least in part, was through the use 
of photographic methods of reticle production. 
It appeared, moreover, that photographic 
methods possessed a number of advantages not 
found in the customary engraving-etching pro¬ 
cedures: (1) The reproduction of a complicated 
pattern would appear to require no more effort 
than a simple one. (2) There could be produced 
patterns containing features ordinarily diffi¬ 
cult to achieve. (3) It should be possible to 
make more perfectly matched reticle pairs than 
had previously been produced. (4) It should be 
possible to produce sharper line edges than 
formerly. (5) The apparatus required for most 
photographic reproduction methods should be 
comparatively simple and inexpensive. 

Research on photographic methods of reticle 
production was carried on at Edward Stern 
and Company, Inc., 13 of Philadelphia (Contract 
OEMsr-293), and at the California Institute 
of Technology 14 (Contract OEMsr-389) under 
Service Directive NO-98. These laboratories 
had as their purpose under these contracts the 
testing of certain new photographic processes 
of reticle manufacture and the determination 
of the possible usefulness of those methods. 



418 


OPTICAL TECHNIQUES 


In all, six processes were tested. Four of 
these had been devised by the British Scientific 
Instrument Research Association, one was de¬ 
veloped in the Eastman Kodak Research Lab¬ 
oratories, and one was developed at the Cali¬ 
fornia Institute of Technology [CIT] especially 
to meet the demand for reflex-sight reticles. 

There are other photographic processes in 
operation which were not tested in these ex¬ 
periments. Two of these methods involve the 
production of the pattern by deposition of a 
metal through a mask or stencil produced on 
the glass by photographic means, and later re¬ 
moved. The quality of reproduction is excel¬ 
lent. 

The Keuffel and Esser process, in its two 
modifications, is capable of producing reticles 
of high quality suitable for side illumination 
and scales with opaque lines, respectively. The 
permanence of the work is very satisfactory 
and the process has been used for production in 
limited quantity. It is not certain how adapt¬ 
able it would be to the production of reticles in 
large quantities. 

The Bausch and Lomb process produces 
reticles reasonably satisfactory for side illumi¬ 
nation, but very fragile in character. 

The Buckbee-Mears process of making metal 
reflex-sight reticles should also be mentioned. 
By this ingenious method, in which a pattern 
is photoetched through a thin metal sheet, a 
very durable and satisfactory reticle was pro¬ 
duced in large quantities. 

The General Bichromated 
Glue-Relief Process 

This general process involves coating the 
reticle blank with a thin layer of bichromated 
glue and exposing it to light through a negative 
of the desired reticle. The exposed reticle blank 
is then washed in water, which removes the 
glue except from those places where it was 
affected by light. The glue remaining after this 
washing is made opaque by depositing in it 
dark material, and baked to increase its resist¬ 
ance to abrasion. 

Variations. Methods of opaquing the glue 
include: 

1. In the British lead-sulfide process, the 
glue forming the reticle lines is made opaque 


through impregnation with lead sulfide by al¬ 
ternate dipping in lead ferricyanide and am¬ 
monium sulfide solutions. 

2. In the Eastman glue-silver process, silver 
is deposited in the glue relief and is built up to 
adequate density by repeated intensification 
through dipping in solutions of mercuric bro¬ 
mide, silver nitrate, and a developer. 

3. The British colloidal-silver process in¬ 
volves the introduction of colloidal silver into 
the glue solution before the reticle blank is 
coated. This silver serves as a nucleus for in¬ 
tensification which is achieved through succes¬ 
sive dipping of the washed reticle in solutions 
of mercuric chloride, silver nitrate, and metol- 
hydroquinone developer. 

Methods Employing Etching of an Opaque 
Subcoat Under a Glue Resist 

This general process involves providing the 
reticle blank with a subcoat of suitable opaque 
material before applying the film of bichro¬ 
mated glue. The glue is exposed to light through 
a positive of the reticle to be copied, and the 
unaffected glue subsequently washed away. The 
pattern is then developed in the subcoat by 
etching away those portions left uncovered by 
the glue resist. 

Variations. Subcoat materials include: 

1. The British silver-line process makes use 
of a subcoat of chemically deposited silver 
which is etched away with alcoholic ferric ni¬ 
trate. 

2. In the CIT lead-sulfide-mirror process, an 
opaque subcoat of lead sulfide is chemically de¬ 
posited and subsequently etched with bromine, 
potassium bromide solution. 

The Photoetching Method 

In processes of this general type, the reticle 
is provided by photographic methods with an 
etching resistant material which is imperme¬ 
able and resistant even to hydrofluoric acid. It 
is then etched with this acid, the resist is re¬ 
moved, and the lines are filled with some opaque 
material in the customary manner. 

Variations. Etching processes include: 

1. The indirect or double-etching process is 
essentially an extension of the preceding proc¬ 
ess. A pattern in silver is developed upon the 





METHODS FOR MAKING RETICLES 


419 


reticle surface, as in the British silver-line 
process, by etching a silver subcoat through a 
resist prepared with the use of photoengraving 
glue or other light-sensitive material. The 
silver pattern serves as the etching resist for 
the final etching with hydrofluoric acid. 

2. The direct process is a method in which 
an etching resist is prepared directly by the 
use of a photosensitive resin which is resistant 
to hydrofluoric acid. 


943 The Results of the Investigations 

All phases of the methods tried and thought 
to be promising were thoroughly investigated. 
Considerable time was spent in a search for a 
suitable resist material for the direct process 
method, a subcoat material for the indirect- 
process method, and in the development of the 
lead-sulfide-mirror process. Several advances 
in the general technique of preparation of the 
master drawings and in the production of print¬ 
ing negatives were achieved. 

In the overall view, all of the advantages 
hoped for in the photographic processes were 
not realized. In particular, it was found that 
the reproduction of a complicated pattern was 
more difficult than a simple one. Hand retouch¬ 
ing was always required. The more complicated 
the pattern, the more time-consuming this be¬ 
came. Nevertheless, the photographic processes 
were still more rapid than the conventional 
mechanical methods. It was found that features 
difficult to produce by pantographic methods 
(especially patterns involving both narrow and 
wide lines) were also difficult to produce by 
photographic methods. 

As to individual processes, the following con¬ 
clusions were reached: 

1. The bichromated glue-relief processes in 
general are simple, rapid, inexpensive, and 
suited to nearly automatic operation. The qual¬ 
ity of reproduction is excellent, but the dura¬ 
bility of the pattern is limited. Cleaning of the 
reticles, which are not generally suitable for 
night illumination, requires special care. 

The British lead-sulfide process, one of the 
two procedures considered to show the most 
promise, is very rapid and convenient, but it 


has been found that the reproduction of lines 
much narrower than 0.001 in. may be inferior 
to that obtained with the Eastman glue-silver 
process and the British colloidal-silver process. 
Line opacity attainable with this process is 
limited, and lines free from scum are difficult 
to obtain. Reticles produced by this method are 
not suitable for use in reflex sights because of 
their insufficient resistance to heat. 

The quality of reproduction obtained with 
the Eastman glue-silver process is excellent, 
and the line opacity very adequate. The method 
is applicable either to black-line or reflex-sight 
reticles. It is, however, more time-consuming 
than the British lead-sulfide process. It gave 
results superior to those obtained by the Brit¬ 
ish colloidal-silver process. 

In the British colloidal-silver process, in¬ 
tensification took place more rapidly than in 
the Eastman glue-silver process, but it was not 
possible to secure sufficient opacity to make the 
process applicable to reflex-sight reticles. 

2. The British silver-line process is of very 
limited applicability owing to the inadequate 
adherence of the silver to the blank. It is not 
adapted to production of night-illuminated 
reticles; reflectivity of the silver makes the 
process unsuitable for reflex-sight reticles. 

In the CIT lead-sulfide-mirror process, on the 
other hand, adherence of the sulfide coat is ex¬ 
cellent. The coat is reasonably black; the 
reticles are reasonably permanent. They resist 
salt spray fairly well, and withstand moderate 
heat, but are considered insufficiently stable for 
use in reflex sights, in which the solar image 
may be focused on the reticle. The method is 
probably restricted in usefulness to opaque-field 
transparent-line work with lines not less than 
0.002 in. wide. 

3. In general, the photoetching method 
yields reticles of a permanent character suit¬ 
able for night illumination. Excellent line qual¬ 
ity may be attained under the best conditions. 
The percentage output of acceptable work, 
however, is probably smaller than that achieved 
with conventional pantograph methods. The 
process is far from automatic, and individual 
attention is required for each reticle, particu¬ 
larly in retouching pinholes in the resist. Re¬ 
sist materials developed so far have limited re- 



420 


OPTICAL TECHNIQUES 


sistance to hydrofluoric acid, the only satisfac¬ 
tory etching material, and hence there exists 
an upper limit to the line width which can be 
produced. 

There are at present two resists of this type 
in actual use. The material developed at the 
Keuffel and Esser Company [K & E], and used 
there, has very limited resistivity, and, al¬ 
though it is adequate for the special K & E 
process, would be unsatisfactory for ordinary 
deep etching. The resist developed by Edward 
Stern and Company is probably the best so far 
devised. It appears to give a very clean-cut 
image by automatic development, and has given 


durability to some reflex-sight reticles which 
have been used, but were inferior to metal 
reticles developed by Buckbee Mears for the 
same purpose. The British lead-sulfide and the 
British photoetching processes were considered 
to be the most promising and were tested on a 
small pilot-plant scale. 

In Table 3 estimates are given of the number 
of operator minutes to produce a reticle by each 
of the processes tested, and of the percentage 
yield of acceptable work. 

In Table 4 will be found a comparison of line 
widths achieved by the three most promising 
methods. The pattern used for these compari- 


Table 3. Working times and yields of various photographic reticle processes. 



British 

lead-sulfide 

British 

colloidal- 

silver 

Eastman 

glue-silver 

British 

silver-line 

CIT lead- 
sulfide-mirror 

British 

photo¬ 

etching 

Operator minutes per 
reticle* 

6 

7 

12 y 2 

7 

io y 2 

15-20 

Percentage yield of ac¬ 
ceptable work 

60-70 

60-70 

50-70 

60-70 

60-80 

20-30 


* These times do not include periods during which the blanks were not actively handled in printing, baking, etc. They are based on 
processing in batches of six, and could therefore be reduced by handling larger batches in some of the steps, such as intensification and 
silvering. 

In the photoetching process much time is consumed in cleaning, inspection, and spotting out. Presumably, this could be reduced with 
further work on the subcoat material. Rejects after the first etching of the silver subcoat may amount to 30 or 40 per cent. These may be 
turned back for reworking so that the blanks are not lost. In the Minneapolis Honeywell and Buckbee Mears plants, where reticles are 
produced essentially by this process, the completed work which is acceptable is said to average about 80 per cent. 


acceptable results with a deep-etching method. 
Its resistivity to hydrofluoric acid is none too 
great, however, and some trouble is experienced 
with pinholes which require considerable hand 
retouching before the final step. 

The British photoetching process is compli¬ 
cated and involves many steps in each of which 
trouble may be experienced. While excellent re¬ 
sults may be obtained, the yield of acceptable 
work may be small. Much depends on the skill 
and ingenuity of the personnel. 

Of the processes tried, the British silver¬ 
line and the British colloidal-silver were found 
to have the most restricted applicability. The 
Eastman glue-silver process was compared with 
the British colloidal-silver process and found 
to be superior to it, but the former was not in¬ 
vestigated extensively inasmuch as it was al¬ 
ready being used successfully in production in 
the United States. Reticles produced by the CIT 
lead-sulfide-mirror process were superior in 


sons consisted of a grid of lines of three widths 
denoted in the table by A, B, and C. 

Figures 30, 31, 33, and 34 exhibit the results 
of photographic processes, and illustrate some 
of their distinguishing characteristics. In Fig¬ 
ures 33 and 34 a faint bulge is discernible at 
each T junction. This is characteristic of any 
photographic process in which the printing 
negatives are made by optical projection, and 
results from the limited resolving power of 
lenses. It can be avoided by making the nega¬ 
tives by some mechanical process. 

In Figures 30 and 31 the horizontal base line 
appears to be slightly wider than the other 
lines. This is also discernible in the originals, 
but not to the extent visible in the plates, where 
it has been exaggerated by astigmatism in the 
microscope through which the photographs 
were taken. The widening is not due to any 
fault of the photographic reticle process, but 
to the difficulty of producing lines of constant 













LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


421 


width in the master drawings. This irregularity 
in line width could undoubtedly be avoided 
through application of the skill and ingenuity 
gained with experience. 

The beautiful clean-cut and crisp reproduc¬ 
tion which is obtained by the glue-relief meth¬ 
ods, and which is seldom, if ever, approached 
by any method involving deep etching, is illus¬ 
trated in Figures 30 and 33. 

Figures 32 and 35 show characteristics of 
average work in the process involving mechani- 


is being used in commercial production on a 
very considerable scale. From a practical point 
of view, it must be regarded as successful. It is 
likely that in certain special fields (for ex¬ 
ample in the production of rangefinder scales 
and reticles) the photographic methods have 
gained a firm position, but it is impossible to 
say to what extent they will displace panto- 
graphic methods. It is perhaps significant that 
in the Minneapolis Honeywell plant the two 
methods have been employed simultaneously. 


Table 4. Line widths. 


Process 

Line widths (microns) ± Z* 

Line A Line B Line C 

Remarksf 

Theoretical 

16 

23 

45 

assuming ideal reproduction of drawing 

Eastman glue-silver 

18 

25 

49 

line quality acceptable 

British lead-sulfide 

20 

26 

50 

line quality acceptable except that finest 
lines may show occasional defects 

British Photoetching 

1. Silver resist (stencil) 

15 

23 

47 


2. Etched line itself 

a. 4-sec vapor etch 

17 

25 

29 

line quality acceptable, filling unsatisfactory 
at crossovers 

b. 8-sec vapor etch 

20 

27 

53 

line quality acceptable, filling satisfactory 

c. 12-sec vapor etch 

29 

40 

61 

serious deterioration of line quality, rough 


edges 


* Variation in line width ± Z microns, 
t Crossovers were reasonably well produced in all cases. 

cal engraving of a wax resist and etching of 
the pattern so formed. 


9 ' 4 * 4 Conclusions and Recommendations 

It is difficult to make an objective comparison 
between the capabilities of mechanical and pho¬ 
tographic methods. This difficulty arises in part 
from the fact that the methods of reticle inspec¬ 
tion have not been completely objective, and as 
a result, the requirements have not been en¬ 
tirely uniform. One consequence of this is that 
a reticle process may be blamed for defects 
which existed in the original blank, and for 
which the process is not responsible. On the one 
hand, the photographic methods have not 
played as important a part in the reticle pro¬ 
gram as it was originally anticipated. On the 
other hand, reticles made by the lead-sulfide 
process and other methods giving equally 
fragile products are being used to some extent 
by the Services, and the photoetching process 


In spite of the fact that they did not gain 
general acceptance during World War II, the 
photographic methods of reticle production are 
still attractive. Particularly so is the general 
method of directly producing an etching stencil 
on a surface which is light sensitive and at the 
same time resistant to hydrofluoric acid. It is 
highly probable that many resists exist which 
are far superior to any yet found, and that a 
search for such resists would be profitable. 
Such a search should be undertaken, it would 
appear, with the cooperation of a competent 
chemist experienced in the field of plastics. 


95 LOW-REFLECTION AND HIGH- 
EFFICIENCY FILMS 

In recent years great improvements in the 
performance of optical instruments have been 
made possible through the application of thin 
films of various materials to their optical ele¬ 
ments. Through the use of such films on glass 






422 


OPTICAL TECHNIQUES 



Figure 30. Eastman glue-silver process. (En- Figure 32. Reticle produced by pantograph, 

largement is 17X.) (Enlargement is 17X.) (Frankford Arsenal.) 



Figure 31. Photoetched reticle. (Enlargement is Figure 33. Eastman glue-silver process. (Enlarge- 

17X.) ment is 56X.) 





























LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


423 


it is possible to decrease or increase surface re¬ 
flection, to make polarizing beam splitters, neu¬ 
tral filters, and so forth. In view of the general 
importance and the numerous applications of 
such films, research on various aspects of their 
development was carried on by the Office of 
Scientific Research and Development under 
Project AC-11 and CE-27, through research 
contracts with the California Institute of Tech¬ 
nology 15 (Contract NDCrc-118), Yard, Inc. 16 


transmitted by instruments containing various 
numbers of optical elements. It is apparent 
from an inspection of these data that great 
difficulty would be experienced in using under 
conditions of poor illumination an instrument 
containing ten or more optical elements. This 
difficulty of obtaining sufficient light intensity 
for clear visibility because of loss of light 
through surface reflections in the optical in¬ 
strument can, to a large extent, be eliminated 



Figure 34. 
56X.) 


Photoetched reticle. (Enlargement is 


Figure 35. Reticle produced by pantograph. (En¬ 
largement is 56X.) (Frankford Arsenal.) 


(Contract OEMsr-529), and the University of 
Rochester 17 (Contract OEMsr-160). 


951 Low-Reflection Films 

No doubt the most widespread and best 
known application of thin films to optical in¬ 
struments has been in the reduction of reflec¬ 
tions from lens surfaces. While the amount of 
light lost by reflection from the two surfaces 
of a single lens is not ordinarily large from 
most points of view, the amount of light lost 
through air-glass reflections in an optical in¬ 
strument, in general, rapidly becomes large as 
the number of optical elements is increased. In 
Table 5 are tabulated the percentages of light 


by coating the lenses with films of the proper 
material and of the proper thickness, as may 
be seen from Table 5. 

Simple Theory of Low-Reflection Films 

In Figure 36 is represented a glass surface 
coated with a thin film. The index of refraction 
of the glass (at wavelength A) we represent by 
n d\' The i n d ex of refraction of the dielectric 
material forming the film we represent by n n , 
which we stipulate must be less than n gx . The 
index of air we take as unity. The arrows A 
and B represent beams of light reflected nor¬ 
mally from the air-film surface and the film- 
glass interface, respectively. 

From Fresnel's law of reflection we can 
write that the amplitude r 4x , of light of wave- 


istrict: 










424 


OPTICAL TECHNIQUES 


length A reflected from the air-film surface, is 
given by the equation 


r A \ = 


Uf\ — 1 


?Vx + 1 

Likewise, the amplitude r 5x , of light of wave¬ 
length A reflected from the film-glass interface 
is given by 


n f \ 

Tb\— -;-. 

n g \ + ft/x 

If none of the light in the beam entering the 
instrument is to be lost through backward re¬ 
flection, we must arrange conditions so that the 
two reflected 11 beams A and B interfere with 



INDEX OF REFRACTION^ 1.0 



‘HI 

(Ml 



INCIDENT 

1 1 

LIGHT 

1 1 



Figure 36. Kays of light reflected normally from 
two surfaces of a thin film on glass. 

each other destructively. For this interference 
to take place, the two reflected beams must be 
out of phase, and must have equal amplitudes. 
Thus 

n f \t = | A, 

where t represents the thickness of the film, 
and 

n f \ — 1 = n g \ — n f \ 

Uf\ + 1 n g \ + rif\ 

From these equations we find that 

t-\-± 

4 n f x 

and, 

n f \ = Vn g \ • 

Inasmuch as the wavelength and index of 

11 Multiple reflections are neglected in this brief sum¬ 
mary. Investigation shows that the essential results of 
this discussion are not altered by their inclusion. 


refraction appear in these expressions, it is 
clear that complete elimination of reflection can 
be accomplished for only one wavelength of 
light. Partial elimination of reflection, how- 


Table 5. Light transmission of an optical instru¬ 
ment containing various numbers of optical ele¬ 
ments (calculated). 


Number of 
optical 
elements 

Overall transmission 
(per cent) 

uncoated coated 

Relative 
increase 
(per cent) 

1 

92.0 

99 

7.6 

2 

84.5 

98 

16.0 

3 

78.0 

97 

24.3 

4 

71.6 

96 

35.5 

5 

66.0 

95 

44.0 

6 

60.5 

94 

55.5 

7 

55.7 

93 

67.0 

8 

51.3 

92 

79.5 

9 

47.2 

91 

93.0 

10 

45.5 

90 

100.0 

20 

21.6 

81 

279.0 


ever, will be accomplished for a band of wave¬ 
lengths of considerable width on either side of 
the wavelength of zero reflectivity. 

At angles of incidence other than normal, the 
theory of reflection becomes more complicated. 
Polarization effects must be considered, and 
the index requirements of the materials of 
which the films are made become more strin¬ 
gent. It can easily be shown that in the case of 
low-reflection films for oblique rays the thick¬ 
ness of the film may be determined from the 
equation 


cos r\ 

where t 0 is the thickness of film for zero angle 
of incidence, and r x is the angle of refraction 
of a ray (of wavelength A) in the film. This 
angle can be computed by means of the expres¬ 
sion 


sin ^ 

-= sin r x , 

n f \ 

i being the angle of incidence of the ray under 
discussion. If the film is to be nonreflecting for 
a ray striking the film at an angle of incidence 
i, the index of refraction of the material of 
which the film is formed must be related to the 
index of refraction of the glass n gX , through the 
equation 




sin 2 A 
ntx 2 / 



























LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


425 


If we choose n gx = 1.51, then for 
i = 0°, n f x = 1.23 
i = 45°, n f \ = 1.15 
i = 90°, n/x = LOO. 

Several methods of producing films having 
the properties required for the elimination of 
reflections have been tried: (1) Dilute solu¬ 
tions of various acids have been used to dis¬ 
solve out certain of the metallic oxides from a 
thin surface layer of the glass treated. A porous 
layer of silica is thus formed; air enters the 
cavities in the silica and a film of index of re¬ 
fraction effectively lower than that of the glass 
proper results. (2) Very thin layers of various 
materials have been applied to glass by a dip¬ 
ping process. Layers of the required thickness 
of one-quarter wavelength 0 can be built up by 
successive dipping. A variation of the dipping 
method involves spinning the film onto the 
glass surface by means of a rotating device. 
(3) Thin films of various metallic salts have 
been deposited on glass by evaporation in a 
vacuum chamber. 

The first of these methods, while apparently 
offering a very permanent treatment, does not 
produce effective low-reflection films on all 
types of glass. It involves, moreover, a very 
special technique for its successful application, 
and, in addition, the lenses to be treated re¬ 
quire special handling during manufacture and 
after treatment. The method has never 
achieved great popularity. 

The process of spinning a film onto a lens has 
been used to some extent commercially and by 
the Services, but the film thus formed is less 
efficient in eliminating reflections and has 
poorer mechanical properties than that pro¬ 
duced by the evaporation process. This latter 
process, from the point of view of efficiency of 
film produced and adaptability to quantity pro¬ 
duction is, so far, the most satisfactory process 
known. 

Among the various materials (generally al¬ 
kali metal and alkali earth fluorides) used in 
the evaporation process to form the film, mag¬ 
nesium fluoride, calcium fluoride, cryolite 
(Na 3 AlF G ), and chiolite (2NaF*AlF 3 ) have 

° In this discussion optical thickness will always be 
understood unless it is specifically stated otherwise. 


proved to be the most useful and practical. 
Cryolite is the material most used in Europe. 
Each of these materials has certain advantages 
and disadvantages. Films made from calcium 
fluoride are very efficient, but, unfortunately, 
are very easily damaged. Films of magnesium 
fluoride have better mechanical properties but 
poorer optical properties than films of calcium 
fluoride, and so forth. Among the primary ob¬ 
jectives of the research on low-reflection films 
carried out by NDRC have been the improve¬ 
ment of the efficiency of the films in preventing 
reflection and the increase of their resistance 
to abrasion, sea water, and so forth. 

In the search for more efficient films many 
new materials, mainly silicofluorides and fluo¬ 
rides containing two metals, were tested. None 
of these were found to be superior to the ma¬ 
terials already in use; most of them were, in 
fact, inferior. 

Two lines of attack on the problem of the 
improvement of the mechanical properties of 
the films were followed. The first of these was 
one originally suggested by Cartwright and 
Taylor in U. S. Patent No. 2,207,656, where it 
is stated that over a fluoride film (which had 
purposely been made a little less than one-quar¬ 
ter wavelength thick) might be deposited a 
very thin protective layer of quartz or zircon. 
A search was made for materials that had the 
protective properties of quartz or zircon, but 
were more suitable for easy evaporation in a 
vacuum chamber. While no such materials that 
were completely satisfactory were found, there 
was an indication that very thin coats of cer¬ 
tain metals evaporated onto the fluoride film 
might be changed into hard transparent pro¬ 
tective oxide coats when exposed to the atmos¬ 
phere. From the experiments performed, how¬ 
ever, it was not possible to arrive at a definite 
conclusion as to the true merits of such a pro¬ 
cedure. 

The second general line of attack on the 
problem of hardening the fluoride film and of 
improving its mechanical properties in general 
involved bombardment of the film with ions in 
a partial vacuum, treatment by mechanical rub¬ 
bing, treatment with heat, and combinations of 
these. Different types of films reacted differ- 
ehtly to the various treatments. In general, it 




426 


OPTICAL TECHNIQUES 


was possible to find a technique of treating any 
film so that its mechanical properties would be 
improved and its optical properties would not 
be harmed at all, or only slightly. 

The process which appears to be the most 
satisfactory for producing low-reflection films 
of average efficiency in eliminating reflections 
and of good mechanical properties is one devel¬ 
oped at the Naval Gun Factory and also investi¬ 
gated by NDRC. The procedure involves a heat 
treatment and the use of specially purified mag¬ 
nesium fluoride in an evaporation process. Be¬ 
fore evaporation of the fluoride, the optical ele¬ 
ments in the evaporating chamber are heated 
to a temperature of approximately 200 C by 
irradiation from Nichrome wires. When they 
reach this temperature, the fluoride is evapo¬ 
rated. The coated elements are then baked at a 
temperature of 200 C for a period of approxi¬ 
mately 1 hour. Films treated in this manner 
show no structure even under a magnification 
of 15,000x> are unharmed by immersion in con¬ 
centrated nitric acid for a period of 12 hr, show 
no effects when subjected to salt spray for 
1,000 hr (they do show some deterioration after 
1,500 hr exposure), and are not affected by dis¬ 
tilled water over long periods of time. Such 
films reduce the amount of white light reflected 
from glass of index of refraction 1.7 from 6.7 
per cent to approximately 0.5 per cent, and for 
glass of index of refraction 1.52 from 4.2 per 
cent to approximately 1.4 per cent. 

Cryolite, deposited on glass treated in the 
manner described, does not form a film so hard 
or insoluble as magnesium fluoride, but there is 
some indication that the cryolite is more effi¬ 
cient in reducing reflection. This reflection re¬ 
duction factor depends greatly upon the index 
of the film, which is very sensitive to the 
vacuum conditions at the time of evaporation. 
The higher the pressure at the time of evapora¬ 
tion, the greater the reflection reduction factor, 
but, unfortunately, the softer and less resistant 
the film deposited. 

The Evaporator and the 
Evaporation Process 

The evaporators used in the formation of low-reflec¬ 
tion and other films are now so generally well known 
because of the widespread usage of coated optics by the 
Services that no detailed description of the apparatus 


with which the greater part of the work discussed in 
this report was done will be given. Only its main 
differences from conventional evaporators will be 
pointed out. 

The apparatus used in these experiments was de¬ 
signed primarily for research, but it was also found to 
be highly satisfactory as production equipment. Its 
chief point of departure from a conventional evapora¬ 
tor is that, instead of consisting essentially of a glass 
or metal bell jar resting on a metal base plate, it was 
made in the form of a cylinder from a piece of drawn 
brass tubing 12 in. in diameter, and closed at the 
bottom by a soldered brass plate. On the top of the 
cylinder, a shoulder was constructed with provision 
for a lightly greased soft-rubber gasket on which could 
be placed a flat 14-in. disk made of heat-treated Libby- 
Owens Ford %-in. glass which formed the top of the 
tank. Atmospheric pressure on the outside of this glass 
plate was sufficient to seal it as soon as the fore pump 
in the vacuum system was turned on. This plate could 
very quickly and easily be cleaned after each evapora¬ 
tion. The apparatus as a whole was very easy to clean, 
and this constituted one of its greatest advantages over 
the conventional type of evaporator. 

The main pump lead and the filament leads come in 
through the side of the cylinder rather than through 
its base. Valves in the vacuum lines are metal stop 
cocks with steel cores and brass shells to prevent bind¬ 
ing. In the bottom of the cylinder is located a ground 
joint which earn easily be rotated from the outside with¬ 
out affecting the vacuum. To this joint on the inside of 
the cylinder is fastened a number of attachments for 
turntables used in the production of films of uniform 
thickness over large areas, and for various other ex¬ 
perimental purposes. 

The vacuum system, which has functioned very satis¬ 
factorily, consists of a 4-in. all metal Distillation 
Products, Inc. [DPI] vertical diffusion pump operating 
with Butyl Sebacate “Special” oil, or Amoil, and a Cenco 
Hypervac-20 fore pump. No traps were used in the sys¬ 
tem, although there were indications at times that a 
trap to prevent back streaming of the oil in the diffusion 
pumps would have been advantageous. 

As is usual in such evaporators, pirani and ion 
gauges are used for determining the degree of vacuum 
attained. Low-reflection films are usually deposited at a 
pressure of 10— 3 mm of Hg or less. Other materials, 
for proper film formation, must be evaporated at 
pressures of 5 X 10— 4 to 10— 5 mm of Hg. 

The cleaning of the glass to be coated is very im¬ 
portant. A film deposited on improperly cleaned glass 
(if it forms at all) is likely to develop blisters and pin¬ 
holes or to peel off. The standard cleaning procedure 
used during the greater portion of this research has 
involved washing the glass to be coated in a 20 per cent 
sodium hydroxide solution and then in 10 per cent 
nitric acid. The glass is scrubbed in each bath with 
diaper cloth. Washing is continued until a water film 
will cling to the whole surface, without drawing away 
at any point. If oils or grease are on the glass, they 



LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


427 


must be removed with benzene and acetone. The glass 
is finally rinsed in tap water, sprayed with doubly dis¬ 
tilled boiling water, and dried in a filtered hot air 
stream. 

If pinholes are to be avoided, it is necessary that the 
surface of the glass to be coated be held upside down 
during the last stages of cleaning, and kept this way 
until the film has been deposited, as otherwise dust 
particles will settle on the surface. 

Magnesium fluoride and cryolite are customarily 
evaporated in the tank from small mullite crucibles. A 
tungsten filament, placed as shown schematically in 
Figure 37, serves as source of heat for the evapora¬ 
tion. 



Figure 37. Schematic diagram of method of 
evaporating fluoride and of determining thickness 
of film deposited. 

Figure 37 also shows the general method of determin¬ 
ing the thickness of the film on the glass by viewing the 
color of the light reflected from the film-glass surface. 
Ordinarily, the film is deposited to a thickness such 
that light in the wavelength rangeP 5,300 A to 5,400 A 
(to which the eye is most sensitive) is very weakly 
reflected. Light of other wavelengths will be more 
strongly reflected. If white light is incident upon a low- 
reflection film of such thickness, the light reflected by 
the film will appear colored, for the green light (5,300 
to 5,400 A) in the reflected beam will be very weak. To 
judge the thickness of the film, one thus examines the 
color of the light reflected from it. Light reflected from 
a film of the proper thickness for maximum trans¬ 
mission of light in the range 5,300 to 5,400 A appears 
purplish in color (or between red magenta and blue 
magenta as some writers say). A film for maximum 
transmission of 4,500 A appears amber or yellow by 
reflected light; a film for maximum transmission of 
6,000 A appears blue by reflected light, and so forth. 


p These figures represent a compromise between the 
wavelengths of maximum sensitivity of the eye for day 
and night vision. 


9 5 2 High-Efficiency Partially Reflecting 
Films on Glass 

In many optical instruments (reflex sight 
systems in particular) it is desirable to have a 
surface that will transmit part of the light in¬ 
cident upon it and reflect the rest. Frequently, 
thin “half-reflecting” films of silver or other 
metals have been used for this purpose, but 
these have several disadvantages. Silver tar¬ 
nishes easily, and is inefficient. A silver film 
which reflects as much light as it transmits, 
will absorb 20 per cent of the incident light, 
reflecting only 40 per cent of the light, and 
transmitting 40 per cent. Silver is thus said to 
have an efficiency of 80 per cent. On the same 
basis aluminum has an efficiency of 70 per cent; 
most other metals have efficiencies which lie be¬ 
tween 60 and 70 per cent. In the form of thin 
partially reflecting films, metals are therefore 
wasteful of light. 

The disadvantages involved in the use of thin 
partially reflecting films of metal, can be over¬ 
come to a large extent through the use of trans¬ 
parent dielectrics which have almost no absorp¬ 
tion coefficient and are therefore extremely effi¬ 
cient. In order to obtain a highly reflecting sur¬ 
face, it is desirable to evaporate onto the glass 
to be treated a dielectric film of refractive in¬ 
dex higher than that of the glass, and of such 
thickness that the reflected beams A and B, as 
illustrated in Figure 36, are in phase and there¬ 
fore interfere constructively. The proper thick¬ 
ness of the film in this case is one-quarter wave¬ 
length, as it was for the low-index material 
which was to prevent reflection. If the film illus¬ 
trated in Figure 36 has a higher index than the 
glass, beam A will undergo a phase change of 
t r upon reflection, but beam B will not undergo 
any. A phase change of tt will be introduced, 
however, in B, due to the difference in optical 
path, if the film is one-quarter wavelength 
thick. Hence, under such conditions, the beams 
are in phase when B emerges from the film.* 

ilf a low-index film were deposited on the glass 
rather than a high-index film, the film thickness neces¬ 
sary to cause constructive interference would be one- 
half wavelength. It has, however, been found more 
satisfactory to employ high-index films in view of their 
greater durability, and in view of the possibility of 
using multilayer films (to be discussed presently) to 
increase the reflectivity to a high value. 













428 


OPTICAL TECHNIQUES 


For making such a high-reflection high-effi¬ 
ciency film, zinc sulfide with an index of refrac¬ 
tion of 2.8 has been found satisfactory. A 
single layer of zinc sulfide deposited in a film 
one-quarter wavelength thick will reflect, on 
the average (the index of ZnS varies somewhat 
with the vacuum conditions), approximately 25 
per cent of the incident white light. The trans¬ 
mitted light will appear very slightly tinged 
with color. This appearance of a colored trans¬ 
mitted beam is completely analogous to the 
appearance of a purple or magenta colored re¬ 
flected beam in the case of low-reflection films. 
The high-reflection film, like the low-reflection 
film, is of the correct thickness for only one 
color of light, and in the case under discussion, 
the color is that to which the eye is most sensi¬ 
tive. The light transmitted by the high-reflect¬ 
ing film, like that reflected by the low-reflecting 
film, appears very slightly purplish or magenta 
in color. The light reflected by the high-reflec¬ 
tion film, like that transmitted by the low- 
reflection film, is not colored to any noticeable 
degree. 

High-reflecting high-efficiency quarter-wave 
films can also be made by fuming titanium di¬ 
oxide onto glass. Such a film, which reflects 30 
per cent or more of the incident light, is harder 
and far more permanent than a zinc sulfide 
film. Films of titanium dioxide deposited on 
properly cleaned glass will not show pinholes 
even if cleaned with Bon Ami (if the pressure 
applied during rubbing is not too great), and 
can easily withstand washing with water, dilute 
acid, acetone, or any other mild cleansing 
agent. 

The most satisfactory method of fuming titanium 
dioxide is as follows: A 10-in. evaporating dish is half 
filled with titanium tetrachloride which is heated to a 
temperature of approximately 60 C. A jet of compressed 
air passing through a funnel 3 in. in diameter is played 
on the surface of the liquid. A cloud of dense white 
smoke arises from the dish, where the chemical re¬ 
action taking place is 

TiCl 4 + 2H 2 0 Ti0 2 + 4HC1. 

The glass to be coated is heated to a temperature of 
200 C and plunged into the cloud. The film will be de¬ 
posited more uniformly if the glass is moved in and 
out of the cloud in a random fashion during the time 
of application of the film. Likewise, it has been found 
that a film deposited very quickly is more uniform than 
one deposited slowly. 


The temperature of the glass is an important factor 
in determining the hardness and index of refraction of 
the film deposited. Films deposited on glass at room 
temperature are soft and foggy. The vapor pressures of 
the titanium tetrachloride and of the titanium dioxide 
and the temperature of the tetrachloride are also very 
important in determining the results obtained. Experi¬ 
ence with the process will allow one to adjust the condi¬ 
tions so that the most satisfactory film will be formed. 

The thickness of the film at any time during its 
formation can be judged from the reflections from the 
surface. When the film has the proper thickness it will 
reflect a bright white color. If it appears somewhat 
yellow, the film is too thick. 

In applying the film, rubber gloves should be worn 
to protect the hands from acid fumes, and all work 
should be done under a hood with a strong air intake. 

The percentage of light reflected by these 
high-efficiency films may be increased through 
the use of a multilayer film made up of alter¬ 
nating layers of high- and low-index materials. 
Two such multilayer films are shown sche¬ 
matically in Figures 38 and 39. 

Figure 38 shows a film of high-index ma¬ 
terial overlayed with a film of low index. The 
reflected rays B and C undergo a phase change 
of t t upon reflection; ray A undergoes no phase 
change upon reflection. If the high-index film 
is one-quarter wavelength thick, A and B will 
be in phase, and the light is strongly reflected; 
if the low-index film is one-half wavelength 
thick, A, B , and C are all in phase and the light 
is even more strongly reflected. 

Figure 39 shows a film composed of three 
layers. A second high-index film has been added 
to the multiple film illustrated in Figure 38. In 
this new multiple film the beam C no longer 
experiences a phase change. If the low-index film 
is one-half wavelength thick as in the case 
previously discussed, B and C will be out of 
phase. If the low-index film is made one-quarter 
wavelength thick, rays A , B , C, and D will all be 
in phase and the light will be very strongly re¬ 
flected. 

A simple multilayer film of alternate quarter- 
wave layers of zinc sulfide and cryolite reflects 
an increasingly greater percentage of light as 
more layers are added. It also becomes more 
selective in its transmission and reflection as 
the layers are added. This, unfortunately, limits 
many of its applications. In Figure 40 are illus¬ 
trated the transmission curves of multilayer 
films containing various numbers of zinc sul- 



LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


429 


fide layers. The narrowing of the range of 
wavelengths transmitted as the number of 
layers increases is very noticeable. 

Through use of the proper number and thick¬ 
nesses of alternate layers of zinc sulfide and 
cryolite, it is possible to produce filters r of 
almost any desired transmission curve. In order 



air: index of 

REFRACTION = 1.0 

| M H M | 

| INCIDENT LIGjHTJ I 

Figure 38. Rays of light reflected from two-layer 
film on glass. 


value of reflectivity for white light. Multilayer 
films made up with titanium dioxide are far 
more durable than those made with zinc sulfide. 
In the half-reflecting form, a titanium dioxide 
multilayer film is quite neutral, having only a 
slight brownish yellow tinge. Its efficiency is 
approximately 99 per cent. 


INDEX OF 
REFRACTION 
= n gA 


INDEX OF 
REFRACTION 

= nhX - 


BLOCK OF 
GLASS 


FILM OF 
HIGH-INDEX 
MATERIAL 


INDEX OF_ 

REFRACTION_ 

= n lA I 

INDEX OF ^- 

REFRACTION= n hX 


_ FILM OF 
LOW-INDEX 
MATERIAL 


RAY 
B , 


RAY 

A 


FILM OF 
HIGH-INDEX 
MATERIAL 


AIR: INDEX OF 
REFRACTION = 1.0 



Figure 39. Rays of light reflected from three-layer 
film on glass. 


to obtain a multilayer filter having a sharper 
wavelength cutoff than those exhibited by the 
films whose transmission curves are illustrated 
in Figure 40, one of several procedures may be 
followed: The thickness of either the cryolite 
or zinc sulfide may be changed, the layers may 
be changed to a higher order of interference, * 8 
or more layers may be used in the film. 

A high-efficiency multilayer film can also be 
made up of alternate layers of titanium dioxide 
and cryolite. In Figure 41 are represented sche¬ 
matically some of the types of titanium dioxide- 
cryolite films that have been tested. Beside 
each filter will be found its observed and com¬ 
puted (multiple reflections were neglected) 

r Including neutral filters. 

8 If a cryolite layer three-quarters wavelength thick 
rather than one-quarter wavelength thick is deposited, 
the percentage of light reflected for that wavelength is 
not changed. The width of the reflection band will be 
greatly narrowed, however. This band will become still 
narrower if the film thickness is made five-quarters 
wavelength thick, and so on. These same effects are true 
for changing the thickness of the sulfide film to obtain 
higher order interference. 


9 ' 5 ' 3 Polarizing Beam Splitters 

In some instruments it is desirable to have a 
beam splitter that reflects half the incident 
light and transmits the remainder, completely 
plane polarizing both the reflected and trans¬ 
mitted beams. 

Normal white light incident upon and re¬ 
flected from a piece of ordinary plane glass at 
the angle of polarization (the Brewsterian 
angle) is completely plane polarized. The trans¬ 
mitted light is not completely plane polarized, 
however, for although glass at the Brewsterian 
angle will reflect only the component of unpol¬ 
arized light whose electric vector is perpen¬ 
dicular to the plane of incidence, its reflectivity 
for that component is only 15 per cent. The re¬ 
mainder of the light of that component is there¬ 
fore transmitted, and is mixed with the other 
completely transmitted component whose elec¬ 
tric vector is parallel to the plane of incidence. 
By means of a multilayer film, the reflectivity 
of the glass for the component whose electric 











































430 


OPTICAL TECHNIQUES 


vector is perpendicular to the plane of inci¬ 
dence can be increased, and therefore the 
purity of the polarization of the reflected and 
transmitted beams can also be increased. 

A successful polarizing beam splitter based 
upon this principle has been constructed by de¬ 
positing alternate layers of zinc sulfide and 
cryolite on the hypotenuse faces of two 45-de¬ 
gree—90-degree prisms, and cementing them 
together as shown in Figure 42. In such a de¬ 
vice there will be only one angle at which the 
beams are completely polarized, that angle de¬ 
pending upon the refractive index of the glass 
and the refractive indices of the materials of 



Figure 40. Transmission curves of multilayer 
(zinc sulfide-cryolite) films composed of various 
numbers of zinc sulfide layers. 


which the multilayer film is composed. Over a 
total angle of approximately 10 degrees, how¬ 
ever, the degree of polarization in both beams 
should be good. The degree of polarization of 
the transmitted beam will be determined by the 
number of layers in the film, and by the thick¬ 
nesses of those layers. 

If we stipulate that the angle cf> in Figure 42 
shall be 45 degrees, it can be shown that the 
conditions for polarization of the beams re¬ 
quire that the following relationship must hold 
between the indices of refraction of the glass 
and the materials of the film layers: 


2 ni 2 n h 
n t 2 + n h 2 ‘ 


where n g is the refractive index of the glass, n, 
the refractive index of the low-index material, 
and n h the refractive index of the high-index 
material, all at the same wavelength. Zinc sul¬ 
fide, which has a refractive index of 2.3, has 


TYPE REFLECTIVITIES 

_ EXPERIMENTAL THEORETICAL 


GLASS | 30 % 31 % 


J-Hs ..1 

GLASS 

Ti°2 I 


43% 45% 


GLASS 

I ™2 I 

EEEr CEMENTEEEE 

| GLASS 1 15 % 


GLASS 

I t~o 2 I 

== CEMENT == 

I Ti °g I , 

GLASS | 27% 


GLASS 


CRYOLITE 

T -°2 I 53% 


GLASS 


| CRYOLITE I 

I T '°2 I 

= cemenT^ 

I CRYOLITE | 


GLASS | 5 6 % 


18% 


31 % 


61% 


6 0 % 


GLASS 

TiOg T 

CRYOLITE 


^=-CEMENT= 

GLASS ] 4 4 % 


48 % 




Ti °2 I . 

GLASS 
T10 2 

CRYOLITE | 

tio 2 


=CEMENT£= 

TiOg I 


1 CRYOLITE 



64% 


Figure 41. Types of multilayer titanium dioxide- 
cryolite films tested. 

























































































LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


431 


been found to be satisfactory for use as the 
high-index material. If we take n h to be 2.3, and 
if we assume n g to be 1.55, then n l must be 1.25; 
if n g is assumed to be 1.65, n t must be 1.35. 

The thicknesses of the films to be deposited 
may be computed from the readily derivable 
equations. 

J-!—; 

4 rih cos xi 4 ni cos X 2 

• ^g • , • W'h 

sin xi = — sin <t>; sin X 2 = — sm xi- 
n h ni 

The angle <f>, as has already been pointed out, 
we take as 45 degrees. 

Zinc sulfide has a high dispersion, v — 17. 
In order to compensate for this and to make 
certain that all wavelengths of light reach the 
interfaces of the high-index and low-index ma¬ 
terials at approximately the critical angle for 



Figure 42. Schematic diagram of polarizing beam 
splitter. 


polarization and reflection, glass of a particular 
dispersion must be used for the prisms. It may 
be shown that if all wavelengths do reach the 
interfaces at the critical angle, the following 
relationship must exist between the dispersion 


of the glass v g , the dispersion of the material of 
the high-index film v h , and the indices of refrac¬ 
tion already introduced: 

= n g (nr + n h 2 ) v h {n g - 1 ) 

V ° n h (2nt 2 - n g 2 ) (n h - m) 

If n h — 2.3, v h = 17, and n x = 1.25, we can pre¬ 
dict that the glass most suitable for the beam 
splitter will have the optical constants n = 1.55 
and v == 48.5. Two glasses exist which have 
approximately these constants. Extra light flint 
has the constants n = 1.56 and v = 45.5; light 
barium crown has the constants n — 1.57 and 
v = 56.8. The light barium crown would show a 
slight dispersion effect; the extra light flint 
would be very suitable for the polarizing beam 
splitter, since its constants match quite closely 
those determined from theory. 

If a low-index material having an index of 
1.35 rather than one having an index of 1.25 
is combined with the zinc sulfide, glass having 
the constants n — 1.65 and v = 46.5 is required 
for the best results. These constants are 
matched most closely by those of extra dense 
flint glass for which n = 1.65 and v — 33.8. The 
difference between the dispersions of the the¬ 
oretically best glass and the available extra 
dense flint is rather large; hence a dispersion 
effect is to be expected in a beam splitter made 
of this glass. 

Beam splitters were made up on prisms of 
these glasses at the University of Rochester. 
The results obtained are summarized in Table 6 
which gives the ratio of the intensity of the 
undesired component to the intensity of the de¬ 
sired component in both the transmitted and 
reflected beams at a 45-degree angle, and in 
three colors. As was anticipated, the polarizer 
made of the light barium crown glass is the 
most satisfactory.* The degree of purity of 
polarization * 11 achieved in all three is, however, 
quite good. 


t Only three layers of zinc sulfide were put down on 
each prism for this polarizer; four were used with the 
other prisms. 

11 It can be shown that the reflected beam will never 
be completely polarized; a small percentage of the 
undesired component will always be found in that beam 
owing to the reflection taking place at the glass-zinc 
sulfide interface. 















432 


OPTICAL TECHNIQUES 


Table 6. Purity of polarized light reflected and transmitted by polarizing beam splitter. 

Type of Low-index Ratio undesired to desired component at 45 degrees 

glass material Red Green Blue 


extra light flint 


/ n — 1.56\ 

cryolite (n ~ 

- 1.25) 

0.0035 

0.0065 

0.0103 

Refl. 

\v = 45.5 ) 



0.0138 

0.0031 

0.0545 

Trans. 

light barium crown 
/n = 1.57\ 

cryolite ( n - 

- 1.25) 

0.002 

0.02 

0.002 

0.015 

0.002 

0.02 

Refl. 

Trans. 

\p = 56.8 ) 







extra dense flint 

lithium 


0.0033 

0.0065 

0.0035 

Refl. 

/n — 1.65 \ 

fluoride ( n ^ 

- 1.35) 

0.014 

0.013 

0.0103 

Trans. 


9 5 4 Metallic Films 

The properties of various metallic films de¬ 
posited by the evaporization process have been 
studied during the course of this work. 

Aluminum 

Although the reflectivity of aluminum is less 
than that of silver for visible light, it does not 
tarnish so easily as silver, and is therefore fre¬ 
quently used for front surface mirrors. Evapo¬ 
rated in any vacuum better than 10 -3 mm of 
Hg, aluminum will reflect 90 per cent of the 


visible light. This reflectivity falls off slightly 
in the near infrared and in the ultraviolet. By 
evaporating aluminum at different pressures 
the ultraviolet reflectivity can be greatly 
changed. A film evaporated under a pressure of 
10~ 3 mm of Hg will reflect only 65 per cent of 
the incident light of wavelength 2,650 A. A film 
deposited at a pressure of 5 X 10 -5 mm of Hg 
will reflect 90 per cent of the incident light of 
wavelength 2,650 A. Reflectivity curves for alu¬ 
minum films deposited under different pressures 
are shown in Figure 43. 



WAVELENGTH IN A 

Figure 43. Reflectivity of aluminum films deposited at various evaporating pressures. 



























LOW-REFLECTION AND HIGH-EFFICIENCY FILMS 


433 


It was found that the frequent blistering of 
aluminum films subsequent to the evaporation 
could, in large part, be prevented by heating 
the glass to be coated to a temperature of 120 C 
during the evaporation of the aluminum. The 
best heating cycle for the particular size and 
shape of element being coated can readily be 



4 200 5000 5800 6600 7400 


wavelength IN A 

Figure 44. Transmission curves for various 
metals evaporated at various pressures. 

• 

determined by experimentation. The glass must 
not be coated when at a temperature higher 
than 120 C, as mirrors formed at these higher 
temperatures have a bluish scattering cast. For 
best results the film should be deposited quickly 
and the glass surface to be coated should be at 
least 6 in. from the filament. 

Silver and Copper 

Second surface mirrors can be coated very 
successfully by the evaporation process. A film 


of silver is deposited first, and then a thin coat¬ 
ing of copper is evaporated over the silver to 
protect it from oxidation. These films are de¬ 
posited in the same vacuum, the silver never 
being exposed to air until after the protective 
coat of copper has been deposited. As a final 
protection, the copper is coated with Nicholas 
black rubberized lacquer and baked for 3 hours 
at 70 C. 

Blacks 

It is often desirable to have a nonreflecting 
highly absorbing surface on which a pattern 
can be engraved. Zinc, tellurium, antimony, 
bismuth, gold, silver, copper, aluminum, and 
chromium all produce good sooty black surfaces 
if evaporated at a pressure of a few millimeters 
of Hg. The surface to be blackened must be 
placed quite close to the source of the metal if 
an opaque film is desired; the films are all very 
soft and do not withstand high temperature. 

AgCl, precipitated out of AgN0 3 solution 
with HC1 and then melted in air, evaporates to 
a black that is not sooty and reflects less light 
than glass. This film is hard and will withstand 
considerable heat, but cannot be coated over 
aluminum since a chemical reaction takes place 
which destroys both films. 

Natural Density Filters 

If Chromel A wire (80% nickel, 20% chro¬ 
mium) is evaporated from tungsten at a pres¬ 
sure of 10 -3 mm Hg, a film of pure chromium 
will be deposited, as shown by the fact that the 
film has the same transmission curve as chro¬ 
mium and shows no chemical trace of nickel. 
If the pressure is reduced, however, to 10 -4 mm 
of Hg or lower, a good deal of nickel is evapo¬ 
rated along with the chromium and the result¬ 
ing film will show the same transmission for 
all parts of the visible spectrum. This neutrality 
continues throughout the ultraviolet to 2,500 A 
and to at least 1.3 ^ in the infrared. (See Fig¬ 
ure 44.) Like most neutral films made of a 
metal deposited on glass, the degree of neutral¬ 
ity attained with Chromel A is a function of 
the density of the filter. Films of Chromel A 
are quite hard and their reflectivity is rather 
high. 

























434 


OPTICAL TECHNIQUES 


955 Conclusion 

The many varied uses of thin films in the 
field of optics are only beginning to be realized, 
and the techniques involved in their production 
are far from being in a state of final perfection. 
Much research work remains to be done in this 
field of optics, many important problems re¬ 
main to be solved, many even to be discovered. 


95 6 Recommendations by NDRC 

1. The search for new and more satisfactory 
materials for forming low-reflection and high- 
efficiency films by the evaporation method should 
be undertaken. While several searches for such 
new materials have been made, an extensive 
systematic one for new materials has never 
been made. In such a systematic study of the 
properties (both optical and mechanical) of the 
films tested, an attempt should be made to draw 
conclusions about groups or classes of com¬ 
pounds so that predictions of properties can 
be made. 

2. A search for materials that might be used 
in the dipping or spinning process should be 
started. There is no obvious a priori reason to 
believe that a dipping method at least as good 
as—possibly even better than—the present 
evaporation process cannot be found. Even the 
acid etching method of forming low-reflection 
films cannot be ruled out as a completely unde¬ 
sirable process, and it, too, should have further 
attention. 

3. The possibility of changing the reflectivi¬ 
ties of metallic films by changing the conditions 


under which they are evaporated should be in¬ 
vestigated. The results on aluminum discussed 
in this report are of considerable importance. 
A further study of the phenomenon not only 
for aluminum but also for other metals should 
be started at once. 

4. The formation of protective coats of quartz 
or ceramic materials over films that are fragile 
or subject to tarnish should be re-examined. 
Some work on this subject has already been 
done, but many questions remain to be investi¬ 
gated. The study should include an examination 
of the whole problem of the evaporation of very 
refractory materials, including the effects of 
such materials on the heating filament. Data 
pertaining to these problems would be of great 
interest in many fields other than optics. 

5. Sputtering as a method for forming films 
of materials not amenable to the evaporation 
technique should be investigated. Films of the 
same material formed by sputtering and evapo¬ 
ration may differ in their properties sufficiently 
to be of interest. 

The problems that have been suggested here 
are all fundamental. Many more problems in 
the improvement and application of polarizing 
beam splitters, multilayer film filters, and other 
multiple films should be investigated. There are 
also many practical problems in technique which 
need further work. In particular might be men¬ 
tioned the desirability of an objective, possibly 
wholly automatic, device for determining the 
thicknesses of the films, as they are deposited. 

It seems likely that a number of research 
workers could very profitably be kept busy for 
some time in the investigation and application 
of thin films in the field of optics. 



Chapter 10 

OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 

By James G. Baker a 


U nder Contract OEMsr-160 the Institute of 
Optics at the University of Rochester de¬ 
veloped a number of unique variants of low- 
power telescopes and binoculars. These devices 
resulted from an extensive program devoted to 
improved night vision. 

Telescopes designed for use at low levels of 
illumination must have characteristics not usu¬ 
ally found in systems for daytime use. The most 
important of these are large exit pupils and 
large real and apparent fields. The former 
makes it possible for the enlarged pupil of the 
dark-adapted eye to be filled with light trans¬ 
mitted by the instrument and also makes it 
easier to locate the exit pupil of the instrument 
with the eye. The wide real field provides a 
great advantage in searching for dimly illumi¬ 
nated objects and for silhouettes, and the asso¬ 
ciated wide apparent field reduces the impres¬ 
sion that one’s field of view is very much 
restricted by the telescope. The development of 
antioscillation mountings for telescopes has in¬ 
troduced a third important requirement on the 
telescope design, namely, that the instrument 
have large eye relief so that the eye may be 
placed at the exit pupil without any part of the 
head touching the antioscillation-mounted parts 
of the instrument. 


101 WIDE-FIELD SYSTEMS AND 
SCHMIDT ERECTORS 

In the design of wide-field telescopic systems 
of medium and high power, most of the diffi¬ 
culties arise in the design of the eyepiece. In 
most cases the residual aberrations of even the 
very best eyepiece design are larger than the 
residual aberrations of the objective, which is 
usually a cemented doublet. However, in low- 
power systems of wide field, the characteristics 
of the objective must also be considered. More¬ 
over, it is possible by means of very complicated 
a Harvard College Observatory. 


forms of objective to obtain improved flatness 
of field, but the possibilities depend markedly 
on choice of magnification. 

The general principles of an eyepiece imply 
difficulty. The effective stop of the system is far 
removed from the elements, which in turn 
means that both distortion and astigmatism 
can reach large proportions. Much of the posi¬ 
tive power of an eyepiece must be placed at low 
ray-height, which means that a large Petzval 
sum is inevitable. Because of the need for field- 
lens action of the eyepiece as a whole, the in¬ 
clusion of negative power near the focal sur¬ 
face for the purpose of flattening the field be¬ 
comes impractical. Finally, the negative type 
of eyepiece with a real focal plane between the 
lenses must be ruled out because of the small 
eye relief afforded. 

It is necessary to design compound lenses 
that control the course of highly refracted rays 
in the outer part of the field. The requirements 
of color correction in a limited space and flat¬ 
tening the field at the expense of astigmatism 
mean that dense flint lenses must often be used, 
together with highly curved surfaces. 

Fortunately, the aperture of any pencil is 
limited by the iris of the eye. Usually, spherical 
aberration and even coma of the eyepiece can 
be neglected, except where the amounts left in 
the design are excessive or where very large 
exit pupils are demanded. The most important 
aberrations of an eyepiece are therefore astig¬ 
matism, curvature of field, and lateral color. 
Very often, distortion must also be kept within 
specified limits. 

An Eyepiece with Aspheric Surface. Two 
wide-angle eyepieces were designed under Con¬ 
tract OEMsr-160. 1 The first employs one as¬ 
pheric surface and its characteristics are shown 
in Table 1. 

Figure 1 shows a cross section of this design. 
The second surface from the left is parabolic, 
rather than spherical, and aids in the correction 
of the 80-degree total field. 


435 



436 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


The eyepiece unfortunately retains a large 
amount of lateral color, due to the impossibility 
of having the negative flint lens closer to the 


Table 1 . Characteristics of wide-angle eyepiece. 


Focal length 

25 mm 

Exit pupil 

7 mm (governed by eye at night) 

Apparent field 

80 degrees total 

Eyepoint distance 

22.7 mm 


eye. The extreme wide angle, as used with a 6x 
objective combination, required a special de¬ 
sign for the objective. The wide angle of the 
system also enlarged the erecting prism, which 
in turn required that the objective be of the 


f - 25 8 MM 

EYEPOINT WITH 6X = 22.7 MM 



FOCAL LENGTH =15.5 MM 


Figure 2 . Objective A 3 for use with aspheric eye¬ 
piece. 


inverted telephoto design, consisting of two 
separated doublets. 

Figure 2 shows the special objective for the 
6X system. The combination of this objective 

f ' = 24.8 MM 

WORKING DISTANCE = 9.7 MM 

PARAXIAL EYEPOINT = 17,5 4-24^8 
MP 



f ' = 24.8 M M 

WORKING DISTANCE =7.3 MM 

PARAXIAL EYEPOINT = 18.5 + 2^8 
MP 



f'= 22.78 MM 
EYEPOINT = 17 MM 



Figure 3. Types Il-b, 37b, and 41 eyepieces. 

with an erecting Porro system and the aspheric 
eyepiece yielded a real field of 14.28 total angle 
at 6X- The large apparent field of the eyepiece 
gives the observer the impression of actually 



















































WIDE-FIELD SYSTEMS AND SCHMIDT ERECTORS 


437 


being nearer the object, with unobstructed 
vision. 

Eyepieces Il-b, 37b, and J>1. An early eye¬ 
piece design developed under the contract was 
a modified Bausch and Lomb Erfle eyepiece, 
scaled to 25 mm equivalent focal length. The 
preliminary form was modified only slightly 
and is called Il-b. Eyepieces 37b and 41 differ 
in eyepoint and compactness. Both are con¬ 
sidered superior to Il-b. Only spherical surfaces 
are employed. 

Figure 3 shows the cross sections of the eye¬ 
pieces. The general Erfle design is favorable 
for the elimination of both lateral and longitu¬ 
dinal color, although only the first is serious. 

Compounded Systems. These three types of 
eyepiece have been combined with different 
prism and objective combinations to form 10X, 
7X> 6x> and 3X telescopes. The optical charac¬ 
teristics are summarized in Table 2, and the 
various cross sections are reproduced in Fig¬ 
ures 4, 5, 6, and 7. 

Testing of the several completed instruments 
was carried out at the University of Rochester. 
Optical bench measures were similar to those 
for standard lenses, except that a real stop of 
8 mm aperture was placed at the eyepoint. Data 
gathered in these tests were of great use in 
further designing. 

In visual testing, an effort was made to carry 
out observations as objectively as possible. 111 A 
chart was made up containing several sets of 
forty numbers arranged in various orders. 
Each set had four of each number from one to 
ten, in order to provide the same number of 
difficult numbers in each complete set of forty. 
The observer read off the numbers of succes¬ 
sively smaller size until he began to make 
misses. The image size of the letters was re¬ 
corded, and the angle subtended by the diagonal 
computed. Figure 8 shows a comparison of re¬ 
sults with the NDRC 7x50 and Zeiss binoculars. 

Flight tests of the 6x42 and Il-b system at 
night indicated the great advantage of this 
type of wide-angle night glass for submarine 
detection. 2 The Navy soon took steps to procure 
more than one thousand units for night use. 
An added feature of tests made both with the 
6x42 and 10x50 was the use of antioscillation 
mountings for aircraft work. 


Figure 9 shows two views of the 3X telescope 
developed in connection with the flightsight 
(a reflex gunsight with radar indication, nor¬ 
mally of unit power). Ttie objective is a Cooke 
triplet, mounted between a forward Porro prism 
in parallel light, and a pair of mirrors between 
the elements serving as the second Porro. Such 


Table 2. Optical characteristics of University of 
Rochester telescopes. 


a. 

10x50 telescope* 





eyepiece 

37b /' 

17.2 

mm 


objective 

doublet /' 

172 

mm 


exit pupil 

5 mm 




real field 

7 degrees 




product of field by magnification 70 




eye distance 

14.6 mm 



b. 

7x50 telescope 





eyepiece 

Il-b or 37b /' 

24.8 

mm 


objective 

doublet f' 

172.2 

mm 


exit pupil 

7.1 mm 




real field 

9.8 degrees 




product of field by magnification 70 




eye distance 

17.6 mm with Il-b 





18.9 mm with 37b 



c. 

6x42 telescope 





eyepiece 

Il-b or 37b f 

24.8 

mm 


objective 

doublet 




exit pupil 

7 mm 




real field 

11.6 degrees 




product of field by magnification 70 




eye distance 

18.4 mm with Il-b 





19.66 mm with 37b 



d. 

3X telescope 





eyepiece 

37b f 

24.8 

mm 


objective 

Cooke triplet 




exit pupil 

7 mm 




real field 

23 degrees 




product of field by magnification 69 




eye distance 

24 mm (approx.) 




* Has not been constructed. 


Notes: Types a, b, and c make use of Porro Type 1 prism 
erectors. In d the real field is so large that the simple doublet 
objective is inadequate. The objective is a Cooke triplet, using 
three separated simple elements. This lens was designed with mirrors 
between objective and eyepiece to avoid the use of a heavy high- 
index prism needed for control of marginal rays (see Figure 7). 

an arrangement permits the saving of weight, 
but even more important, allows for control of 
maximum illumination over the large real field 
of 23 degrees. 

Also developed for auxiliary use with the 
flightsight is the system shown in Figure 10. 
The light first strikes a penta-prism, then a 
Cooke triplet objective, and then a roof prism. 
The system provides for a large real field of 
23 degrees with a considerable offset between 
the optical axes of objective and eyepiece. In 






438 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


both 3X instruments, lateral color is small and 
field curvature moderate. 

One of the most striking optical systems de¬ 
veloped under Contract OEMsr-160 is a mo¬ 


by the arrangement of the diagonal faces of the 
prisms. This great disadvantage has been over¬ 
come by coating the surfaces of the prisms with 
nonreflecting films. Further modification of the 



Figure 4. Optical system of 10x50 binocular. 


FIELD OF VIEW = 9.8° 
EYE DISTANCE - 17.6 M M 



FIELD OF VIEW =11.6° 
DISTANCE =18.4 MM OR 19.7 MM 



OBJECTIVE PRISM EYEPIECE 

Figure 6 . Optical system of 6x42 binocular. 


nocular telescope making use of a perfected 
Schmidt prism erector. In the past the principal 
objection to use of a Schmidt erector, Figure 11, 
has been the presence of serious ghosts caused 


Schmidt prism erector permitted a shortening 
of the light path in glass and made it possible 
to incorporate the prism in a wide-angle mo¬ 
nocular. 



























































WIDE-FIELD SYSTEMS AND SCHMIDT ERECTORS 


439 


REAL FIELD 23° 

37b EYEPIECE 
FOCAL LENGTH 24.8 M M 





Figure 7. Optical system of 3X monocular. 



Figure 8. Resolving power of 7x50 and Zeiss 
binoculars. 




Figure 9. 3X telescope for flightsight. 



Figure 10. 3X offset telescope for flightsight. 








































440 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


Figure 12 shows two views of this very com¬ 
pact type of wide-angle monocular. A service 
prototype was constructed, making use of the 
Il-b eyepiece and 42 mm aperture doublet, re¬ 
sulting in a 6x42 system with 11.7-degree real 
field. A 3x objective consisting of two sepa¬ 
rated doublets is interchangeable with the 6X 
objective in cell, and parfocalized. The latter 
combination gives a real field of 23 degrees 
with exceptionally fine correction. 

The II-c 10x glass, used in the night flight 
tests along with the 6x42, employed the com¬ 
plete optical system of the Zeiss Dekar. This 
instrument was later fitted with a new eyepiece 
of the 37b form, scaled to 19 mm focal length. 


Substantial production of the 6x42 Porro- 
type instrument in antioscillation mount was 
undertaken by the Air Forces. The production 
of the 7x50 wide-angle binocular for general 
use was undertaken by the Navy Bureau of 
Aeronautics, although partially redesigned for 
production by the Bausch and Lomb Optical 
Company. 

The 6x1*2 Monocular. This system is an ad¬ 
vanced form of the 6x42 telescope already de¬ 
scribed. The chief alteration is the substitution 
of a Schmidt erector for a Porro-type prism, 
accompanied by minor changes in the form of 
objective. The objective was designed to correct 
for the spherical and chromatic aberrations of 


FIELD VIEW =11.6° 

EYE DISTANCE = 18.4 MM 



OBJECTIVE ‘PRISM EYEPIECE 

Figure 11. Optical system of 6x42 binocular telescope with Schmidt erecting system. 


The modified Dekar has very good performance 
but is not quite equal to the performance ob¬ 
tained with the 10x50 design. The 10x50 has 
50 per cent greater eye relief and an optical 
performance markedly superior to that of the 
standard Zeiss Dekar. 

During the last two years of the war other 
types of instruments were developed. 3 Among 
these were 3X, 6X, and 7x telescopes with un¬ 
usually wide fields. In the case of the 7X the 
product of field in degrees by the magnification 
has been made approximately* 85 by the use of 
one aspheric surface in the eyepiece. The 
aspheric surface was produced by a molding 
process which has been much refined by the In¬ 
stitute of Optics and used in quantity produc¬ 
tion of other instruments. 


the prisms and eyepiece. This monocular has 
a real field of 11.6 degrees, an eye relief of 
19.66 mm, and an exit pupil of 7 mm. An Erfle- 
type eyepiece identical with that of the older 
instrument was used. 

The optical system is as well corrected as 
possible, short of more elaborate design. Spher¬ 
ical aberration is imperceptible. Axial color has 
been reduced to secondary spectrum level which 
cannot be eliminated with available glasses. 
Coma of the system is completely corrected by 
the objective. Lateral color from the eyepiece 
and prisms cannot be eliminated entirely by 
the objective, but the residual is not objection¬ 
able. The tangential curvature is eliminated 
for the system, but there exists a large sagittal 
curvature owing to the succession of positive 
















7x50 BINOCULARS WITH 10-DEGREE FIELD 


441 


powers. Eighty per cent of the field curvature 
is due to the eyepiece. Astigmatism is the most 
objectionable error remaining in the system. 

The 3x21 Monocular. This monocular was 
similar to the 6x42, except that a new objective 
was provided, along with a change of glass 
type from DF-3 to LF-2 for the Schmidt prisms. 
Although an elaborate form of objective might 
have been used for the purpose of flattening the 
overall field, the particular application did not 



Figure 12. 6x42 monocular telescope. 


require the extra improvement. Consequently, 
the system was fitted with a simple cemented 
doublet, and was found to be adequate. 

The 3x21 monocular was thereafter elabo¬ 
rated to include a 5-element objective with 
overcorrected Petzval curvature. It was found 
possible to design such an objective to eliminate 
the large errors of the eyepiece, and therefore 
to produce a system nearly fully corrected for 
flat field free from astigmatism. The report 3 
states that at the edge of the field the astig¬ 
matism is less than 1 diopter. Coma in the 
objective was corrected to nearly zero. In addi¬ 
tion, lateral color was adjusted until it became 


less than 2 min for the entire system. Tests 
showed that this instrument has a clear image 
field of constant quality throughout. The sys¬ 
tem has a real field of view of 23 degrees, an eye 
relief of 20 mm, and an exit pupil of 7 mm. 

A 7x35 Monocular with Aspheric Eyepiece. 
This monocular was a 7X system with 5 mm 
exit pupil and 16 mm eye relief. The product 
of field by magnification for the instrument was 

85.4 degrees. This apparent field was made pos¬ 
sible only by means of using one aspheric sur¬ 
face in the eyepiece, namely, the first surface 
of the eye lens. It was found that the aspheric 
surface removed the distortion without mate¬ 
rially affecting the astigmatic correction, and 
that the spherical aberration of the principal 
rays was greatly reduced, which in turn per¬ 
mits the eye position to remain fixed for oblique 
rays. 

Correction of lateral color in this system re¬ 
quired unusual extremes. The partial correction 
left over from the eyepiece was entirely elim¬ 
inated in the objective by the use of two sep¬ 
arated elements and a special chromatic plate. 
The objective corrected the system for spherical 
aberration and longitudinal color simultane¬ 
ously. Figure 13 shows a cross section of the 
final design. 

A 7x50 Monocular with Parabolic Surface in 
Eyepiece. A 7x50 monocular was required for 
use as a wide-field night glass. It was consid¬ 
ered very important to obtain a long eye relief. 
In order to obtain this, an eyepiece of 25.8 mm 
focal length was designed, with the curved sur¬ 
face of the eye lens ground to a paraboloid. As 
in the system previously described, the objective 
is separated and the chromatic plate is used to 
correct for lateral color. A real field of view of 

12.4 degrees is obtained along with an eye relief 
of 22 mm and an exit pupil of 7 mm. The optical 
system is shown in Figure 14. 

The report 3 states that the performance of 
this system is excellent, although the trans¬ 
mitted image appears yellow due to a long light 
path through the dense flint Schmidt prisms. 
It is recommended that the 7x35 be used in¬ 
stead of this 7x50, and scaled up to comparable 
size if necessary. 

A 3X Monocular. The 3x21 monocular de¬ 
scribed above was scaled down to have a real 






442 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


field of 23 degrees and eye relief of 15.2 mm, 
along with an exit pupil of 5.4 mm. It was in¬ 
tended that this monocular be as light as pos¬ 
sible and strapped to an operator’s head. It was 
planned to insert an aspheric eyepiece with a 
focal length of 16.24 mm, which would result 
in a 3.69X telescope with the same real field as 
the 3X telescope. The end of the contract pre¬ 
vented completion of this design. 


justify either the large field of view or Ameri¬ 
can reproduction. Instead, efforts were made to 
design a glass of higher quality with a 70-degree 
apparent field. These efforts were successful, 
and later crystallized in a production model. 

Mechanical design features of the wide-angle 
7x50 glass were adapted to mass production, 
and departed slightly from requirements for 
minimum weight. In the end the manufactured 


V = 64.5 


V = 29.3 





2/ = 38.5 


V = 55.0 


V =55.0 


2/ =38.5 


V = 55.0 


V = 55.0 


SCHMIDT PRISM SYSTEM 
PRISM LENGTH = 110.7 
GLASS DF-2 r» D = l.6l7 

Figure 13. 7x35 monocular telescope. 


102 7x50 BINOCULARS WITH 

10-DEGREE FIELD 

Under Contract OEMsr-579 the Bausch and 
Lomb Optical Company undertook requested 
variations in the design and construction of 
binocular systems. 4 With respect to optical de¬ 
sign there were developed a 7x50 wide-field 
binocular with 10-degree field and a binocular 
with a 10-mm exit pupil, both intended for 
night use. A Bausch and Lomb eyepiece with 
large eye relief was fitted to several experi¬ 
mental binoculars. Several special eyepieces 
were designed and constructed for the use of 
Contract OEMsr-1229 at Brown University. 

A study made of the Zeiss 8x40 binocular, 
rated for an apparent field of 90 degrees but 
which proved to have only 83 degrees, showed 
that the mediocre definition obtained did not 


glass weighed 68 ounces, compared to 49 ounces 
for the standard 7-degree glass, and 36 ounces 
for the fragile Zeiss 8x40. 

The Zeiss tapered Porro prisms caused vig¬ 
netting of rather serious proportions far off 
axis. Bausch and Lomb reduced this vignetting 
by returning once more to untapered prisms. 
The 7x50 glass of the Institute of Optics suf¬ 
fered partially by making use of the tapered 
Zeiss prisms. 

A considerable amount of time was spent at 
Bausch and Lomb on computations of aspheric 
eyepieces. It is reported that no important im¬ 
provement in definition could be obtained to 
justify the complication, although it proved 
possible to obtain better transmission far off 
axis. 

A 10x50 Binocular with 7-Degree Field. Two 
samples of a Bausch and Lomb binocular meet- 























FOUR DESIGNS FOR TELESCOPE OBJECTIVES 


443 


ing the above specifications were delivered to 
NDRC for the Bureau of Aeronautics. The 
change consisted primarily of substitution of 
higher power eyepieces. 

A 7x50 Binocular with Reduced Diameter at 
Eye End. Two samples of a binocular meeting 
these specifications were made up and delivered 
to NDRC in February 1944. These binoculars 
differed from the standard 7x50 only in the 
respect that the metal rims of the eyepieces 
were cut back to allow as much eye and nose 


vision. These binoculars consisted of standard 
7x50 systems equipped with scaled up Kellner 
eyepieces. The real field remained unchanged. 
These samples were shipped in January 1944 
to NDRC. Later, four other samples were made 
up and delivered to NDRC in April 1944. 

A Dummy Binocular. In accordance with ex¬ 
perimental investigations on the influence of 
exit pupil diameter on the effectiveness of bin¬ 
oculars for night vision, Bausch and Lomb 
suggested that binoculars with no objectives or 



FIELD = 6.2°(HALF) 

EYE DISTANCE 22 MM 
DIA OBJECTIVE 50 MM 

Figure 14. 7x50 monocular telescope. 


room as possible. To facilitate such work, fixed 
focus eyepieces were adopted. 

A 7x50 Binocular with Increased Eye Dis¬ 
tance. Newly designed Bausch and Lomb eye¬ 
pieces with large eye relief were fitted to two 
otherwise standard 7x50 binoculars. No details 
of the eyepieces are available since they are 
not NDRC designs. Two binoculars so equipped 
were shipped in April 1944, and two more in 
July 1944. 

A Binocular with 10-mm Exit Pupil. Two 
sample binoculars were made up with 10-mm 
exit pupils for experimental work in night 


eyepieces could be used. Two dummy binoculars 
with the same size and field as the standard 
7x50 were supplied in February 1944. The field 
inversion caused by the Porro prisms alone was 
removed by substitution of rhomboid prisms in 
the binocular bodies. The field of vision was 
partially restricted by these prisms. Nine pairs 
of exit-pupil diaphragms were also supplied, 
ranging in steps of 1 mm from 2 to 10 mm. 

Additional Eyepieces. For use in the experi¬ 
mental investigations on binocular performance 
carried on at Brown University, Bausch and 
Lomb under Contract OEMsr-579 supplied 
































444 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


three pairs of eyepieces of 14-mm focal length, 
and three pairs of eyepieces of 32-mm focal 
length, mounted to be interchangeable with the 
regular eyepieces of the Mark I, 7x50 binoc¬ 
ular. These eyepieces provide powers of 14 X 
and 6x respectively, when combined with the 
7x50 objectives. 

The eye distances and apparent fields of view 
for these eyepieces were as follows: 

eye distance apparent field 
14X 11.5 mm 30 degrees 

6X 17.8 mm 41.5 degrees 

The report 4 states that there are no special 
features of these eyepieces in either optical or 
mechanical design to justify further comment. 

10 3 FOUR DESIGNS FOR TELESCOPE 
OBJECTIVES 

Under Contract OEMsr-474 Harvard Uni¬ 
versity was requested to make up several long- 
focus collimator lenses for use wherever needed 
by Army, Navy, or other NDRC projects. 5 Only 
one such collimator was made up, but four 
separate designs were submitted. There is 
nothing essentially new about these objective 
designs, but it is instructive to reproduce them 
here as carefully computed standard systems 
for visual use. 

The best monochromatic correction has been 
taken for e light at 5,461 A, which is very close 
to the maximum spectral sensitivity of the eye. 
For all designs, F and C have been combined 
accurately for the 0.707 zone at //15, and the 
rim ray at //15 has been combined with the 
paraxial focus. Table 3 summarizes the optical 
constants of these objectives. 

In every case the values listed in Table 3 are 
in units of the equivalent focal length. The ob¬ 
jectives are listed in order of increasing quality. 
Objectives A and B are of the cemented type. 
Type B has less than half of the zonal aberra¬ 
tion of lens A, and yet has shallower curves. 
The coma is very nearly of the same value but 
of opposite sign. Type C has the spherical aber¬ 
ration slightly reduced relative to type B, and 
is fully corrected for coma. It has broken con¬ 
tact at the inner surfaces and is called a Fraun¬ 
hofer-type objective. Type D has only half the 


zonal aberration of Type C, and is equally well 
corrected for coma. The two elements of type D 
are air-spaced by such an amount that the in¬ 
ternal radii are identical when the aberrations 
are eliminated. Further increase in air space 
would eliminate the zonal aberration altogether 
at //15, but would lead to a noticeable increase 
in lateral color of the system. 

Figure 15 shows a 6-in. lens of type D in a 
special cell. The completed lens yielded theo¬ 
retical definition. The details of the cell design 
are shown clearly in Figure 15. It should be 
noted that the retainer rings do not bind 
against the elements but, by careful lapping in 
optical fashion, square against a shoulder just 
short of touching the lens surface. Provision is 
made for adjusting the air space for maximum 


Table 3. Optical constants of four simple objec¬ 
tives. 


Surface 

Radii 

Separations 

Indices 

e 

Glass types 

A. 

1 . 

1.0177 

0.0070 

1.51899 

BSC-2 


2 . 

—0.2877 

0.0050 

1.62115 

DF-2 


3. 

—0.7336 

0.9972 



B. 

1 . 

0.4645 

0.0070 

1.51899 

BSC-2 


2 . 

—0.4314 

0.0050 

1.62115 

DF-2 


3. 

—5.2140 

0.9922 



C. 

1 . 

0.6083 

0.0070 

1.51899 

BSC-2 


2 . 

—0.3544 

0.0001 




3. 

—0.3585 

0.0050 

1.62115 

DF-2 


4. 

—1.4936 

0.9941 



D. 

1 . 

0.5814 

0.0069 

1.51899 

BSC-2 


2 . 

—0.3585 

0.0031 




3. 

—0.3585 

0.0050 

1.62115 

DF-2 


4. 

—1.6189 

0.9868 




performance, whereafter a spacer ring of suit¬ 
able thickness is inserted for true centering of 
the elements. 


10 4 TANK TELESCOPES 

Under Contract OEMsr-160 with the Univer¬ 
sity of Rochester and Contract OEMsr-1078 
with the Yerkes Observatory, the design and 
construction of several tank telescopes were 
accomplished. In 1943 the Frankford Arsenal 
was much interested in procuring an improved 







TANK TELESCOPES 


445 


tank telescope to match the acknowledgedly su¬ 
perior performance of the corresponding Ger¬ 
man and English equipment. A start had al¬ 
ready been made at the Arsenal itself toward 
incorporation of a Cooke triplet objective in¬ 
stead of the conventional doublet. The result¬ 
ing performance, however, still was below re¬ 
quirements. 


10,41 Rochester Modifications 

The tank telescope in production at Frank- 
ford Arsenal was examined at the University 
of Rochester and found to have the following 
specifications. 6 The apparent field of the eye¬ 
piece was 67 degrees, the exit pupil 7 mm, the 
overall length 28 in., and the maximum lens 
diameter 1.5 in. The optical system gave 3X 
and employed lens erection. 

The time allotted for change in design was so 
short that it became necessary to make the 
quickest possible improvements 2a rather than 
to redesign the system. Moreover, the produc¬ 
tion of the optical parts had already been ini¬ 
tiated, which made it desirable to conserve the 
existing design as far as possible. 

The chief aberrations of the existing 3X 
(T-76) tank telescope were found to be spheri¬ 
cal aberration and color, both greatly in excess 
of tolerance. In addition, the eye relief was 
somewhat insufficient and the Petzval curva¬ 
ture of the system too large. 

It proved possible to eliminate the color and 
most of the spherical aberration by means of a 
new erector system, consisting of new doublets 
and an achromatizing zero power plate inserted 
only for expediency. The resulting instrument 
was fully corrected for color and nearly cor¬ 
rected for spherical aberration. No improve¬ 
ment in the Petzval sum was possible by this 
interchange of doublet erectors, nor was it 
feasible to increase the eye distance. 

The modified telescope was taken to the 
Frankford Arsenal for comparative tests. The 
results can be summarized as follows. The 
NDRC telescope in principle had transmission 
equal to that of the Bausch and Lomb sample, 
but was handicapped provisionally by uncoated 
surfaces and the presence of the chromatic cor¬ 


recting plate. The competing system of the 
Eastman Kodak Company was in general better 
corrected, but gave markedly reduced trans¬ 
mission and color to the image. The superior 
correction of this latter system was obtained 
at the expense of more complicated construc¬ 
tion with more air-glass surfaces and unusual 
glass types. 

The eye distances of all three telescopes were 
very nearly identical. The Eastman sample re¬ 
quired that the eye be moved in order to see the 
entire field. The Eastman sample did not vi¬ 
gnette as rapidly as the other two near the axis, 


- 


\ | ^ 




Figure 15. 6-in. aperture objective lens. 


but vignetted more rapidly in the outer part of 
the field of view. The Eastman design showed 
considerable lateral color, but the Bausch and 
Lomb and NDRC models none. All the tele¬ 
scopes showed the same axial color, due mostly 
to secondary spectrum. The Eastman telescope 
showed marked spherical aberration compared 
to both the others. 

Following these tests a redesign succeeded in 
eliminating the chromatic plate, and in replac¬ 
ing the original Arsenal doublets by new dou¬ 
blets with better spherical and chromatic cor¬ 
rections (see Figure 16). The resulting instru¬ 
ment yielded performance identical with that 
of the temporary model, and hence was deemed 
suitable for production purposes. The triplet 
objective and Erfle eyepiece were unaltered. In 
the meantime, modifications of the design at the 
Arsenal were believed to have accomplished a 
similar overall improvement. The final produc¬ 
tion therefore was in accordance with the Ar¬ 
senal design. 

The 5X Tank Telescope. The scaling up of 
the objective used in the 3X telescope was not 
advisable owing to excessive spherical aberra¬ 
tion. Consequently, it was considered necessary 
to redesign not only the erecting system for a 




















446 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


5x system but the objective as well. 6a Figure 17 
shows a cross section of the 5x optical arrange¬ 
ment. The doublet erectors are identical and 
work at //5. The Erfle eyepiece is identical with 
that used in the original T-44 and T-76 tele¬ 
scopes, even though it would have been desir¬ 
able to enlarge the element diameters for re- 


over, the shallowness of the curves of the in¬ 
strument will ensure that production units will 
be of uniformly high quality. 

In the case of the 3X telescope a slight im¬ 
provement in quality at the edges of the field 
would have resulted if the objective of the T-76 
had been altered. Inasmuch as the objectives 


26.297" 




POWER * 3X # 

FIELD OF VIEW - 23* 

DIA OF EXIT PUPIL » 0.276 

APPARENT FIELD OF VIEW - 64° 30* 


Figure 16. 3X tank telescope with a 23-degree field. 


duction of vignetting had time limits permitted. 

The optical performance is summed up in 
Table 4. 


Table 4. Optical performance of the 5X telescope. 


1 . 

Spherical half apert. 

sph. aber. 

tolerance 


aberration: (in.) 

(in.) 

(in.) 


0.69 

0.022 

0.01 


0.48 

—0.014 

0.015 


0.25 

—0.01 

0.02 

2 . 

Chromatic half apert. 

chr. aber. 

tolerance 


aberration: (in.) 

(in.) 

(in.) 


0.69 

0 

0.0000098 

3. 

Petzval sum: 0.764. 



4. 

Eye distance: 1.2 in. 



5. 

Oblique aberrations: Ray-tracing calculations show 


that the only important 

residual aberration off 


axis is astigmatism. When the eyepiece is focused 
for collimation of the axial bundle of rays, the 


sagittal rays at the edge of the field 

are out of 


focus by 2 diopters. 



6 . 

Lateral color: Essentially zero over the entire field. 

7. 

Secondary spectrum: Customary residual. 


Conclusions. These telescopes do not repre¬ 
sent the best that can be obtained in optical 
performance. Inevitably, some choice must be 
made between complication and practicality. 
More complicated systems could be designed 
with better image quality at the edges of the 
field. Even for the existing instrument, how¬ 
ever, the image quality at the center of the 
field is designed to be as good as possible. More- 


were already in production, it seemed wiser to 
make use of the existing design. 

1042 Yerkes Tank Telescope 

The 3x tank telescope problem was consid¬ 
ered so urgent that the Yerkes Optical Bureau 
under Contract OEMsr-1078 was also called on 
for an improved design. 7 The general restric¬ 
tions were identical with those imposed on the 
Rochester modification, although the Yerkes 
program called for a complete redesign from 
the very beginning. The aberrations already 
found and described above in connection with 
the Arsenal design were evaluated at Yerkes 
and either eliminated or improved in the re¬ 
design. 

The requirements met by the Yerkes design 
included a 21.5-degree real field, a 7.8-mm exit 
pupil, and 2-in. diameter lens elements. The 
difficulties outlined by Yerkes to be overcome 
involved removing both the spherical aberra¬ 
tion and the chromatic difference of spherical 
aberration from objective and erector system, 
the reduction of the Petzval sum to a minimum 
consistent with other details of performance, 
and the artificial flattening of the apparent field 
by introduction of a suitable amount of nega¬ 
tive astigmatism. 

























TANK TELESCOPES 


447 


The final lens design is shown in Figure 18. 
The objective is a widely separated Cooke trip¬ 
let at //5 for the axial bundle. The entrance 
pupil lies in the middle of the objective. The 
focal point lies 0.8 in. in front of a collective 


The spherical aberration is almost completely 
corrected at the reticle. The astigmatism is 
greatly overcorrected at the reticle in order to 
compensate for negative astigmatism in the 
eyepiece. 



POWER = 5X 

FIELD OF VIEW =13° 

DIA OF EXIT PUPIL =0.276 
EFL OF OBJECTIVE =5.525 
EFL OF EYEPIECE =1.625 

Figure 17. 5X tank telescope with a 13-degree field. 


lens which tends to reduce the angular aper¬ 
ture of the bundle. The principal ray is bent so 
that it strikes almost centrally on the erecting 
system. 

The erecting system consists of two sepa¬ 
rated doublets, each with a negative lens in 


The spherical correction of the system is 
achieved by proper choice of separations and 
powers. The objective is corrected within the 
Rayleigh limit. The correction of the erecting 
systems is aided materially by means of the 
slight convergent effect of the collective lens 



- 27.5" - 

Figure 18. Yerkes 3X tank telescope. 


front, and working at //3.5. Between the erec¬ 
tors the light is parallel. The reticle is flat, and 
is viewed by an eyepiece similar to an Erfle 
design. 

The aberrations in each component of the 
system are uncorrected in order to provide a 
greater degree of freedom in the overall design. 


which lies away from the focal plane of the 
objective. In both of the erectors the negative 
flint lens is placed in front with an appreciable 
air space. The optical system as a whole is ex¬ 
tremely well corrected. 

It proved possible by choice of these various 
separated elements to reduce the Petzval sum 












































448 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


of the system appreciably as compared to a 
system of all positive lens elements. The final 
system gives a much flatter field than the T-93 
Arsenal design. 

Investigations Leading to the Final Design. 
Much effort was put on reduction of the Petzval 
sum. Toward this end, triplet objectives with 
still greater air spaces were attempted. How¬ 
ever, the longer triplets were afflicted with 
higher order coma and had to be discarded. It 
usually proved possible to control the higher 
order spherical aberration. 

Similar attempts were made to construct the 
collective lens of three separated elements. 
These calculations showed that such a collective 
lens could not be used because of the large re¬ 
fractions involved. Consequently, the collective 
lens, which constitutes a chief source of the 
large Petzval sum, was made of a single high- 
index element. 

Many calculations were expended on improv¬ 
ing the erecting system. A fully symmetrical 
system of four separated elements was tried in 
hopes of procuring extremely good correction 
at unit magnification. Unfortunately, when the 
erecting system was almost exactly corrected 
for zonal spherical aberration, there remained 
positive astigmatism of considerable magni¬ 
tude. Although use of an aspheric surface on 
the eyepiece might help, this system was aban¬ 
doned. 

The erecting system chosen, as shown in Fig¬ 
ure 18, provides a control on both astigmatism 
and errors in the aperture. Indeed, the erecting 
system was considered to include the collective 
lens and was corrected for overall perform¬ 
ance. Although the total field curvature is al¬ 
ready large at the reticle, the markings are 
only in the central one-fifth of the field. Since 
the spherical aberration of the system is well 
corrected at the reticle, the system does not 
have an axial parallactic shift anywhere in the 
exit pupil. 

The eyepiece design was based on the Erfle 
type but was the subject of a separate investi¬ 
gation. For the sake of convenience and effec¬ 
tiveness, the calculations were based on the 
equivalence of the three eyepiece elements to 
two thin air-lenses of positive power. By 
changing the shape factors of the two air ele¬ 


ments, it proved possible to introduce varying 
degrees of astigmatism into the eyepiece. 

It is stated in the report 7 that if an aspheric 
surface could be used in the eyepiece as a 
source of large negative astigmatism, it would 
be possible to compensate the excess positive 
astigmatism that constituted the chief fault of 
the 4-element symmetrical erector. Moreover, 
if this aspheric surface could have been used, 
the overall performance with respect to coma 
and the chromatic variation of the aberra¬ 
tions might have been considerably improved. 

The paraxial eye distance was set at 1.5 in., 
but at large field angles the spherical aberra¬ 
tion of the pupil reduced this value to 1 in. The 
lateral color, common to most Erfle eyepieces, 
is large at half field and becomes small or of 
opposite sign at full field. The primary cause is 
chromatic difference of distortion. It is prob¬ 
able that some secondary spectrum in the lat¬ 
eral color may be present also. 

During the design work there was some un¬ 
certainty as to the amount of anastigmatic flat¬ 
tening of field that could be introduced. To de¬ 
termine the answer, two eyepieces were con¬ 
structed having different amounts of negative 
astigmatism. The first of these, T 14.57, had 
the larger negative astigmatism and required 
less eye accommodation to reach the surface of 
least confusion. The second, T 14.64, proved to 
give better definition at large field angles with¬ 
out exhausting the ability of the eye to accom¬ 
modate to the field curvature. Indeed, still an¬ 
other eyepiece, T 14.65, with even less astigma¬ 
tism, was made up and found to be satisfactory. 
The usual practice of flattening the tangential 
focal surface therefore led to unnecessarily 
large negative astigmatism. 

The test models were built with oversize 
lenses. It was possible to obtain an exit pupil of 
7.8 mm with slight vignetting nearly to the 
edge of a 23-degree true field. In a production 
model with heavier cells and smaller lenses in 
the collective and second erector, it would still 
be possible to obtain an exit pupil of more than 
7 mm with a true field of 21 degrees. The limi¬ 
tation on the size of the axial pupil is set by 
the variation of spherical aberration with 
color, which is already detectable at the edge of 
the 7.8-mm pupil. The axial performance is ex- 



TANK TELESCOPES 


449 


cellent with the reticle sharply defined. The re¬ 
solving power at full field is enough to detect 
a separation of 0.6 mils in object space, con¬ 
trasted to an axial resolving power of 0.07 mils. 

Table 5 gives the optical constants of the sys¬ 
tem. All dimensions are in inches, and all 
glasses standard Bausch and Lomb types. 

Table 6 contains a comparison of perform¬ 
ance data for T-14.64 and the T-93 tank tele¬ 
scopes. Complete data are tabulated in the 
original report. 7 


the field. Owing to the use of lighter glasses 
and one less element, however, it is likely that 
the total transmission of the T-93 design is 
slightly better than that of the NDRC design. 


1043 The Bifocal Bipower Telescope 

The tank gunner is confronted with the dual 
problem of finding his target and obtaining 


Table 5. Optical constants of the Yerkes 3X tank telescope. 


Lens 

Function 

Outside 

diameter 

(in.) 

Radii 

Thickness 

(in.) 

Space 

(in.) 

Glass 

type 

1 

Objective 

1.25 

2.549 

—3.053 

0.26 

0.346 

DBC-3 

2 

Objective 

1.25 

—1.050 

1.951 

0.10 

0.348 

DF-1 

3 

Objective 

1.25 

6.92 

—1.080 

0.32 

4.170 

BSC-2 

4 

Collective 

1.76 

flat 

—2.47 

0.30 

5.607 

EDF-3 

5 

Erector 

1.60 

15.1 

1.739 

0.13 

0.250 

EDF-1 

6 

Erector 

1.60 

2.162 

—4.087 

0.33 

1.797 

DBC-2 

7 

Erector 

1.76 

—71.4 

2.208 

0.13 

0.310 

EDF-1 

8 

Erector 

1.76 

3.361 

—2.778 

0.42 

7.737* 

DBC-3 


Reticle 

2.00 

flat 

flat 

0.20 

0.5f 

BSC-2 

9 

Field 

2.30 

—3.982 

1.600 

0.12 

cemented 

DF-3 

10 

Field 

2.30 

1.600 

—2.111 

1.00 

0.01 

DBC-1 

11 

Center 

2.30 

9.29 

—3.189 

0.37 

0.01 

DBC-1 

12 

Eye lens 

2.03 

3.663 

—1.942 

0.55 

cemented 

DBC-1 

13 

Eye lens 

2.03 

—1.942 

—9.38 

0.12 

1 *2$ 

EDF-1 


* Adjust to focus on reticle, 
f Adjust to focus eyepiece. 

$ Mean eye relief. 


Table 6 . Optical performance of the T 14.64 and 
T-93 tank telescopes. 


Ray height Color 
(in.) 

Axial (mils) 
T-14.64 T-93 

Full field (mils) 
T-14.64 T-93 

0.42 

C 

—0.5 


. . • 



D 

0.4 

—5.0 

—7.0 

—3.2 


F 

—0.5 

—8.7 

—12.8 

... 

0.21 

D 

—0.2 

2.5 

0.0 

—1.4 


F 


1.6 


... 

0 

D 

0.0 

0.0 

0.0 

0.0 

—0.21 

D 

0.2 

—2.5 

2.5 

21.6 


F 


—1.6 


... 

—0.42 

C 

0.5 





D 

—0.4 

5.0 

7.3 

257 

F 

Accommodation 

0.5 

8.7 

—1.7 


(diopters) 


3.2 

0.6 

0.0 

0.0 

Distortion 


49 


... 

—20 


It is evident that the NDRC telescope consti¬ 
tutes a real improvement over the original 
Arsenal design, especially near the center of 


accuracy of fire. In the speed and maneuver of 
battle the gunner should be distracted from his 
gunnery as little as possible. Consequently, it 
was proposed by Army Ordnance in February 
1944 that a tank telescope should be con¬ 
structed, having a 5x magnification in a 
limited portion of the field, and a 1.5 X magni¬ 
fication over as large a field of view as prac¬ 
tical. 7 ' 1 

Figure 19 shows the proposed NDRC design 
for such a telescope, as worked out under Con¬ 
tract OEMsr-1078. The objective, erecting sys¬ 
tem, and eyepiece are standard. The collective 
lens, however, consists of a simple positive lens 
with a central hole containing a telephoto-like 
magnifier. This inserted triplet system pro¬ 
vides effective 5x magnification over a limited 
field, and has a virtual image plane coplanar 
with the image plane of the objective. 










450 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


The field of view is divided as shown in 
Table 7. 


Table 7. Characteristics of the bifocal bipower 
telescope. 



True field 

Apparent field 


(degrees) 

(degrees) 

5 X magnification 

0 to 1.5 

0 to 7.5 

Dark zone 

1.5 to 7.5 

7.5 to 11.2 

1.5 X magnification 

7.5 to 20 

11.2 to 30 


The obscured zone in the apparent field arises 
from the vignetting of the small lenses, and 
hence does not have sharp boundaries. It is 
obvious that there must be a region of obscura¬ 
tion in the true field, even if the apparent field 
is continuous. Because this factor is a serious 
one, no final design was attempted. 

10.4.4 The Split-Field Tank Telescope 

In order to overcome the difficulty of obscu¬ 
ration of part of the field present in the pre¬ 
ceding design, it was proposed by the Ordnance 
Department that a split-field telescope of two 



Figure 19. Magnifying system for differential 
magnifier in bipower telescope. 


powers be constructed. It was believed that the 
usual apparent circular field of the individual 
telescope could be divided into two equal halves, 
the upper operating at 5x, the lower at 1.5X• 
Instead of dividing the fields for continuity at 
the common border, it was believed more prac¬ 
tical to make use of two separate optical axes 
intersecting the respective fields at their cen¬ 
troids. Thus, an object like a tank would appear 
in the centroid of each field simultaneously at 
the respective magnifications. 75 


Figure 20 shows the cross section of the opti¬ 
cal design. The two objectives are telephoto and 
inverted telephoto, respectively, in order that 
coplanar focal planes might be achieved at two 
magnifications. The semicircular field lenses to¬ 
gether make up a complete circle central in the 
tube. These semicircles are cut from a centered 
lens system, relative to the individual optical 
axes. Consequently, the two entrance pupils are 
imaged at infinity with the chief rays parallel 
to the optical axis of the entire instrument. 

The second collective re-images the pair onto 
the entrance pupil of the erector system, and 
thence through the eyepiece. The final exit 
pupil is therefore circular and coincident with¬ 
out loss of light. The rays through this exit 
pupil from the two halves of the field differ in 
angle rather than position. 

The general specifications of the single test 
model made up under Contract OEMsr-1078 
are given in Table 8. The single test model was 
demonstrated and delivered to Army Ordnance. 


Table 8. Characteristics of the split-field tele¬ 
scope. 


Magnifications 

5X and 1.5X 

Number of lenses 

19 

Length 

27.5 in. 

Tube 

2.75 outside diameter (up to eye¬ 
piece) 

Exit pupil 

5 mm round and centered on axis 

Eye relief 

1.6 in. 

Apparent field 

True fields 

65 degrees 

1.5X 

20 degrees radius 

40 degrees diameter along cut 

5X 

6 degrees radius 

12 degrees diameter along cut 


Aberrations of the Split-Field Tank Tele¬ 
scope. The calculation of such a lens system pre¬ 
sents problems. The multiplicity of optical axes 
and lack of rotational symmetry mean that in¬ 
dividual parts of the system must be highly 
corrected for best results. Zero field in the ob¬ 
jectives corresponds to about half field in the 
eyepiece. Fortunately, the telephoto objectives 
are well adapted to highly corrected fields, and 
at the same time for parfocal objectives having 
entrance pupils in the optimum location. 

The eyepiece is approximately of the ortho- 
scopic type with a large field free from astigma- 






















ANTITANK TELESCOPE 


451 


tism and with good eye relief. Such an eyepiece 
has a fairly pronounced curvature of field, but 
when used with the split-field telescope the 
trouble was not marked. 

Table 9 gives the calculated aberrations of 


Table 9. Aberrations of the split-field tank tele¬ 
scope at full field (mils in image space). 


Aperture 

(mm) 

Color 

Erectors 

and 

eyepiece 

5X 

objective 

1.5X 

objective 

2.50 

D 

—1.2 

0.2 

3.8 

1.25 

D 

—0.1 

—0.1 

1.8 

0.00 

C 

1.1 




D 

0.0 

0.0 

0.0 

—1.25 

D 

—0.5 

—0.1 

—0.5 

—2.50 

D 

—1.7 

—0.7 

4.6 


out departing appreciably from the 5x optical 
system. It was planned to place the 1.5X field 
above, and the 5X below. Contrary to the field 
arrangement of the first split-field design, the 
5x field had its optical axis unaltered in the 
center of the apparent field, and on the division 
line. The optical axis of the 1.5X field, however, 
lay at the centroid of the upper half of the 
apparent field. The true fields for the 1.5X 
were 20 degrees along the cut and 10 degrees 
up and down, and for the 5x> 6 degrees along 
the cut and 3 degrees up and down. 

The T-118 telescope made use of a 4-mirror 
erecting system. The modification into the split- 
field form unfortunately required separate mir¬ 
ror erectors and separate objectives. 



Figure 20. Optical system of split-field telescope. 


the system. For more complete tabulation the 
reader is referred to the original report. 7 

10 5 ANTITANK TELESCOPE 

The T-118 antitank telescope developed by 
the Polaroid Corporation was designed for very 
large exit pupil and eye relief. 70 The system 
measured 20x7x7 in. and used an objective 4 in. 
in diameter at 5x overall magnification. The 
T-118 model employed plastic lenses, with the 
exception of one glass protective lens. The opti¬ 
cal properties include an exit pupil of 20 mm, a 
true field of 6 degrees, an apparent field of 30 
degrees, and an eye relief of 4 in. 

The purpose of the modification developed at 
Yerkes under Contract OEMsr-1078 was to pro¬ 
duce a split-field telescope of 1.5x and 5x with- 


The 1.5X objectives consisted of an inverted 
telephoto lens, covering a 20-degree field at //5. 
The two separate optical systems form separate 
images on the two semicircular coplanar reti¬ 
cles required. The eyepiece shows both fields 
simultaneously. The reticle contains two pat¬ 
terns, one for each field of view. The two exit 
pupils coincide. 

Two objectives were designed, ITP-12 and 
PL 11, the latter containing plastic elements 
with the exception of the protecting front ele¬ 
ment. 

Eccentric Collective. Since it was undesir¬ 
able to alter the fundamental design of the 5x 
side of the field, the optical axis of that objec¬ 
tive was left coincident with that of the eye¬ 
piece. However, the 1.5X objective was placed 
so that its optical axis lay 0.55 in. above the 



























452 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


axis of the eyepiece. Since the entrance pupil 
of the 1.5X was therefore centered 0.55 in. 
above the axis of the eyepiece, the closer to the 
eyepiece, the otherwise uncorrected exit pupil 
would have been below that of the 5X by 0.1 in. 
and about 0.8 in. to the right. This disadvan¬ 
tage was overcome by making use of an eccen¬ 
tric collective, which combined prismatic effect 
and power to readjust the pupil. The exit pupils 
of the two final systems were therefore per¬ 
fectly coincident. 

Table 10 gives the computed aberrations of 
the ITP-12 objective. 


Table 10. Aberrations of ITP-12 objective in 
mils in image space. 


Aperture 

(in.) 

Color 

Axial 

Half field 

Full field 

0.4 

D 

—0.1 

0.5 

0.4 (vignetted) 


F 

—0.1 



0.2 

D 

0.1 

0.2 

0.1 

0.0 

D 

0.0 

0.0 

0.0 

—0.2 

D 

—0.1 

—0.3 

—0.2 

—0.4 

D 

0.1 

0.2 

1.3 (vignetted) 


10 6 SPECIAL BINOCULARS 

At the request of the Army Ordnance De¬ 
partment a periscopic binocular was developed 
under Contract OEMsr-160 for use by tank 
commanders. 8 This instrument has 7X magni¬ 
fication and a 10-degree real field. The binocu¬ 
lars have 7-mm exit pupils, and long eye relief, 
so that they are usable under unfavorable light¬ 
ing conditions and under conditions of consid¬ 
erable shock. A number of unusual mechanical 
requirements were met, including provision for 
quick replacement in the field of the top prism 
system which is exposed to destruction by shell 
fire. 

Substantial production of 7x50 wide-field in¬ 
struments was undertaken by Army Ordnance. 
The production was very considerably modified, 
however, to include a small unit-power system 
as well. This unit-power system had been in¬ 
cluded in the original design of the Institute of 
Optics, but was removed on Army request. It 
was later reinstated after further designs and 


samples were submitted. The entire program 
began in November 1942. 

Under Contract OEMsr-579 the Bausch and 
Lomb Optical Company delivered a 7x50 binoc¬ 
ular to be attached to a special device on the 
deck of a submarine. 441 This binocular was built 
to withstand a pressure of 200 psi without 
damage that would prevent subsequent surface 
use. 

Also under Contract OEMsr-579 the Bausch 
and Lomb Optical Company delivered one 
sample each of a standard 6x80 and a 7x50 
glass equipped with nonfocusing eyepieces. 4b It 
was believed that since most military users had 
normal or nearly normal eyesight, fixed focus 
binoculars would not only prove practicable but 
would lead to simplifications in construction 
and to reducing water ingress and other con¬ 
tamination. 


10.6.1 The University of Rochester 
Periscopic Binoculars for Tank 
Commanders 

A 7x binocular with an 8-in. vertical offset 
between the horizontal optical axes of the ob¬ 
jectives and the eyepieces was required. It was 
also required that the binoculars have 7-mm 
exit pupils, a 10-degree real field, and a large 
eye relief. Image quality was to match that of 
the standard 7x50 hand-held glass. Mechanical 
requirements stipulated a 12-degree minimum 
angle of depression and a 20-degree upward tilt, 
an inter pupillary distance adjustable from 58 
to 72 mm, an eyepiece adjustable for focus in a 
range of 5 diopters on either side of infinity 
focus, and adjustments for alignment. All ex¬ 
posed parts were to be covered with plastic and 
were to be easily replaceable. Space between the 
objectives was to be provided for a unit-power 
rear vision or forward viewing optical system. 

The system adopted is shown in Figure 21. A 
modified Porro prism system was used for 
image erection with the entrance prism split to 
leave 8 in. between the first and second reflec¬ 
tion for the periscope offset, and the objectives 
placed between the two parts of the prism. In 
order to allow for passage of the oblique rays 
with little vignetting, the prisms were made 







SPECIAL BINOCULARS 


453 


very large. The optical system proved to give near the edge of the field. Comparison with a 
better image quality than that of the standard 7x50 Zeiss showed that the NDRC periscope 
7x50 binoculars. All other requirements of the binoculars were as good put to the full field. 



OBJECTIVE 
f'« 172.3 M M 
V'= 165.4 

CLEAR APERTURE 
50 MM 




device were met in the mounting. Figure 22 
shows a view of the completed instrument. 

Extensive optical tests were made on the sys¬ 
tem. The most noticeable defect in the image 
quality proved to be the rapid deterioration 



Figure 22. T-9 periscope binoculars. 

Field tests by the Army at Fort Knox 
brought forth a number of minor difficulties, all 


















































454 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 




SECTION B-B 



Figure 23. 7x50 pressureproof binoculars. 




























































































































































OPTICAL DESIGN WITH PLASTICS 


455 


of which were overcome by slight alterations 
in design. A corresponding Arsenal model was 
put into production during 1944. 

10 . 6.2 Bausch and Lomb Pressureproof 

Binoculars 

The task of pressurizing the 7x50 binoculars 
was entirely a mechanical problem. For the 
sake of simplicity and speed of delivery, the 
eyepieces were of fixed focus. Figure 23 shows 
a sketch of the instrument. The jacket consists 
of a cylinder closed at the eyepiece and by a 
cover welded into a recess in the main jacket. 
This cover carried a hood for the mounted eye¬ 
piece which terminated in a cover glass sealed 
in with a suitable gasket. The objective end of 
the jacket was also closed by a cover, likewise 
sealed to the jacket by a gasket and held in 
place by a number of screws. This cover in turn 
contained a thick cover glass sealed in with a 
gasket. This construction permitted insertion 
of the optical parts, which were mounted on a 
skeleton framework of cast iron. 

The jacket and associated exposed parts of 
the sample binocular were made of mild steel. 
In production it was intended to make use of 
stainless steel to resist the corrosive sea. 

Tests. Since the optical system was standard, 
no tests other than pressure tests were con¬ 
ducted. The tests proved that the binoculars 
could withstand the pressure limit assigned 
without suffering permanent injury to the per¬ 
formance. 

10.6.3 Fixed-Focus Binoculars 

The problems met by the Bausch and Lomb 
fixed-focus binoculars were: (1) adjustment 
of the eyepiece to a predetermined number of 
diopters plus or minus relative to the position 
of infinity focus, (2) reliable clamping in this 
position, (3) adjustment of the eyepiece to 
some other position in case of repair or other 
need, and (4) water-tightness. 

Figure 24 shows two views of the fixed-focus 
eyepiece. The simplifications achieved in pro¬ 
duction would have lowered the cost of the 
binoculars by only $1.50 without greatly ac¬ 
celerating output. However, the physical ad¬ 


vantages offered by the simplification might be 
worth while. 


FOR 7X50 BINOCULAR 



FOR 6X30 BINOCULAR 



Figure 24. Fixed-focus eyepieces. 


Final delivery of the 6x30 and 7x50 coated 
binoculars with eyepieces set for —0.75 D was 
made in July 1943 to NDRC. 

io.7 OPTICAL DESIGN WITH PLASTICS 

Optical design work on plastic optics carried 
out at the Polaroid Corporation was very ex¬ 
tensive. 9 In most instances it was found that 























































456 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


design problems were greatly complicated by 
the absence of a variety of optical properties 
and the unsuitability for many applications of 
the indices of the plastics available. In addition, 


0.070 

-1- 

—I-1— 

-1-1-1- 1 tt 

0.060 



STYRENE 

0.050 




0.040 

0.030 


CHM 

— __ 

0.020 



DENSE 

0.010 


* 

• FLINT 

0 



‘ 

- 0.010 

-0.020 

# • 

i i 

i i i i i 

65 

60 55 

50 45 40 35 30 




V 

Figure 

25. 

Optical 

constants of glasses and 


plastics. 


there were a number of practical considera¬ 
tions that limited the scope of the designs. 
Among these were change of focus with tem¬ 
perature, bubbles, scattering, surface changes 
on molding, and quality of surface. 

The optical constants of CHM (n D 20 = 1.5064, 
v — 56.9) and of styrene (n B 20 = 1.5916, v — 
31.0) are adequate for the design of ordinary 
visual instruments. Figure 25 shows the index 
for the D line plotted against v for representa¬ 
tive optical glasses. The positions of CHM, 
styrene, and EDM are also shown. CHM-styrene 
pairs are “old glass” combinations and there¬ 
fore do not yield the advantages of some of the 
“new glass” combinations. 

Styrene has in its favor an unusually low 
v-value for its index. For many purposes achro- 
matization with styrene leads to rather shallow 
curves, which properly used may reduce higher 
order aberrations otherwise accentuated by the 
relatively low indices of the two materials. 
CHM has in its favor unusual properties for re¬ 
duction of secondary spectrum. Combined with 
styrene, a CHM objective will have about two- 
thirds of the secondary spectrum of a normal 
glass pair. Combined with dense flint, CHM 
will remove nearly all of the secondary spec¬ 
trum. 

The Polaroid report 9 states that while the 
CHM-styrene combination is comparable to an 
ordinary glass pair for the design of visual in¬ 
struments, plastic combinations are not as sat¬ 
isfactory for the design of camera lenses. A 
plastic with high v-value and high index was 
sought at great length without material suc¬ 


cess. Consequently, resort was made to combi¬ 
nations of glass and plastic. 

Reflector Aerial Gunsights. Certain designs 
do not require a lens with flat field but do re¬ 
quire lenses with a low ratio of overall length 
to clear aperture. Such a lens is desirable for 
reflector gunsights. A material with low v-value 
leads to feasible achromatization with shallow 
curves. Without this low v- value, the lens curva¬ 
tures are so steep that, to obtain sufficiently 
clear aperture for the lens system, the lenses 
themselves must be thicker than the desired 
overall length. Ideally, a plastic or other ma¬ 
terial with r-value lower than 25 is desirable. 

Wide-Angle Eyepieces. In the design of wide- 
angle eyepieces it is desirable to achromatize 
with shallow curves so that aberrations do not 
become excessive. A low v-value makes this 
possible. A plastic material is required that pos¬ 
sesses a r-value considerably below 25. 

Under Contract OEMsr-70 Polaroid under¬ 
took development of plastics with unusually 
low r-values. Although several such materials 
were synthesized, production in large quantities 
would have been handicapped by the extra steps 
required, compared to CHM. It is believed that 
if the demand for a high-index material of low 
F-value proves sufficiently great, small-scale 
production can be accomplished. 

10,71 Athermalization 

The changes in volume and index of refrac¬ 
tion with temperature of plastic materials are 
large. The position of the focal surface of a 
lens system of plastic therefore depends mark¬ 
edly on temperature. In military instruments it 
is often inconvenient or impossible to adjust 
focus for any cause. Consequently, steps were 
taken at Polaroid to develop plastic systems 
with stabilized thermal properties. The main 
conclusions are as follows: 

1. Athermalization can be accomplished in 
a plastic system by the addition to the lens sys¬ 
tem of one or more glass lenses which have a 
negligibly low coefficient of thermal expansion. 

2. It is possible to athermalize a plastic sys¬ 
tem by using a housing composed of alternate 
sections of metal and plastic. The effect of a 
cumulative contraction adequately compensates 




PRECISION THEODOLITE TELESCOPES 


457 


for changes in the elements. The procedure of 
compensation analogous to that of a compen¬ 
sated pendulum can also be effected. 

3. Partial athermalization for focal distance 
can be obtained quite simply by using an alumi¬ 
num housing. In a telescope with plastic ele¬ 
ments housed in an aluminum tube, a tempera¬ 
ture drop would reduce the back focal distance 
of an element, while at the same time the alumi¬ 
num tube would undergo a compensating con¬ 
traction. 

4. A plastic mirror system may be com¬ 
pletely athermalized by making the connection 
between the mirrors, and from the mirror to 
the focal surface, of the same plastic material 
as the mirror. This is also essentially true of a 
system like the Schmidt where the refracting 
element has negligible power. 

The thermal correction of an optical system 
is very analogous to the usual chromatic cor¬ 
rection. The constant v as defined from the dis¬ 
persion of a glass and its index is replaced by 
a quantity v T , also characteristic of the mate¬ 
rial. This v T is used in the same fashion. An 
athermal system requires that the v T ’s of the 
two materials differ widely if athermalization 
is to be achieved. The report 9 states that for 
plastic materials known at present, the v r ’s 
differ only slightly. 

To athermalize a system one or more glass 
lens elements must be introduced, the glass 
serving as the thermal crown, and the plastic 
as the thermal flint. The v T for glass is approxi¬ 
mately 10 times that for plastic. Athermaliza¬ 
tion can be accomplished by using a single ele¬ 
ment of glass as the outside lens of the system 
in order to serve as protection for the plastic 
elements. A glass element is chosen whose focal 
length is approximately the same as that of the 
whole system. One example of a well athermal¬ 
ized system, in the absence of temperature 
gradients, is the objective lens of the T-108 
telescope. 

10 8 PRECISION THEODOLITE 

TELESCOPES 

The Army Engineer Board requested NDRC 
through the University of Rochester to design a 
compact internal focusing telescope of the 


highest possible optical performance for use 
in production of a precision theodolite. 10 

The optical specifications for the telescope 
were as follows: 

1. The objective was to be of 1.5 in. aperture. 

2. The eyepiece was to yield a large field of 
view and have an eye distance greater than 
0.33 in. 

3. The telescope was to be inverting, and to 
focus internally. The overall length was not to 
exceed 7% in. 

4. The system was to be free of scattered 
light. 

5. The exit pupil was to exceed 0.06 in. 

6. The magnification was to be 25 to 30X* 
The system was to be sensibly free from spheri¬ 
cal aberration, chromatic aberration, astigma¬ 
tism, coma, curvature, and distortion. 

7. The resolving power was to be 4 sec of 
arc or better. 

8. The minimum sighting distance was to be 
10 ft. 

9. The telescope was to be anallatic so that 
stadia might be read within 1 ft at all dis¬ 
tances exceeding 10 ft. 

Four foreign theodolite telescopes of high 
quality were sent to the University of Rochester 
for tests. It was requested that the new tele¬ 
scope equal or exceed the optical performance of 
the best of these. The four types received were: 

1. C.T.S. b level (41730) 

2. Tavistock T-65 (39481) 

3. Zeiss II (Nr 34174) 

4. Wild T-2 (7158) 

Inasmuch as the eyepiece in the theodolite 
works at very moderate aperture, the overall 
performance of the telescope is largely depend¬ 
ent on the objective. Tests were made in which 
the eyepiece was replaced by a 100 X micro¬ 
scope. The telescope was directed toward a dis¬ 
tant point-source, and studied first through a 
monochromatic green filter, and then without 
filter. 

These tests showed that the best results were 
achieved with the C.T.S. level and Tavistock, 
but that the Zeiss theodolite offered the most 
practical design. The variations from perfec¬ 
tion appeared to be due mostly to manufactur- 

b Cooke, Troughton, and Simms (Optical firm in York, 
England). 




458 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


ing difficulties. Accordingly, the final specifica¬ 
tions were to design a telescope with dimen¬ 
sions similar to the Zeiss instrument and with 
optical performance equal or better. Only opti¬ 
cal glasses of American manufacture were to 
be employed. The number of glass-air surfaces 
was to be the same or fewer, compared to the 
Zeiss system. 

Design . The final design contains a triplet 
air-spaced objective, focal length 122 mm, with 
an effective aperture of 40 mm. The focusing is 
done with an internal negative lens of focal 



Figure 26. Optical 

length —47.6 mm. This lens is made a separated 
doublet with positive and negative crown ele¬ 
ments. The eyepiece is a standard orthoscopic. 

The three separate parts of the theodolite 
must be corrected individually for the perti¬ 
nent aberrations. Even though the design may 
be sufficiently perfect theoretically, it presents 
a manufacturing problem of considerable mag¬ 
nitude. The optical system collectively must 
meet the Rayleigh tolerance. 

The final optical characteristics of the tele¬ 
scope were: 

1. Magnifying power, 27.2. 

2. Exit pupil, 1.47 mm or 0.058 in. 

3. Real total field, 1.5 degrees. 

4. The telescope is anabatic to 1 per cent 
from 3 m to infinity. 

5. Overall length of the optical system, 6.5 in. 

A sample was made up by the W. and L. E. 

Gurley Company. After a number of tests, and 
two modifications of the sample, a sufficiently 
good model was obtained. The original sample 
submitted was undercorrected spherically by 6 
times the Rayleigh limit. The reworked model 
was still undercorrected slightly, due appar¬ 
ently to slight turned edges. A final slight 
change of design produced a bit of overcorrec¬ 
tion. The observed angular resolution was very 


close to the theoretical requirements and equal 
to or better than the Zeiss resolution. 

Although in the early tests the Zeiss instru¬ 
ment excelled for freedom from scattered light, 
the NDRC telescope after thorough internal 
blackening and light trapping proved to be 
equal or superior to the Zeiss instrument in this 
respect. A further small change was made in 
the eyepiece in order to obtain a colorless and 
sharply defined field stop. Figure 26 shows a 
cross section of the final optical system. 

The theodolite telescopes were later manu- 


FOCAL 



system of theodolite. 

factured in large quantities for use with the 
theodolites manufactured by the Gurley Com¬ 
pany. 

109 SUBMARINE PERISCOPES 

The problems raised by the Eastman report 
(see reference 1 in Chapter 18) on periscope 
photography were brought to the attention of 
the Yerkes Optical Bureau under Contract 
OEMsr-1078 for study and possible solution. 11 
These problems were threefold: (1) secondary 
color, (2) larger eye relief, and (3) curvature 
of field. Altogether, three existing Kollmorgen 
periscopes were investigated. It is stated that 
a completely new design would have made it 
easier to correct troubles, and that only limited 
improvements could be made in the existing de¬ 
signs. 


Improved Periscope Design 

The properties of the existing Kollmorgen 
periscopes are listed in Table 11. The designa¬ 
tion N refers to the number of elements and D 
to the diameter of the top part of the periscope 











SUBMARINE PERISCOPES 


459 


tube. All these periscopes are provided with 
magnifications of 6X and 1.5X the latter by 
insertion of an inverted Galilean telescope be¬ 
fore the top objective. Photographic work is 


Table 11. Properties of the Kollmorgen peri¬ 
scopes. 


Type 

N 

D (mm) 

Exit pupil (mm) 

1.4 

19 

44 

4 

1.9 

10 

44 

4 

IV 

10 

76 

7 


accomplished exclusively with the 6X- The di¬ 
ameter of the true field of view is 8 degrees, 
the apparent field 48 degrees. The length of 
tube is 40 ft. All systems have considerable 
vignetting of the exit pupil near the edge of 
the field. Type 1.4 is the most used, but also the 
most complex. 

Aberrations of the Three Periscopes. The ex¬ 
isting imperfections of the periscopes listed 
above were determined by ray tracing. In the 
following tables (Tables 12, 18, 14) depicting 
the results, a refers to the intercept of the in¬ 
dividual ray in the exit pupil, as measured from 
the optical axis in a perpendicular direction for 
a fixed position of the exit pupil. The angle U 
refers to the off-axis field angle of the principal 
ray in object space. The aberrations are given 
in terms of mils in image space of the com¬ 
puted ray’s direction, referred to the principal 
ray. Curvature of field is not taken out, since 
the purpose of the investigation is photo¬ 
graphic rather than visual. 


Table 12. 
(mils). 

Aberrations 

of the type 

1.4 periscope 


Axial, U\ — 0° 


Color a 

= 1.0 mm 

a 1.5 mm 

a — 2.0 mm 

C 

0.3 

0.7 

1.7 

D 

1.0 

1.8 

3.0 

F 

0.1 

0.2 

0.0 

g 


—5.1 


h 

—7.6 

—12.1 

—17.5 


Off-axis 





Three-fourths 



Half field 

field 

Color 

a (mm) 

Ui = 2° 

Ih = 3° 

F 

1.0 

1.0 

0.7 

F 

0.0 

0.0 

0.0 

h 

0.0 

0.9 


F 

—1.0 

—0.7 

—1.2 


Adopted entrance pupil, 170 mm in front of first lens. 


Table 13. Aberrations of the type 1.9 periscope 
(mils). 


Color 

Axial, Ifi == 0° 

a = 1.0 mm a = 1.5 mm a = 2.0 mm 

C 

—0.1 0.0 

0.4 

D 

0.5 


F 

—0.2 —0.2 

0.1 

G' 

—3.1 


h 

—4.1 —6.1 

Off-axis 

—8.2 


Three-fourths field 

Color 

a (mm) 

CJ 

M 

II 

CO 

o 

F 

0.67 

—0.6 

F 

0.33 

—0.2 

F 

0.00 

0.0 

F 

—0.33 

0.3 

F 

—0.67 

0.5 


Adopted entrance pupil, 230 mm in front of first lens. 


Table 14. Aberrations of the type IV periscope 
(mils). 


Axial, U\ — 0° 

Color a = 1.75 mm a = 2.62 mm a — 3.50 mm 


C 

0.9 1.5 

2.9 

D 

1.8 


F 

0.0 0.0 

0.2 

G' 

—3.7 


h 

—5.1 —7.9 

—10.6 


Off-axis 



Three-fourths field 

Color 

a (mm) 

Ui = 3° 

F 

1.75 

—0.7 

F 

0.88 

—0.6 

F 

0.0 

0.0 

h 

0.0 

2.8 

F 

—0.88 

0.6 

Adopted entrance pupil, 290 mm in 

front of first lens. 


Secondary Color and Its Correction 

An investigation was made of correction of 
secondary color for periscope purposes. The 
problem required substitution of at least a 
triplet objective in order to meet all require¬ 
ments of focal lepgth, and full color correction. 
Even with a fluorite, BSC-1 and EDF-2 com¬ 
bination, it proved necessary to use more than 
three elements in order to reduce the large lens 
powers. It was necessary to introduce overcor¬ 
rected secondary color in the objective in order 
to reduce the secondary color of the system as a 
whole. 

Type l.Jf Periscope with the P-55 Fluorite 
Corrector. The Yerkes report 11 states that one- 






















460 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


third of the secondary color of the type 1.4 peri¬ 
scope arises in each of the two large doublets, 
while the remaining one-third arises in all the 
other lenses. It was believed that any attempt to 
remove the color aberrations should be for 100 
per cent correction. Anything less was believed 
to reduce the loss of contrast at the expense of 
loss of definition, until some small value of the 
circle of confusion had been reached. 

Only two reasonable possibilities existed for 
elimination of the color aberrations. The first 
of these involved the insertion of a zero-power 
correcting combination between the large dou¬ 
blets at the point where the ray height reached 
a maximum. The early calculations showed, 
however, that even at the optimum location, 
success was exceedingly doubtful. The second 
possibility involved replacing one of the large 
doublets by a correcting lens of equal power, 
but overcorrected color. This second possibility 
was pursued. 

Many computations were required before a 
result satisfactory from all points of view was 
obtained. Methods were developed for complete 
control over the spherical aberration, the zonal 
spherical, the paraxial color, chromatic varia¬ 
tion of spherical aberration, and the coma. The 
final residual aberrations were even balanced 



FLUORITE BSC" 1 EDF-2 BSC"1 FLUORITE 

Figure 27. P-55 fluorite corrector for type 1.4 

periscope. 

for secondary color variation in the spherical 
aberration. Figure 27 and Table 15 contain the 
final specifications for the fluorite corrector. It 


should be noted that the aberrations of this 
corrector are magnified 30X in final image 
space. Hence, the effort put into the design 
work was very much worth while. 

For similar reasons the construction work 
was critical. This work was carried out at the 
Harvard laboratory under Contract OEMsr- 
474. 12 In the Harvard report it is stated that 
the curves were held to better than one part in 
2,000, that the thicknesses were held to less 
than 0.002 in. from assigned values, that the 
centering was satisfactory, and that the glass- 
air surfaces were figured individually to better 
than one-quarter wave. On the other hand, con¬ 
siderable difficulty was experienced with the 
fluorite-air surfaces of which there were four. 
The chief trouble lay in the occurrence of 


Table 15. Optical data for P-55 fluorite corrector 
for type 1.4 Kollmorgen periscope. 


Lens* 

Radius 

(mm) 

Separation or 
thickness 
(mm) 

Material 

1 

163.1 

17.0 

fluorite 


—674.0 

10.1 


2 

—186.7 

12.0 

BSC-1 


368.6 

20.0 


3 

—154.0 

12.0 

EDF-2 


—121.6 

0.02 


4 

—414.7 

12.1 

BSC-1 


191.8 

4.0 


5 

229.9 

18.0 

fluorite 


—259.6 

2,060.5 



* All lenses have 115 mm diameter. 


numerous small pits at the center of minute de¬ 
pressions of fair diameter. These depressions 
were several fringes deep and as much as % in- 
in diameter. In addition, it was difficult to hold 
the surfaces spherical within several fringes. 
The final image of a star examined under 50 X 
seemed somewhat diffuse. 

Table 16 gives the results of ray tracing with 
the new corrector in place. The monochromatic 
performance is slightly inferior to the earlier 
design, but the color has been improved mark¬ 
edly. The off-axis performance is adversely 
affected by slight coma and lateral color intro¬ 
duced by the corrector. This lateral color is of 
a secondary nature, introduced by the refrac¬ 
tion of principal rays through the corrector 
above the axis. To remove this lateral color, 























SUBMARINE PERISCOPES 


461 


both large doublets would have to be replaced, 
or else a corrector put at a pupillary point. In 
spite of the lateral color introduced, the off- 
axis chromatic performance is still much bet¬ 
ter than given by the original system. 


Table 16. Aberrations of 
type 1.4 periscope (mils). 

P-55 

corrector 

with 


Axial, Ui = 

: 0° 



Color 

a = 1.0 mm a = 

: 1.5 mm a = 

2.0 mm 

C 

—0.1 

0.4 


0.8 

F 

0.1 

0.5 


-0.6 

h 

—0.4 

0.1 


0.9 


Off-axis 







Half field 


Color 

a (mm) 


U! = 2° 



F 1.0 0.4 

F 0.0 0.0 

h 0.0 —5.1 

F —1.0 2.4 

The Type 1.9 Periscope and the H-12 Cor¬ 
rector. This periscope differs from Type 1.4 in 
that it consists of but a single erecting telescope. 
Most of its secondary color arises in the two 
large erecting doublets, and it can be corrected 
most conveniently there. Consequently, it be¬ 
comes possible to replace these doublets by 
fluorite, BSC-1 doublets with elimination of 
secondary color of the system. The EDF-2 ele¬ 
ment needed for the earlier corrector is not 
needed here, inasmuch as no overcorrection for 
secondary color is necessary. Table 17 contains 
the optical data for the Type 1.9 periscope re¬ 
designed doublets. 


Table 17. Optical data for the H-12 fluorite cor¬ 
rector for the type 1.9 periscope. 


Lens 

Radius 

(mm) 

Separation 

or 

thickness 

(mm) 

Material 

Outside 

diameter 

(mm) 

1 

piano 

523.8 

15.0 

4.0 

BSC-1 

147 

2 

520.5 

—692.1 

18.0 

7297.0 

fluorite 

147 

3 

692.1 

—520.5 

15.0 

4.0 

fluorite 

115 

4 

—523.8 

piano 

12.0 

2027.0 

BSC-1 

115 


The lateral color of the corrected system is 
less than that of the original periscope design. 
Moreover, the axial performance is good over 


the entire range of colors. The doublets are 
fully corrected for color, spherical aberration, 
and coma. The required diameters, unfortu¬ 
nately, are at least 4.5 ih., and one up to 5.8 in. 
would be desirable. However, it should be 
pointed out that provision of a good periscope 
system for so complex and expensive an object 
as a submarine cannot be ignored at any cost 
normal to optical applications. 

Table 18 contains the results of ray tracing 
for the H-12 and Type 1.9 periscope, for axial 
and off-axis aberrations. 


Table 18. Aberrations of H-12 corrector and type 
1.9 periscope (mils in image space). 


Color 

Axial, Ui = 0° 

a = 1.0 mm a = 1.5 mm a = 2.0 mm 

C 

<tq 

© 

© 

1 

—0.7 

D 

—0.7 


F 

—0.1 0.1 

0.1 

G' 

0.2 


h 

—0.2 —0.3 

Off-axis 

—0.1 


Three-fourths field 

Color 

a (mm) 

Ui = 3° 

F 

0.67 

—0.2 

F 

0.33 

—0.1 

F 

0.00 

0.0 

h 

0.00 

0.3 

F 

—0.33 

0.2 

F 

—0.67 

0.3 


The Type IV Periscope with the G-8 Cor¬ 
rector. The corrector adopted for the Type IV 
periscope is exactly analogous to that used in 
the 1.9. For complete transmission of the 7-mm 
exit pupil, lens diameters of 6.0 in. are needed. 
The aberrations are given in Table 19. The 
large exit pupil causes slightly larger residual 
aberrations, such as chromatic difference of 
spherical aberration. The off-axis performance 
is good. The lateral color is absent. The differ¬ 
ence between primary and secondary curva¬ 
tures of field yields a uniformly illuminated 
oval disk measuring about 0.2x2.2 mils at three- 
fourths field. 

Revised Eyepiece of Larger Eye Relief 

In June 1945 a request was made by the 
Navy for revision of the eyepiece in the Type 
1.4 periscope in order to provide 5 to 10 mm 
more eye distance. The eyepiece designed to fill 
this need can be used with or without the chro- 

















462 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


matic corrector. Where the original design em¬ 
ploys a curved face on the prism, the redesign 
utilizes a flat prism face as shown in Figure 28. 
This modification alone produces 7.7 mm addi- 


Table 19. Aberrations of G-8 corrector and type 
IV periscope (mils in image space). 


Color 

Axial 

a — 1.75 mm a =■ 2.62 mm 

a = 3.50 mm 

C 

—0.6 —0.9 

—0.6 

D 

—0.6 


F 

CO 

1 

rH 

© 

i 

—0.2 

G' 

—0.3 


h 

—0.3 —0.7 

Off-axis 

—1.2 


Three-fourths field 

Color 

a (mm) 

o 

CO 

II 

£ 

F 

1.75 

—1.2 

F 

0.88 

—0.3 

F 

0.00 

0.0 

h 

0.00 

0.2 

F 

—0.88 

1.0 



Figure 28. Type RE-4 eyepiece. 


tional eye relief. The redesigned eyepiece has 
slightly more negative astigmatism and conse¬ 
quently a flatter mean field than that of the old 
eyepiece. 

The calculations involved in the eyepiece de¬ 
sign gave more information on the off-axis per¬ 
formance of the system for photographic use. 
The field is highly curved. For example, if a 
camera of 50-mm focal length is used, the con¬ 
fusion disk will be approximately 0.06x0.27 


mm, owing mostly to errors in the secondary 
plane. 

Correction for Field Curvature 

A great quantity of detailed information is 
contained in the Yerkes report 11 on field-flat¬ 
tening devices for the various periscopes. Space 
is too short to give more than a brief account. 

It is possible to place a field-flattening lens 
in the primary focal plane of the periscope and 
to carry on photography with the eyepiece re¬ 
moved. The success of this arangement depends 
in practice on the optical quality of the peri¬ 
scope up to this point and on the curvature of 
field present. In the 1.4 periscope it is felt that 
such a field-flattening lens cannot be used. How¬ 
ever, in the 1.9 type, its use is practicable. 

The most successful device for improvement 
of periscope photography is the introduction of 
a new camera lens design called the PD-5. This 
lens is designed for use at //12 and has a 
greatly overcorrected Petzval sum. By design 
of the camera lens it is possible to obtain a flat 
overall field of high correction for the entire 

REAR VERTEX 




Figure 29. PD-5 and PC-60 field-flattening camera 
lenses. 

field. Computation of the aberrations of the 
overall system show that a uniform image qual¬ 
ity of 0.02 mm can be obtained over three- 
fourths of the field in F light by means of this 
















































SPECIAL PERISCOPES 


463 


special camera adapted to the 1.4 periscope. 
These results indicate a resolution of at least 25 
lines per mm to be compared to the 16 lines per 
mm observed by Eastman investigators 11 at the 
center of the field. 

Figure 29 shows a cross section of two such 
cameras designed for use with the 1.4 peri¬ 
scope. The PD-5 is for use with the original 
periscope and RE-4 eyepiece. The PC-60 lens 
is to be used with the periscope as modified by 
the P-55 corrector and RE-4 eyepiece. Appar¬ 
ently, the best performance in F light is given 
by the first mentioned combination, but over a 
range of colors, as in practice, the second com¬ 
bination is better. No tests have been made as 
yet with any of these combinations, and in¬ 
deed, only the PD-5 prototype has been con¬ 
structed. 

Summary 

Type l.U Periscope 

P-55 Fluorite color corrector, replaces lower 
erector doublet. 

RE-4 Eyepiece with longer eye relief, 

PD-5 Camera lens for flat field with Type 1.4 
periscope. 

PC-60 Camera lens for flat field with Type 1.4 
periscope and P-55 fluorite corrector. 

Type 1.9 Periscope 

H-12 Fluorite color corrector, replaces both 
erector doublets. 

Field-flattening lens for standard cam¬ 
era with Type 1.4 periscope (only 
basic data for design). 

Field-flattening lens for film before 
the eyepiece. 

Type IV Periscope 

G-8 Fluorite color corrector, replaces both 
erector doublets. 

Field-flattening lens for standard cam¬ 
era with type 1.4 periscope (only basic 
data for design). 

Field-flattening lens for film before 
the eyepiece. 

Redesign of Submarine Periscopes 

In the course of the development work under 
Contract OEMsr-1078, a number of general 


principles appeared that must be borne in mind 
in subsequent efforts. 

1. In periscopes where the size of the top 
part of the tube is of paramount importance, 
it is still worth while to make the optical parts 
as large as possible for the assigned tube size. 
For example, the optical disadvantages of the 
Type 1.4 system are large compared to the Type 
1.9, yet the objective lens sizes differ by only 
10 mm. 

2. In those periscopes containing one erect¬ 
ing telescope, 1.9 and IV, the use of fluorite 
apochromatic doublets is recommended. The 
objective need not use fluorite but could make 
good use of a flat field, as from a Cooke type of 
system. In any redesign of these periscopes, 
special attention should be paid to the location 
of the objective with respect to the top prism 
and to the sizes of the lenses for minimum vi¬ 
gnetting. 

3. A periscope with two complete telescopes 
like the 1.4 poses difficult design problems. If 
possible, the upper objectives and erectors 
should be redesigned with flat fields for pho¬ 
tography in front of the eyepiece. If not, and if 
normal objectives with positive Petzval sums 
are used, they should still be redesigned with 
apochromats for freedom from secondary spec¬ 
trum. 

Photography with Type 1.4 redesigned will 
still require a special field-flattening camera 
like PD-5 after the eyepiece. Before the eye¬ 
piece, if the upper objectives have been care¬ 
fully designed with flat fields, it may become 
possible to use a simple field-flattening lens. 

4. Redesign of eyepieces with additional eye 
relief does not offer much difficulty. 

5. Photography will be most efficacious 
through a periscope with the maximum possible 
exit pupil diameter in order to obtain short ex¬ 
posure times with cameras of long focal length. 
At present Type IV should use exposure times 
of about one-third of either Type 1.9 or 1.4. 
Vignetting should be minimized. 


10 10 SPECIAL PERISCOPES 

During the course of World War II need 
arose for a number of special periscopes either 



464 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


for protection as in trench warfare, or for of Rochester. Some of the construction work 
extending the field of the observer to otherwise was carried out at Harvard under Contract 
inaccessible spots. This work was accomplished OEMsr-474. 



under Contract OEMsr-1078 at Yerkes and 
under Contract OEMsr-160 at the University 


lo.io.i Aircraft Periscope 

In April 1944 a request was received (Proj¬ 
ect NR-111) from AsDevLant at Quonset Point, 
Rhode Island, to design and construct a unit- 
power periscope for the purpose of extending 
the view of the bombardier in the TBF-1 patrol 
plane. The periscopic sight was to be linked di¬ 
rectly to a bombsight and searchlight for night 
patroling. 

The optical design for the proposed periscope 
was undertaken at Yerkes and completed in a 
very short time. 13 The mechanical details were 
handled by the Harvard group in full coopera¬ 
tion with representatives of AsDevLant, who 
furnished the major portion of the design. 14 

Figure 30 shows a cross section of the final 
periscope design. Figures 31 to 34 show several 
views of the periscope in position in the air- 



Figure 31. View of AsDevLant periscope as in¬ 
stalled in the TBF plane. 

plane. Figures 35 to 40 show several views of 
the periscope with detailed indexing of the im¬ 
portant parts. 

Optical Design. The requirements for the 
unit power periscope for AsDevLant were: (1) 
24-degree total field, (2) large exit pupil, (3) 
large eye relief, and (4) an offset of 36 in. be¬ 
tween the forward lines of sight at scanning 
prism and observer’s eye. 





















































SPECIAL PERISCOPES 


465 



The characteristic feature of the design is 
the use of cemented doublets placed at suffi¬ 
cient distances from the pupillary points to 
control the astigmatism. It was found that by 
proper choice of glass types it was possible to 
design a cemented doublet free from astigma¬ 
tism. 


Figure 32. Cockpit view of AsDevLant periscope 
as installed in the TBF plane. 


Figure 33. Cockpit view of AsDevLant periscope 
as installed in the TBF plane. 


Figure 34. View of AsDevLant periscope in use 

by bombardier in the TBF plane. 

form of a black interrupted cross and concen¬ 
tric circles. The curved rear side of the collec¬ 
tive lens accommodated the calculated field cur¬ 
vature at that point. 

The outstanding aberration of this unit- 
power system is unobjectionable spherical ab¬ 
erration off the optical axis. At full field there 
is a residual field curvature of only 0.3 diopters. 
At full field there is considerable difference 
between primary and secondary curvatures, 
but in view of the relatively small part of the 
exit pupil and the resultant reduction of the 
aberration disk, there is little effect on the 
sharpness of the image. The prototype showed 
no appreciable visual errors and left nothing 


The upper prism of the 36-in. periscope was 
made large enough to permit scanning in eleva¬ 
tion. The instrument as a whole rotated in azi¬ 
muth. The final design shown in Figure 30 gives 
excellent definition over the entire field. The 
exit pupil is 0.8 in. in diameter, and the eye 
relief 0.4 in. Ten lenses were used with maxi¬ 
mum diameter 2.5 in. The reticle was engraved 
on the back side of the collective lens in the 










466 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


to be desired in performance. It is believed, 
however, that any future design should be 
based on a larger apparent field in order to 
facilitate use in the daytime. At night, the nar¬ 
row beam of the searchlight required only a 
small field, and the boresighting of searchlight 
and periscope meant that the observer was 
always looking along the beam. 



Figure 35. Assembly view of AsDevLant peri¬ 
scope. 

Mechanical Design. The prototype built in 
April and May 1944 proved to be satisfactory 
optically but lacked the necessary mechanical 
qualities for good linkage with bombsight, 
searchlight, and scanning prism. Representa¬ 
tives of AsDevLant prepared detailed draw¬ 
ings of a new mechanical design improved 
with respect to linkage, accuracy, facility of 


adjustment, waterproofing, and ease of opera¬ 
tion. In all sixteen units were built and deliv¬ 
ered to AsDevLant. Of this work NDRC con¬ 
tributed only the prototype complete and the 
optical parts of the remaining fifteen units. 
Service tests showed satisfactory results. The 
entire sixteen units were produced in the pe¬ 
riod from August to November 1944. 



Figure 36. Assembly view of AsDevLant peri¬ 
scope. 

The mechanical design consists of three main 
subassemblies: (1) the periscope optical sys¬ 
tem, (2) the housing unit, and (3) the control 
unit. The housing unit serves to protect the 
optical system and the mechanical parts. The 
upper end of the housing contains an elliptical 
window made of selected plate glass. The prism 
is mounted directly behind this window and is 





SPECIAL PERISCOPES 


467 



COVER TUBE 


UPPER BEARING 
GREASE FITTING 


SCANNING PRISM 
HEAD 


PRISM PIVOT 
BUSHING 


COVER AND 
ENTRANCE WINDOW 


FLANGED MOUNTING 
CASE 


SEARCHLIGHT 
AZIMUTH CARD 


TELESCOPE 

EYEPIECE 


SEARCHLIGHT 
ELEVATION CARD 

ELEVATION 
BY-PASS VALVE 


ELEVATION 
ISODRAULIC UNIT 


HOUSING UNIT 


LOWER BEARING 
OILER 

COLLAR NUT 


Figure 37. Detailing of AsDevLant periscope 
assembly. 



MOUNTING 


SEARCHLIGHT 


HYDRAULIC 

CYLINDER 


CASE 


SEARCHLIGHT 

AZIMUTH 

HYDRAULIC 

CYLINDER 


PUSH-PULL ROD 
LOCK SCREW 


PRISM OPERATING 
MECHANISM 


SLIDING BLOCK 


Figure 38. Detailing of AsDevLant periscope 
assembly. 



BOMBSIGHT 

HANDLES 


BOMBING KEY 

PERISCOPE 
HANDLES 

CONNECTING 
LINK 

BOMBING KEY 


MK 23 BOMBSIGHT 


TRIGGER SW 


BOMBSIGHT 


Figure 39. Detailing of AsDevLant periscope 
assembly. 


PRISM HOLDER 



PUSH-PULL 
ROD 

ALLEN SET 
SCREW 
HANDLE SHAFT LEVERS 
OPERATING MECHANISM 
LINE "C" 


Figure 40. Detailing of AsDevLant periscope 
assembly. 











































468 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


operated from the handles 36 in. away by 
means of connecting rods. The rotation in azi¬ 
muth is accomplished by swiveling the housing 
and interior unit in bearings housed in a cast¬ 
ing with a flange that is bolted to the airplane. 

The push-pull rod connecting the control 
unit below with the scanning prism passes be¬ 
tween the outer housing and the tube of the 
periscope itself. In order to give the observer 
the convenience of “feeling” the elevation, 



Figure 41. Foxhole periscope, Type 1. 

there is a 2/1 reduction in rotation between the 
handle shaft and the upper prism which also 
allows for the factor of 2 in image shift versus 
prism shift in angle. 

The control unit is connected by isodraulic 
remote control to the searchlight in the bottom 
of the ship. Any movement in elevation or azi¬ 
muth will be transmitted directly to the search¬ 


light. The searchlight is therefore always bore- 
sighted with respect to the cross wires of the 
periscope. The isodraulic units work independ¬ 
ently in elevation and azimuth. The isodraulic 
unit itself has been worked out carefully to 
allow for contraction or expansion of the 
fluid in the cylinders and connecting tubes. Ini- 


n°. i - 


NO. 2 
NO. 3 


18" UNIT POWER 
PERISCOPE 
40° FIELD 
7 MM PUPIL 


EYEPOINT 

<- 

NO. 9 NQ.8 

Figure 42. Foxhole periscope, Type 2. 

tial alignment is provided for by a by-pass 
valve, which allows one piston to be moved at 
one end of the unit without corresponding 
movement of the other piston. 

A Mark 23 bombsight is installed on the 
lower end of the periscope. The bombsight 
comes with a pair of handles like the handles of 
the periscope. Operation of the periscope can 
be carried out with either pair of handles, 
arranged for by a direct connecting link. The 
bombsight handles are used for daytime bomb¬ 
ing, and the periscope handles for a night 
searchlight attack. Bomb release is accom- 



i 


i 

18*0 

l 

NO. 5 










































SPECIAL PERISCOPES 


469 


plished by either of the two right-hand handles. 
The searchlight switch is located only on the 
upper left-hand handle. The searchlight cannot 
be turned on until its covers are open below the 
plane. 

The line of sight is restricted by stops from 
horizontal to 45-degrees dip. Rotation in azi¬ 
muth is 20 degrees on either side. 


10.10.2 Foxhole Periscope 

The Signal Corps requested design and con¬ 
struction of a periscopic attachment to a 35- 
mm motion picture camera for photography of 
combat operations. 13 " 1 The purpose was to set 
the camera and the top prism of the periscope 
about 18 in. above the operator's eye level on a 
rotatable stand, with the periscope used there¬ 
fore as a viewfinder. A real field of 40 degrees 
diameter with magnification of IX or 1.5 X was 
requested. The exit pupil was to be larger than 
4 mm. 

Two alternative designs were submitted by 
Yerkes. Figure 41 shows Type 1. In this system 
simple lenses are used throughout, with the ex¬ 
ception of the hyperchromat at the diaphragm. 
Each of the paired simple lenses is so shaped 
that the system is aplanatic, surface by surface. 
The accumulated longitudinal color is corrected 
at the pupil point by the zero power hyperchro¬ 
mat. The system has curved field but is fully 
free from astigmatism and coma. The spherical 
aberration is sufficiently small, owing to the 
shallow curves employed. The system has a real 
field of 40 degrees diameter, 10-mm exit pupil, 
and 2-in. eye relief. 

Figure 42 shows Type 2. The erecting system 
in this instrument employs a symmetrical sep¬ 
arated triplet hyperchromat. Because this trip¬ 
let has positive power at the pupil, it is neces¬ 
sary to compensate the astigmatism thus intro¬ 
duced by negative astigmatism at the other two 
paired lenses. In this instrument the real field 
is 40 degrees, with 7-mm exit pupil and eye 
relief of 1.6 in. Only seven elements were used, 
which have smaller diameters than those in 
Type 1. The aberrations of Type 2 are some¬ 
what larger than those of Type 1. 


Both designs were submitted to the Signal 
Corps for development. 


10.10.3 ^ 110-in. Periscope for the P-51 
Airplane 

A periscope 110 in. in length was designed at 
Yerkes to enable the pilot of a P-51 airplane to 
extend his view 12 degrees below horizontal. 13b 
The lenses are approximately 8 in. in diameter 
and are to be made of plastic. The eye lens is to 
be fitted into the canopy of the airplane in such 
a way as to cause minimum disturbance when 
the pilot sweeps his vision from looking through 
the Plexiglas to looking through the eye lens 
of the periscope. Although several groups ex¬ 
pressed considerable interest in this develop¬ 
ment, it was not covered by a Service request 
and time did not permit making a model. 

From a schematic lay-out of the nose of the 
P-51b, it appeared that the pilot could normally 
see only about 3.5 degrees downwards below 
the fuselage reference plane. A periscope 84 in. 
long from entrance pupil to exit pupil would 
increase the clear downward vision to 13 de¬ 
grees, and a periscope 115 in. long to about 
25 degrees. 

The proposed design made use of doublets of 
CHM and styrene. The length between lenses 
was 113 in., and between entrance and exit 
pupils 135 in. The real field was 24 degrees 
diameter with an exit pupil of 3 in., and eye 
relief of 11 in. The system employs ten ele¬ 
ments with apertures from 7.3 to 7.8 in. 


10.10.4 P-80 Periscope Design 

In the spring of 1945 a request was received 
from the Air Forces for a periscopic viewing 
system for P-80 airplanes which were to be con¬ 
verted into photographic ships. 15 Such a peri¬ 
scope was to have a total real field of 45 degrees 
with an exit pupil 5 in. in diameter located at 
the eyes of the pilot. No part of the periscope 
was to interfere with any of the pilot’s motions 
and all parts were to be within the plane. An 
unobstructed vertical view was desired. 

Figure 43 shows a cross section of the optical 



470 


OPTICAL SYSTEMS FOR TELESCOPES AND BINOCULARS 


system proposed by the University of Rochester 
under Contract OEMsr-160. No details were 
worked out on the mounting, owing to uncer¬ 
tainty of the structure of the airplane. It is 
believed that such an installation must of ne¬ 
cessity be accomplished at the factory by engi¬ 
neers with complete knowledge of the nature of 
the plane. 

The negative magnification inherent in a 
system meeting the specifications is a great dis¬ 
advantage to the pilot, although no doubt better 
than no viewing system at all. The only possible 
improvement would be to set aside ample space 
in the design of the airplane for a viewing 
screen of large size, installed on the instrument 
panel in line with the pilot’s most convenient 
sight, on which is cast at least a unit-power 
image. In order to obtain sufficient field and 
illumination, a rather complicated periscope 
system with large lens elements would be re¬ 
quired. Sufficient room forward of the instru¬ 
ment panel would have to be set aside for ac¬ 
commodation of the large periscope tube and 
large prisms or mirrors. Every effort should 
be made to avoid slight bendings of the optical 
axes in favor of use of right angle prisms or 
equivalent mirrors. Finally, such a system 
should be equipped for scanning from forward 
horizon to rear horizon by means of a head 
prism in a streamlined blister. 

1011 DISCUSSION 

For the most part the instruments delivered 
to the Services were developments parallel to 
types already in production or under develop¬ 
ment by the several branches of the Services. 
The overlap in effort proved useful, however, 
in stimulating activity and in isolating factors 
that could be improved. Many of the NDRC 
ideas and improvements were incorporated 
either directly or indirectly in Service proto¬ 
types. 

There can be no single group of detailed rec¬ 
ommendations covering so broad a subject as 
summarized in this chapter. It is evident that 
many of the instruments were brought close to 
the limits of perfection possible with existing 
materials, and that the principles evolved in the 
work might readily be applied to incorporation 


of improved materials. The appearance of a 
glass capable of combining with ordinary glass 
types for reduction or elimination of secondary 
spectrum would remove a group of problems 
common to all visual systems involving image 
erection, tube extension, or, as in periscopic sys¬ 
tems for submarines, small tube diameters and 
many elements. Fluorite represents only a par- 



Figure 43. P-80 viewing periscope. 

tial solution to date, and introduces almost as 
many difficulties as it removes. 

It is probable that many of these visual in¬ 
struments might profit by discriminating use 
of aspheric surfaces. Developments along such 
lines should certainly include sponsorship of 
glass-molding techniques of production char¬ 
acter and accuracy. 

Many of the instruments described in this 
chapter have an abundance of glass-air sur¬ 
faces, and elements of high index. For these 
reasons it is probable that transmission has 
been impaired. Further work should always 
keep in mind as careful a balance between per¬ 
formance and transmission as possible, and 
actual models should incorporate the best pos¬ 
sible surface coatings. In view of field experi¬ 
ence that coated surfaces often achieve no per¬ 
manent gain because of contamination in use, 
it would appear that all production optical sys¬ 
tems should employ sealed optics. 

















RECOMMENDATIONS BY NDRC 


471 


1012 RECOMMENDATIONS BY NDRC 

1. A full study of all available designs for 
tank telescopes should be made. Present re¬ 
quirements demand not only a wide field and 
a 7-mm exit pupil, but also that the optical 
system fit inside the present 2-in. diameter tube 
which is now in use. If studies show that a 
satisfactory result cannot be achieved when the 
lenses are limited to 2 in. diameter, then con¬ 
sideration should be given to two possible 
modifications. Either the size of the tube should 
be increased slightly or the field or exit pupil 
should be reduced somewhat until a com¬ 
promise is reached where definition over the 
whole field is adequate, as judged by field tests. 

2. The development of the split-field tank and 
antitank telescopes should be continued and 


models should be field-tested to show whether 
these designs represent promising approaches 
to the problem raised by the need for both high 
and low magnification. 

3. A complete redesign is indicated for sub¬ 
marine periscopes in order to reduce present 
aberrations, particularly in some models. Color 
and curvature of field now limit photographic 
resolution seriously. Fluorite could be used in 
both doublets of the erector system to reduce 
color to an insignificant level. Curvature of field 
can be compensated by using a special camera 
lens, such as has already been delivered for the 
1.4-in. periscope, unless a new design for the 
periscope produces a field sufficiently flat to 
permit using the present camera lens. Rare- 
earth glass would help markedly in designing 
an improved periscope. 



Chapter 11 

PROJECTING SYSTEMS AND OTHER SPECIAL OPTICAL DEVELOPMENTS 

By Theodore Dunham, Jr. and James G. Baker a 


T wo requirements for special projecting 
systems arose under Section 16.1 of NDRC. 
One of these was based on a Service request, 
while the other arose in connection with ex¬ 
perimental work on the binocular testing pro¬ 
gram. These are described in this chapter, to¬ 
gether with two special optical developments 
of different kinds. 

111 WIDE-FIELD PROJECTOR FOR 
DOME TRAINER 

Under Project NA-200, the Special Devices 
Division of the Bureau of Aeronautics requested 
that a projector be designed and constructed 
which would project on the inside of a dome, 
with a radius of 9 ft, moving images of 
aircraft for training purposes. A field of 180 
degrees horizontally and 90 degrees vertically 
was desired, with a relative aperture of about 
f/2. This imposed very severe difficulties in the 



matter of optical design. Nevertheless, an op¬ 
tical system which appears to be entirely satis¬ 
factory was developed. 

a Dr. Dunham is Chief of Section 16.1 NDRC and Dr. 
Baker is at the Harvard College Observatory. The 
first two sections of this chapter have been written by 
Dr. Dunham; the third section by Dr. Baker. 


Figure 1 shows a layout of the optical system 
finally developed by the Yerkes Observatory 1 
under Contract OEMsr-1078, based to a consid¬ 
erable extent on suggestions made by the Spe¬ 
cial Devices Division. An f/2 modified Petzval 
lens with a field of about 14 degrees is used in 
combination with half of a spherical mirror 


Table 1 . Optical design of a wide-field projector 
for dome trainer. 




Thickness 





or 



Ele¬ 

Radius 

separation 



ment 

(in.) 

(in.) 

Glass 

Remarks 

Film 

Flat 

1.795 

air 

Adjust to focus 
image on dome 


—3.884 




1 

—1.765 

0.237 

DBC-1 

1.625" OD 



0.007 

air 



+3.633 




2 

—5.138 

0.236 

DBC-1 

1.625" OD 



0.158 

air 



—2.252 




3 

+24.37 

0.105 

EDF-1 

1.625" OD 



1.891 

air 



—4.989 




4 

+4.499 

0.135 

EDF-1 

2.08" OD, Bevel 
to 1.70" on rear 

5 

—2.265 

0.503 

BSC-2 

2.08" OD 



0.007 

air 



+ 13.44 




6 

—10.16 

0.285 

BF-2 

2.25" OD 



55.35 

air 

Path bent by 
plane mirrors 

7 

—9.265 

144 

Front surfaced mirror 
Approximate distance to 




dome 



12 in. in diameter, which increases the field by 
a factor of ten. The mirror has little effect on 
spherical aberration and coma, and no effect on 
chromatic aberrations. The curvature of field 
of the mirror is opposite in sign to that of the 
lens, and they are made to cancel one another 


472 








HIGH-RESOLUTION PROJECTION LENS 


473 


to a close approximation. The astigmatism of 
the lens is varied to counteract, as nearly as 
possible, that introduced by the mirror. A 6-ele¬ 
ment lens is capable of reducing higher order 
aberrations to the point required by the 330 X 
magnification which is specified for this pro¬ 
jector. There is considerable distortion, but this 
is not serious. 


the glass surfaces of the lens elements. A narrow 
field was entirely acceptable, so that off-axis 
aberrations were unimpprtant. For this reason 
it was satisfactory to use a simple achromatic 
doublet, corrected as well as possible for spher¬ 
ical and chromatic aberration. The design was 
undertaken by the Yerkes Observatory under 
Contract OEMsr-1078. la 



Figure 2. Optical design for wide-field projector. 


The details of the optical design are shown 
in Figure 2 and in Table 1. In the final installa¬ 
tion it is intended that the path should be bent 
by two reflections, as shown in Figure 1, so as 
to yield a compact system with the light path 
clearing both the projector and the head of the 
observer. 

Figure 3 shows the projector set up tempo¬ 
rarily for testing in a room, with the projected 
image of a coarse reticle. Illumination and res¬ 
olution were adequate. It seems likely that this 
design will satisfactorily serve the purpose for 
which it was designed. 


112 HIGH-RESOLUTION PROJECTION 
LENS 

In connection with the binocular testing pro¬ 
gram at Dartmouth College (Contract OEMsr- 
1058), a projector of high optical performance 
was required for projecting on a screen images 
of small black targets with the absolute mini¬ 
mum of light directed into the images as a 
result of aberrations or of actual scattering by 


In view of the fact that the images were to 
be observed at low levels of illumination (sco- 
topic vision), it seemed desirable to adjust the 



Figure 3. Reticle image projected on dome. 


color correction to take account of the shift in 
effective sensitivity of the eye which occurs 
when passing from high to low levels of illu¬ 
mination. At 10 -2 millilamberts, the maximum 

















474 


PROJECTING SYSTEMS AND OTHER DEVELOPMENTS 


absolute sensitivity of the eye 2 is at 5,060 A. 
The relative visual stimuli at low levels of il¬ 
lumination were computed, taking account of 
the energy distribution in a black body source 
at 3,000 K and of the rate of change of refrac¬ 
tive index with wavelength. Figure 4 is a plot 



ANGSTROMS 

Figure 4. Relative visual stimuli at low levels of 
illumination. 


showing the relative response of the eye per 
unit range of refractive index, as a function of 
wavelength. This curve shows that the maxi¬ 
mum stimulus lies at 5,250 to 5,330 A, and that 
the curve is reasonably symmetrical. Hence, 
the minimum focal length of the doublet should 
be placed at about 5,330 A, with the focal 
lengths for F and D approximately equal. If the 
lens is achromatized for the mercury green 
line (5,461 A), then D and F will produce a 
response about 10 per cent as great as that 
produced by light having the wavelength of the 
green line. 

The design for a lens corrected as well as 
possible for minimum wavelength at 5,461 A is 


Table 2. Optical design of high-resolution projec¬ 
tion lens. 


Radius (in.) 

Thickness (in.) 

Glass 

Tin 

V 

+213 ± 10 





+ 9.34 ± 0.03 

0.325 ± 0.01 

DF-1 

1.6058 

38.1 

— 16.85 ± 0.10 

0.525 ± 0.01 

LBC-2 

1.5727 

57.3 


shown in Table 2. The design takes account of 
the finite distance of the image, which in this 
case is 1,100 in. The source is located 30.5 in. 


from the surface of the flint element. The lens 
has a relative aperture of //9. The aberrations 
of this lens, for an axial image, are shown in 
Figure 5. 



Figure 5. Angular ray aberrations for projection 
lens, in mils. (Each interval on abscissa represents 
one-quarter aperture of the lens.) 


A lens was designed at a later date for use 
as a precision collimator at the Massachusetts 
Institute of Technology [MIT] (Contract 
OEMsr-203), which was to deliver parallel 
light. It was found that the only change re¬ 
quired was to increase the radius of curvature 
of the first surface from 213 to 278 in. in the 
design of the projector lens shown in Table 2. 

Models of these lenses were made at the Ray 
Control Company in Pasadena. Some were re¬ 
figured at Harvard (Contract OEMsr-474) and 
were carefully cemented in cells to avoid strain. 
Tests showed their performance to be excellent. 
These designs are likely to be of interest for 
other applications. 


113 SPECIAL OPTICAL DEVELOPMENTS 
n.3.1 ^ Plastic Condenser Lens 

Under Contract OEMsr-474 in the summer 
of 1943 an //1.5 condenser lens of plastic and of 
8 in. clear aperture was completed. 3 The glass 
condensing lens already in use in the field was 
much too heavy for general use and also gave 
too large a circle of confusion for the photo¬ 
electric application involved. The circle of con¬ 
fusion for all light from the condenser was to 
have a diameter of 3 mm. 

A 2-element condenser of Lucite was made 
up in accordance with the following table. 















SPECIAL OPTICAL DEVELOPMENTS 


475 


Table 3. Design data for an 8-in. //1.5 condenser. 


Surface 

Radii 

(in.) 

Separations 

(in.) 

Material 

1 

11 

1.1 

Lucite 

2 

piano 

0.0 


3 

6 

1.0 

Lucite 

4 

11 




The form of condenser lens chosen was not 
color corrected but had very much reduced 
spherical aberration as a result of the division 
of the convergence between two lenses. The 
color residual proved small enough for the pur¬ 
pose. The weight of the final plastic lens system 
mounted in sheet metal was less than 5 lb. The 
system is to be recommended for general use 
where light weight and a 3-mm circle of con¬ 
fusion are to be used. In production, such a 
condenser system should be made of molded 
plastic lenses. 


11-3,2 A Cemented Triplet Coronagraphic 
Objective 

At the request of the Navy, NDRC undertook 
to provide under Contract OEMsr-474 (Har¬ 
vard) a cemented triplet coronagraphic objec¬ 
tive. 351 Most of the work was completed during 
1945 and delivery was made to the Navy late in 
the year. 

The coronagraph is a simple type of telescope 
designed for the purpose of producing an arti¬ 
ficial eclipse of the sun. The light from the sun 
is focused by means of a simple objective lens 
onto an opaque disk just slightly larger than 
the image of the sun. In this way practically 
all of the sunlight is eliminated from the re¬ 
mainder of the optical system. The light from 
the solar corona, the object of study, is col¬ 
lected again by a field lens immediately behind 
the opaque disk and directed toward a projec¬ 
tion lens system which refocuses the corona 
onto a photographic film. Generally a color 
filter, or more recently, a quartz monochromator 
filter (see Chapter 9) is used for the purpose of 
restricting the color range of the scattered light 
within the atmosphere and the instrument. The 
light from the corona is in part made up of 
monochromatic radiations. Consequently, by the 


use of filters, the image of the corona in these 
special wavelengths can be greatly increased in 
contrast, relative to the general background of 
sunlight. 

By far the most sensitive part of the system 
is the objective lens. The coronal light is only 
perhaps a millionth as bright as the sunlight 
over the visual range of spectrum, and much 
of this light comes from a continuous spectrum. 

The slightest defect in the coronagraphic ob¬ 
jective lens will scatter more stray light around 
the occulting disk than comes from the corona 
directly. A bubble only 0.1 mm in diameter 
cannot be tolerated. Seeds, bubbles, striation, 
sleeks, scratches, etc., must all be eliminated by 
provision of a nearly perfect lens. The light 
diffracted from the edge of the objective around 
the occulting disk is trapped farther down the 
tube by making the clear aperture of the fol¬ 
lowing projection system smaller than the 
image of the objective focused on the projec¬ 
tion lens by the field lens. Within this focused 
image of the objective it is also possible to 
eliminate any fixed spots like minute bubbles. 
In general, however, it is almost an absolute 
necessity that the objective lens be nearly per¬ 
fect. 

' Spectrographic use requires that the light 
coming around the edge of the occulting disk 
be focused onto a slit tangential to the image 
of the disk. The spectrograph serves as its own 
filter, but requires that the image of the corona 
in the several colors be in sharp focus on the 
slit face. With a simple objective lens not only 
is the corona image in widely separated colors 
out of focus but also the out-of-focus light of 
the sun in colors other than that for which the 
size and position of the occulting disk are deter¬ 
mined, spills onto the slit. This additional illu¬ 
mination of the grating within the spectro¬ 
graph will cause considerable scattered light 
and reduction of the exposure time below what 
is needed to obtain the coronal lines. 

Owing to the importance of the Navy pro¬ 
gram making use of solar observations, it was 
thought necessary to attempt a nearly perfect 
cemented triplet objective. For this purpose an 
ordinary doublet of crown and flint would have 
been impractical. The coronagraphic lens is 
cleaned rather frequently. A flint-glass surface 







476 


PROJECTING SYSTEMS AND OTHER DEVELOPMENTS 


would not be able to withstand such cleanings 
for long under the critical conditions of use. 

The triplet coronagraphic objective form was 
used in order to have the hardest type of optical 
glass on the outside. The design adopted was 
that of the ordinary cemented doublet with 
nearly equiconvex first element. The rear ele¬ 
ment was scarcely more than a cover plate, al¬ 
though allowance was made in the design for 
its presence. The design is given in Table 4. 

The system is well corrected for spherical 
aberration and color but not for coma. The re¬ 
quired field of view is so slight that coma is of 
no importance. 

The lenses were made of glass selected for 
freedom from all detectable defects. Fine grind¬ 
ing was carried to unusual length in order to 
remove any vestiges of subsurface defects that 


would ultimately impair the performance. The 
assembled lens system gave a sharp clear image. 
At the time of delivery, the meticulous cement¬ 
ing task had not been carried out and was 


Table 4. Optical design of the coronagraphic 
triplet objective, / = 9.6 in. 


Surface 

Radii 

(in.) 

Separations 

(in.) 

Indices 

Glass types 

1 

44.19 

0.89 

1.517 

BSC-2 

2 

—41.04 

0.77 

1.617 

DF-2 

3 

—496.0 

0.89 

1.517 

BSC-2 

4 

—556.2 





expected to be accomplished later under Navy 
direction. The three lenses individually were 
free from detectable defects when illuminated 
in a dark room by means of a narrow beam of 
light from a 250-w sealed beam reflector. 








Chapter 12 

REFLEX SIGHTS 

By John W. Evans a 


F ive organizations under contract with 
OSRD did extensive work in the develop¬ 
ment of reflex sights. They are the Yerkes 
Observatory of the University of Chicago 
[Yerkes], the Mount Wilson Observatory of the 
Carnegie Institution of Washington [MWO], 
the Institute of Optics of the University of 
Rochester [Rochester] and the Polaroid Cor¬ 
poration [Polaroid] under contract with Divi¬ 
sion 16, NDRC, and the Eastman Kodak Com¬ 
pany [Eastman] under contract with Division 
7, NDRC. The work consisted in the design of 
optical systems, and the construction of sample 
sights for optical and service tests. Several new 
types, such as the Lens Mangin, Bowen, Fly’s 
Eye and solid sights, were devised and success¬ 
fully demonstrated, while others were investi¬ 
gated theoretically and rejected as unsatis¬ 
factory. 


12 1 CHARACTERISTICS OF REFLEX 
SIGHTS 

Introduction 

The advent of military aviation and aerial 
gunnery brought peculiar sighting problems 
which were not satisfactorily met by any exist¬ 
ing forms of iron sights or telescope sights. A 
good aircraft sight must fulfill the following 
requirements: 

1. The gunner must be able to see his aiming 
pattern superimposed on the target at infinity 
to obviate the necessity for accommodation in 
sighting. 

2. The gunner should have as much eye free¬ 
dom as possible, i.e., the position of his eye 
should not be rigidly restricted as it would be, 
for example, if he were using a telescope sight. 
The process of bringing the sight to bear on 
the target involves a certain amount of delicate 

a Harvard University. Formerly with University of 
Rochester. 


manipulation which is usually already hampered 
by considerable acceleration in almost any di¬ 
rection. The necessity for holding the eye in a 
fixed position during the operation is obviously 
a serious handicap. 

3. The pattern seen by the gunner should 
provide some aid to deflection shooting. In other 
words, there must be some means of estimating 
or setting the correct lead for a target moving 
across the line of sight at high speed. 

4. The sight must obscure as little of the 
gunner’s field of vision as possible. 

5. The sight must not interfere with other 
instruments on the panel. This usually imposes 
serious limitations on its size. 

The reflex sight was devised to meet these 
requirements. The first model developed by the 
Army Air Forces was the N-l, designed in 
1933. Since that time the reflex sights used by 
the U. S. Army and Navy have undergone a 
series of improvements and modifications for 
specific purposes. At the outbreak of World 
War II NDRC undertook to develop further 
certain types of reflex sights. At the same time 
the Army and Navy expanded their activities 
in developing reflex sights, and several com¬ 
mercial firms initiated development programs. 

While the present report is concerned solely 
with the activity of OSRD, the reader should 
not have the impression that it was the only 
reflex sight work done during World War II, 
since the programs of other agencies were 
equally important. 

At present the reflex sight is used almost 
universally for aiming the guns of all types 
of military aircraft, and is being introduced on 
certain types of antiaircraft guns. At the time 
of the termination of OSRD contracts the de¬ 
velopment of a reflex sight for the infantry¬ 
man’s rifle was in progress. It is perhaps not 
too much to say that a properly designed reflex 
sight is the best optical device for aiming any 
weapon for which a telescope sight is unsuit¬ 
able, and it appears probable that ultimately 


477 



478 


REFLEX SIGHTS 


these two will be the only sights used for mili¬ 
tary purposes. 


12,1,2 General Description of the Reflex 
Sight 

The elements of a conventional reflex sight 
are shown in Figure 1. They consist of an il¬ 



luminator i, a reticle r, a collimating lens l , and 
a reflex mirror p. 

Light from the illuminator passes through 
the reticle to the lens, which collimates it, and 
thence to the reflex mirror, which projects the 
image of the reticle at infinity in the direction 
00'. The reflex mirror, which is usually a plane- 
parallel plate glass, reflects a fraction of the 
light and transmits most of the remainder. 
Thus, on looking through the reflex mirror in 
the direction 00', the observer sees the bright 
image of the reticle pattern at infinity. 

A typical reticle pattern for a gunsight con¬ 
sists of a central dot, the aiming point, sur¬ 
rounded by a series of concentric circles and, 
in some instances, radial lines. The sight is 
aligned with the gun, or boresighted so that 
the aiming point coincides with the line of fire 
at a given range. In shooting at a rapidly mov¬ 
ing target, the problem is to aim ahead of it, 
by the appropriate deflection, to insure that it 


crosses the line of fire at the instant the pro¬ 
jectiles reach the target’s range. This is accom¬ 
plished by leading the target with the aid of 
the circles of the reticle pattern. Each circle 
indicates the appropriate lead for a given ve¬ 
locity across the line of sight, and hence is 
referred to as a speed ring. The apparent radii 
of the speed rings are commonly multiples 
of 70 mils, which is the lead for a velocity of 
100 mph across the line of sight for a mean 
bullet velocity of 2,100 fps. 

The method of aiming is shown in Figure 2, 



Figure 2. The gunner’s view of speed rings and 
target. 

which depicts the gunner’s view through the 
sight. The dotted line indicates the path of a 
target relative to the gun, with a component of 
velocity across the line of sight of 200 mph. The 
gun is aimed by placing the 200-mile speed ring 
on the target in such a position that the pro¬ 
jected velocity passes through the aiming point. 
The gun is then swung to follow with the target 
in this position on the reticle pattern and is 
fired as long as the aim can be maintained or 
until the target disintegrates. 

Recently, lead-computing mechanisms have 
been introduced which automatically deflect the 
sight with respect to the gun. The gunner 
merely keeps the aiming point of his sight on 
the target. Reflex sights for this use do not re¬ 
quire speed rings. 


121-3 Characteristics of Reflex Sights 

The arrangement of optical parts in a reflex 
sight can be varied in many ways, but its func- 








CHARACTERISTICS OF REFLEX SIGHTS 


479. 


tion is always the same—to project a collimated 
image of a reticle pattern in the direction of 
the point of aim. For example, Figure 3 shows 



Figure 3. An unconventional type of reflex sight. 

a variation in which a partially reflecting mir¬ 
ror serves both as collimator and reflex plate. 
Regardless of its form, there are certain fea¬ 
tures of any reflex sight which determine its 
usefulness for a given purpose, and furnish a 
basis for comparison with other sights. They 
can be enumerated as follows: 

1. Brightness and uniformity of the projected 
image. 

2. Eye freedom. 

3. Apparent radius of reticle pattern. 

4. Parallax on and off axis. 

5. Chromatic blurring of the pattern. 

6. Transmission in the direction 00'. 

7. Reliability and ease of repair in case of 
failure. 

8. Size (interference with other equipment). 

9. Obstruction of the observer’s field of view. 

10. Power consumption and heat dissipation. 

A discussion of each of these characteristics 

follows. 

Brightness and Uniformity of 
Projected Image 

The brightness of the projected image deter¬ 
mines the visibility of the reticle pattern against 
the target area. Let B 0 be the brightness of 
the target area, and B 1 the brightness of the 
reticle pattern which would be seen by looking 
directly into the collimator lens of the sight 
without any reflex mirror. Let P be the fraction 
of light reflected and r, the fraction transmitted 
by the reflex mirror. To an observer using the 
assembled sight, the target area appears with 
brightness of B 0 t and the reticle pattern with 
brightness B 0 t + B 1P . The visibility of the pat¬ 
tern can be expressed then as the fraction: 

t t _ Bqt + Bip _ Bip 

B 0 r + Bqt 


Although the eye is quite sensitive to small 
discontinuities in brightness and can readily 
detect differences of 5 per cent, the minimum 
value of V at daylight levels of illumination 
which will permit a pattern of narrow lines of 
the order of 1 mil wide, or small dots, to be 
instantly visible without a short search is prob¬ 
ably about 1.2 to 1.3. As the illumination of the 
landscape decreases, the minimum usable value 
of V increases. In the light of the full moon, 
for instance, V should be at least 3.0 for targets 
against the sky with 1-mil lines. 

The necessary brightness of the reticle pat¬ 
tern is fixed, then, by the brightest target area 
against which it is to be projected. An antiair¬ 
craft sight must be usable against the brilliant 
translucence of a hazy sky in the neighborhood 
of the sun, which may attain a brightness of 
thousands of lamberts. A sight for use on 
ground targets, on the other hand, generally 
requires much less light since ground areas 
rarely exceed 5 lamberts in brightness. How¬ 
ever, sky line targets with background bright¬ 
nesses of 30 or 40 lamberts may be encoun¬ 
tered, and such targets must not infrequently 
be viewed against the much brighter sky near 
the sun. If all contingencies are to be antici¬ 
pated, the sight for ground targets should meet 
the same brightness requirements as the anti¬ 
aircraft sight. In many instances it is sufficient 
to provide a dense neutral filter which can be 
flipped into position between the reflex mirror 
and the target to reduce the apparent bright¬ 
ness of the target area so that a less brilliant 
reticle pattern can be used. 

Unfortunately, no quantitative data are avail¬ 
able on the reticle brightnesses of most of the 
sights which have been developed under OSRD, 
although satisfactory estimates of visibility can 
be made for the daylight illuminated sights. 

A second important characteristic of the pro¬ 
jected reticle image is the uniformity of its 
brightness. Unless due care is taken in design¬ 
ing the illuminator, particularly in a wide-angle 
sight, different parts of the reticle image are apt 
to show differences in brightness. Usually the 
center of the pattern appears brightest. One also 
frequently finds that the brightness of the whole 
pattern varies as the observer’s eye moves 
around in the eye space. It is not difficult to 








480 


REFLEX SIGHTS 


avoid either of these faults singly, but the 
simultaneous elimination of both, combined 
with the achievement of great reticle bright¬ 
ness, requires the even and powerful illumina¬ 
tion of both reticle and the exit pupil. In large- 
aperture wide-angle sights illuminated by elec¬ 
tricity this becomes a major problem. 

The reason for the difficulty lies in the pe¬ 
culiar inefficiency of the reflex sight in utilizing 
the available light. Fortunately it is not difficult 
to use a condenser which directs most of the 
light of the lamp onto the reticle. However, 
only a small fraction of the light which falls on 
the reticle passes through the narrow openings 
of the pattern. Unless a carefully designed con¬ 
denser is used, a large part of the light which 
does get through the reticle openings is gen¬ 
erally outside the solid angle containing the 
collimating lens, and so is wasted. Finally, a 
bare-glass reflex mirror is usually employed 
which reflects only about 9 per cent of the light 
emerging from the lens. 

The ideal illuminator would be one which 
concentrates most of the light of the lamp on 
the openings of the reticle pattern in a solid 
angle which just fills the exit pupil. Such a 
device could doubtless be designed, but it would 
be expensive and bulky. Since plenty of power 
is usually available, it has seemed more prac¬ 
tical to use simple illuminators, and make up 
for the losses by using lamps of relatively high 
wattage in most of the OSRD sights. Accepta¬ 
ble reticle brightnesses are attained, and the 
visibility of the pattern against unusually 
bright target areas can be enhanced to any 
desired extent with the aid of a neutral absorb¬ 
ing filter mounted so it can be moved into posi¬ 
tion just beyond the reflex mirror. This has the 
effect of reducing r without affecting p. 

The brightness of the projected reticle pat¬ 
tern is completely independent of the focal ratio 
of the collimating lens. However, unless the il¬ 
luminator is designed to just fill the lens it will 
be easier to illuminate a sight with a high aper¬ 
ture ratio than one with a small ratio, since the 
latter has the larger reticle area to be illumi¬ 
nated. 

One of the most interesting developments in 
the OSRD reflex-sight projects is the use of 
daylight for illumination. Its prime advantage 


is that the visibility of the reticle pattern can 
be made approximately independent of the 
brightness of the target area. Hence, the pat¬ 
tern remains conspicuous against the most 
brilliant sections of the sky. Attendant advan¬ 
tages are the simplicity of the optical system 
and the reliability of the illumination. Against 
the advantages, there is the disadvantage that 
the reflex mirror must be coated with a par¬ 
tially reflecting film to raise P to the neighbor¬ 
hood of 0.5. This means either a complicated 
sandwich-type mirror with the film protected 
by a covering glass or a toughened film on an 
external surface of a one-piece plate. Even the 
toughest films are not quite as durable as bare 
glass. Since it is doubtful whether even dia¬ 
mond would withstand repeated G.I. cleanings 
without some evidence, the external films pre¬ 
sent a problem. 

If the sight is to be used at night as well as 
by day, it is necessary to provide an auxiliary 
electrical illuminator. But since the target area 
at night is hundreds of thousands of times 
fainter than in daylight the achievement of suf¬ 
ficient reticle brightness is no problem, and an 
extremely simple (but carefully designed) 
illuminator is adequate. For the utmost effi¬ 
ciency in night use, it would also be desirable 
to have a bare-glass reflex mirror readily inter¬ 
changeable with the coated mirror used in day¬ 
light. 

One of the several possible types of daylight 
illuminated sights is shown in Figure 4. Its 
great simplicity is apparent at once. The reticle 
is illuminated by the target area itself. The 
apparent brightness of the reticle B r equals 
kB 0 , where k is the transmission of the sight 
without the reflex mirror for light which enters 
through the reticle. The visibility is then: 

V = fc - + 1. 

T 

It is independent of the brightness of the target 
area. Typical values of k, P , and r are respec¬ 
tively 0.65, 0.4, and 0.6, and V equals 1.43. 

Eye Freedom 

Eye freedom is one of the most prominent of 
the advantages of the reflex sight. There is a 
region referred to as eye space behind the re- 



CHARACTERISTICS OF REFLEX SIGHTS 


481 


flex mirror from every point of which the en¬ 
tire reticle can be seen. The datum of practical 
interest is the diameter of the cross section of 
the eye space perpendicular to the line of sight 
at the observer’s normal eye distance. The lim- 



Figure 4. Daylight illuminated sight. 



iting ray from the edge of the reticle through 
the near edge of the lens in the same meridian 


plane is the boundary of the eye space. Figure 
5 shows the limiting rays and the geometry of 
the eye space. The lettered parts and distances 
are as follows: 

r reticle (of angular radius 8) 
l lens 

V image of lens reflected in first surface of 
reflex mirror 
E plane of exit pupil 
0 eye point at greatest eye relief 
N plane of normal eye position 
d distance V to E 
e distance E to 0 
k distance from edge of sight to 0 
A aperture of lens l 
a aperture of exit pupil 
/ diameter of circle of eye freedom 
p distance N to 0 

The exit pupil of a reflex sight nearly always 
coincides with the lens aperture. However, 
since the Mount Wilson group has made some 
notable exceptions to this rule, the more gen¬ 
eral case is considered. 

The greatest eye relief is simply the distance 
at which the exit pupil appears filled by the 
reticle pattern with an apparent radius of 6. 
Hence, 


e = ~ cot 8. 

The eye space consists of a pair of cones based 
on the exit pupil with apices at O and O', both 
of which are at a distance e from the E plane. 
It is represented in Figure 5 by the shaded 
area. The only part of the eye space of practical 
interest is that portion which lies beyond the 
edge of the sight where it can be reached by 
the observer’s eye. It is completely specified by 
the distance k, the angle 8 and, if the exit pupil 
is between the edge of the sight and 0, as is 
rarely the case, the distance e. If the normal 
eye distance is known, the eye freedom, /, can 
be calculated from the relation: 

f = 2p tan 8. 

The foregoing calculations are based on the 
assumption that the sight is circularly sym¬ 
metrical. If the reticle or collimating lens are 
noncircular, the calculation would have to be 
carried out for the various meridian planes of 
































482 


REFLEX SIGHTS 


interest. Usually it is sufficient to know the 
vertical and horizontal eye freedom. 

Apparent Radius of Reticle Pattern 

The apparent radius of the reticle pattern is 
an important specification of a sight. It de¬ 
pends, however, on the off-axis correction of 
the lens and the capacity of the illuminator to 
fill a large reticle uniformly, and hence is not 
an independent characteristic of the sight 
which can be adjusted at will merely by making 
a reticle of the appropriate size. 

Parallax 

The parallax of a sight determines the accu¬ 
racy of aim. If a sight has the mythical per¬ 
fectly corrected lens, the projected reticle pat¬ 
tern appears to remain perfectly stationary 
against a distant background as the eye is 
moved from one side of the lens aperture to the 
other. If, however, it has an imperfectly cor¬ 
rected lens, the pattern appears to shift against 
the distant background, and the aim will be 
different for different positions of the eye. The 
extreme apparent shift (usually measured in 
mils) as the eye moves across the diameter of 
the lens has been termed the parallactic range, 
denoted by P, by the Yerkes Observatory work¬ 
ers. We shall retain the notation here. 

Generally, P varies from center to edge of 
the reticle pattern. At the center it is due to 
spherical aberration alone, while off the axis 
coma and astigmatism become effective. Curva¬ 
ture of field is effectively compensated by use of 
a curved reticle, and in instances where the 
reticle pattern consists entirely of concentric 
circles or radial lines the parallactic shift due 
to astigmatism can be well corrected by curving 
it to the primary or secondary focal surface. 1 

The tolerances in P usually requested by the 
Armed Forces can be expressed roughly by the 
formula, 

P = 2 0.0150 

where 0 is the angle off axis expressed in mils. 
The reason for the larger tolerance off axis is 
that the accidental error of the gunner’s esti¬ 
mate of deflection increases with the deflection. 
While it is certainly desirable to hold the syste¬ 


matic errors due to imperfection in the sight to 
a minimum, a systematic error of 2 mils (devi¬ 
ation from the mean direction) in the 300-mile 
speed ring has little practical effect when the 
accidental error is probably not less than 15 
mils. 

Chromatic Blur 

Chromatic blurring of the reticle pattern 
occurs when the collimating lens is imperfectly 
corrected for color. An off-axis point in the 
reticle appears as a short radial colored line, 
and a tangential line is broadened out into a 
colored band. The result is that the position of 
the dot or line is indeterminate. The simplest 
and most effective preventative for this defect 
is a collimating lens well corrected for color, 
although the use of color filters is very helpful 
if the illuminator can stand the additional 
strain. Tolerances for chromatic blur between 
the C and F lines should not be greater than 
those for parallax. 

Transmission of Reflex Mirror 

The transmission of the reflex mirror de¬ 
serves consideration in instances where it 
occupies only a small part of the observer’s 
field of view. It is undesirable to have a violent 
discontinuity in the brightness of the landscape 
at the edge of a reflex mirror. It has been ob¬ 
served that a loss of 50 per cent is very rarely 
noticed by military personnel, and it is likely 
that there would be no objection to transmis¬ 
sions as low as 20 per cent unless attention 
were specifically called to the fact that it is only 
20 per cent. 

If a sight is to be used at night, any loss in 
transmission is, of course, objectionable. Tests 
at the University of Rochester of visibility of 
objects against the night sky indicate that the 
range at which a target could be seen is about 
20 per cent less when seen through a 50 per 
cent transmitting plate than with the unim¬ 
peded eye. 

The reason for mentioning low transmission 
here is that it is the unavoidable concomitant 
of high reflecting power in a reflex mirror, and 
any use of partially reflecting films on the mir¬ 
ror to increase the visibility of the reticle pat- 



TYPES OF OPTICAL SYSTEMS FOR REFLEX SIGHTS 


483 


tern by increasing P results in a decrease in t, 
since p + t cannot exceed 1.0. 

Reliability 

The reliability of a sight needs little com¬ 
ment. Perhaps the two most common sources of 
failure of reflex sights are the electric lamps 
and the reflex mirrors. It is wise to provide for 
quick and one-handed replacement of either of 
these parts. 

Size 

For most military purposes the sight should 
be as compact and as light as possible. This is 
particularly true of sights for use in aircraft 
where the user is figuratively slid into place 
with the aid of a shoehorn. The military re¬ 
quirements in size will not be completely satis¬ 
fied until a sight of negative volume is devised. 

One of the most promising means for reduc¬ 
ing the overall size of the sight in fighter 
planes, and thus diminishing interference with 
other instruments and the pilot’s field of view, 
is to eliminate the reflex mirror and to reflect 
the collimated beam from the armor glass. This 
requires a close tolerance on the parallelism of 
the two surfaces of the armor glass, but experi¬ 
ence has already shown that this can be 
achieved. Instrument panels should be designed 
in the future to take account of the specific 
sight whether the armor glass is used or not. 
This is a requirement that, for some strange 
reason, has been almost universally disre¬ 
garded in the past. 

Obstruction of Field of View 

Large obstructions in the observer’s field of 
view are usually very objectionable because 
they can hide a target and delay its discovery. 
As far as possible the body of the sight is 
placed outside the normal field of vision and 
only an unframed reflex mirror is allowed to 
project. 

Power Consumption 

The power consumed in the illuminator of a 
sight is not ordinarily of any importance in it¬ 
self. However, since the heat losses are roughly 
proportional to the power input, an observer 
who works in very close quarters will be inter¬ 


ested in an efficient cool illuminator of low 
power consumption. A 100-w lamp can, under 
some circumstances, be a most uncomfortable 
neighbor. In extreme cases it may be necessary 
to provide a cooling system to remove un¬ 
wanted heat. 


12.2 TYPES of optical systems for 
REFLEX SIGHTS 

Several distinct types of optical systems for 
reflex sights have been investigated under 
OSRD to meet the various requirements of eye 
relief, bulk, and accuracy. They are described 
briefly below with as little reference as possible 
to the specific sights of each type. 


12,21 Lens Sights 

The majority of reflex sights are of the “con¬ 
ventional” type shown in Figure 1, with a lens 
for a collimator, and it may be seen in the table 
in Appendix I to this chapter that the lens 
collimators outnumber all other types combined 
developed by OSRD. Their functioning is per¬ 
fectly straightforward and requires no descrip¬ 
tion. The problem of design is similar to that 
for a camera lens, but is simplified by the fact 
that the field can be given any desired curve. 


12,2,2 Lens-Mangin Sights 

The Lens-Mangin la sight has an ingenious 
optical system originated at the Yerkes Ob¬ 
servatory for use where very large exit pupils 
are necessary. The main work of collimating 
the light is performed by a spherical mirror, 
some of the aberrations of which are corrected 
by refracting surfaces. 

The system is shown in Figure 6. Light from 
the reticle r is reflected by a partially reflecting 
dividing plate d to the Mangin mirror m, which 
has a lens surface on the front and a reflecting 
surface of silver or aluminum on the back. 
After reflection from m the light passes through 
d to a weak lens l and the reflex mirror p. This 
optical system differs from the simple Mangin, 



484 


REFLEX SIGHTS 


which is well corrected for color and spherical 
aberration, in the presence of the lens l, which 
controls the coma. The result is an excellent 



Figure 6. Lens-Mangin sight. 


system with P — 1 mil for rings with radii up 
to 100 mils in focal ratios as high as 1.3. 

The principal disadvantage of the Lens-Man¬ 
gin type of sight is the loss of at least 75 per 
cent of the light in the dividing plate. However, 
it was found possible to obtain satisfactory visi¬ 
bility with moderate power input. 


12 2 3 Solid Sights 

For applications where large eye relief is un¬ 
important (infantryman’s rifle, antiaircraft 
guns, flexible aircraft guns, etc.) sights can be 
made relatively small and in some instances 
it was found that there were both optical and 
mechanical advantages in making the sight a 
solid block of glass or plastic. With one excep¬ 
tion, all of the OSRD solid sights have the same 
general form as that shown in Figure 7. It 
consists of three pieces, two of which are ce¬ 
mented together with a half-reflecting film 
between them to form the reflex mirror d, and 
the third of which is simply a first-surface 
spherical mirror m which bears the main bur¬ 
den of collimation. The lens surface l immedi¬ 
ately in front of the mirror corrects the spheri¬ 


cal aberration, and the collimator is essentially 
a Mangin system with an air lens. In the assem¬ 
bled sight, light proceeds from reticle r, 
through the dividing surface d to mirror m. 
The collimated beam returns from m to the 
observer’s eye by reflection from d. The sur¬ 
faces h and k must, of course, be plane. 

Another form of solid sight consists simply 
of a block of glass with a reticle at one end, 
and a lens surface and one separate lens ele¬ 
ment at the other. The only example of this 
type designed under OSRD is the solid “Fig- 
ure-4” sight shown in Figure 14B. 

The most conspicuous optical feature of the 
solid sight is the fact that its equivalent focal 
length in air is smaller by a factor of n (index 
of refraction) than the focal length in glass. 
This means that the field angles in glass are 
multiplied by n when the light emerges into 
air. The reticle of the solid system is thus 



Figure 7. Solid Mangin sight. 


smaller than that of an air system of the same 
length and field, and the off-axis aberrations are 
smaller. This advantage is accompanied, how¬ 
ever, by the unavoidable introduction of some 
lateral color. 

The mechanical advantage of the solid sys¬ 
tem is obvious. The parts are held accurately in 
position without the necessity of a complicated 
mounting and in small sizes the resulting unit 
is very simple to make and very rugged. Ex- 






















TYPES OF OPTICAL SYSTEMS FOR REFLEX SIGHTS 


485 


perience indicates, however, that the cemented 
diagonal in the Mangin form is a weak point 
which must be protected in mounting. 

In some instances the smaller reticle permits 
a very decided gain in compactness when the 
optical system is to be folded as it is in the 
“Figure-4” sight. 

The solid Mangin sights suffer from the same 
large loss of light as the Lens-Mangin type at 
the dividing surface, and since most of them 
have been designed for daylight illumination 
this is serious. An ingenious device for reduc¬ 
ing this loss has been proposed by the Mount 
Wilson workers. If the dividing surface con¬ 
sists of a series of alternating layers of dielec¬ 
tric material of high and low indices such as 
zinc sulfide and cryolite, and if the refractive 
index of the glass is properly chosen, the light 
is reflected at the Brewster angle, and the re¬ 
flected light is highly polarized while the trans¬ 
mitted light is polarized at right angles to it. If 
now a quarter-wave plate is inserted between 
the dividing surface and the mirror, the plane 
of polarization of the collimated beam is rotated 
90 degrees with respect to that of the light re¬ 
flected by the dividing surface, and is trans¬ 
mitted by that surface with no appreciable loss. 
Thus, it is theoretically possible to increase the 
product of transmission and reflection of the 
dividing surface from 0.25 to 0.50, doubling the 
brightness of the projected image. If the polari¬ 
zation is not complete the product will be some¬ 
where between these limits. Dividing surfaces 
of the required type have been developed at the 
University of Rochester, but have not yet been 
tried in the solid Mangin sights. 


12,2 4 Double Mangin Sights 

It was suggested by the Mount Wilson group 
that a combination of two Mangin mirrors with 
a family resemblance to the Cassagrain tele¬ 
scope might constitute a useful collimator for 
a reflex sight in which a large exit pupil is de¬ 
sired. The reticle is placed in a hole in the 
center of the large primary mirror. The secon¬ 
dary obscures both the direct view of the reticle 
and the inner zones of the primary. To avoid 
the large blind spot the eye is raised well above 


the axis, and the exit pupil is lune shaped. Only 
half of the primary mirror is used. A prelimi¬ 
nary investigation of the aberrations at the 
Yerkes Observatory indicates that for a focal 
ratio of //1 the axial aberrations are small, but 
color is poorly corrected and the field aberra¬ 
tions are large. However the system is ex¬ 
tremely compact. With a focal length of 10 in. 
the sight has approximate overall dimensions 
of 10x5x4 in. (excluding the reflex mirror) 
with an exit pupil about 8x2.5 in. The combined 
effects of aberrations and vignetting are such 
that a field of about 75 mils radius is the maxi¬ 
mum usable. 

It is regrettable that the details of the per¬ 
formance of the double Mangin system have 
not been investigated. On the whole it appears 
to be a promising sight for certain applications. 
For instance, it might be excellent as a lead¬ 
computing sight in a fighter airplane if 
mounted to use the armor glass as a reflex mir¬ 
ror, with the deflection set in by tilting the 
whole sight. 

12 2 5 Schmidt Sights 

The Schmidt optical system shown in Figure 
8 has tempting optical properties for use in re- 



Figure 8. Straight Schmidt sight. 


flex sights. It was found that the off-axis aber¬ 
rations of a system with a focal ratio of //0.7 
are satisfactorily small. For a field of 100 mils 
radius, P = 1.3 mils, and both radial lines and 
speed rings can be used. 












486 


REFLEX SIGHTS 


However the high focal ratio is not a fair in- adaptation of the Schmidt optical principle de¬ 
dication of the compactness of the system, since signed for use in fighter airplanes with a very 
the overall length is slightly more than twice large exit pupil and a minimum of obstruction 
the focal length. Also, the reticle obscures the to the gunner’s view of the instrument panel 
center of the exit pupil, creating a large blind or the field outside the airplane. Its distinctive 
spot. The aspheric correction plate is not the feature is the formation of a definite exit pupil 
serious drawback it would have been before at the normal eye position of the pilot, which 


I 



World War II since methods have been devel¬ 
oped for molding Schmidt plates of the requi¬ 
site quality at the University of Rochester and 
many thousands of plates have been produced 
commercially at a very low cost. 

The disadvantages of the unmodified Schmidt 
system appear to outweigh the advantages, and 
no efforts to develop a reflex sight from it have 
been made. 

However the Bowen sight 2a is a beautiful 


limits the effective aperture to such an extent 
that the usual aspheric element of the Schmidt 
system can be dispensed with. A diagram of the 
main elements of the optical system is shown in 
Figure 9. Its simplicity is apparent. 

The collimator consists of a large spherical 
mirror. An illuminating surface is placed at the 
center of curvature [CC] of the mirror and its 
image reflected by the reflex mirror is the exit 
pupil. The spherically curved reticle is placed 









TYPES OF OPTICAL SYSTEMS FOR REFLEX SIGHTS 


487 


in the focal surface of the collimating mirror 
(very nearly halfway between the mirror and 
CC). Mirror and reticle are concentric. Since 
the axis of the cone of light emerging from any 
point in the reticle is a radius of the mirror, 
the system has no optical axis in the normal 
sense, and hence no coma or astigmatism. The 
parallactic range is therefore the same over the 
whole field, and the reticle size is limited only 
by mechanical conditions. 

Since the spherical aberration of the colli¬ 
mating mirror is not corrected, and is approxi¬ 
mately proportional to the cube of the aperture 
ratio, the diameter of the exit pupil is limited. 
The parallactic range is about 1 mil at //2, the 
aperture ratio adopted for the samples of the 
sight which were constructed. 


12 2 6 Fly’s-Eye Sight 

The advantages in pilot visibility of using 
the armor glass of a fighter airplane for a re¬ 
flex mirror have already been mentioned. The 
accompanying disadvantage is that the aperture 
and hence the volume of the sight must be very 
large in order to attain the necessary eye relief 
when large speed rings are used (as in the 
standard Navy reticle pattern). If the sight can 
be recessed into the fuselage forward of the in¬ 
strument panel, however, large aperture, per 
se, is not difficult to accommodate. The diffi¬ 
culty comes in the volume of sight projecting 
inside the fuselage into a space already 
crowded by the instruments mounted on the 
panel. 

In an effort to secure a large aperture in a 
compact sight of minimum depth the Develop¬ 
ment Department of the Eastman Kodak Com¬ 
pany originated and developed the “Fly’s-Eye” 
Mark 14 sight for use with Navy fighter air¬ 
craft under contract with Division 7, NDRC. 3 
It consists essentially of an array of many 
small reticle and collimator units honeycombed 
together and carefully aligned with each other 
so that the light from corresponding points on 
all the reticles emerges parallel over the whole 
array. Thus, as the observer’s eye scans the 
aperture, passing from one lens to the next, the 
apparent position of the reticle remains fixed 


in space except for minor parallax errors of 
the individual lenses. The small lenses are care¬ 
fully edged to a shape (usually hexagonal) 
which permits them to fill completely the area 
of the array. 

The depth of the collimating system is simply 
the focal length of the individual lenses, and 
the aperture can be made any desired size in¬ 
dependent of the depth. 

The principle of the system will be seen in 
Figure 10. The three essential parts are the 



Figure 10. Elements of the Fly’s-Eye sight. 


lens plate l , the reticle plate r, and the illumi¬ 
nator i. The lens plate carries the small collima¬ 
tor lenses, which will hereafter be referred to 
as lens elements. The reticle plate carries the 
corresponding reticles, one for each lens ele¬ 
ment. The apparent field of the system is re¬ 
stricted mechanically to that obtained with 
reticles of the same diameter as the lens ele¬ 
ments. Wide fields are therefore obtained only 
with lens elements of high aperture ratios. 

Ideally it should be possible to make multiple 
lens collimating systems with depths of only 
IV 2 or 2 mm (diffraction becomes troublesome 
in smaller sizes). But small depths naturally 
require a great many very accurately adjusted 
lens elements and reticles, and the practical dif¬ 
ficulties of assembly and adjustment by the 
present methods become prohibitive. 













488 


REFLEX SIGHTS 


As a sample of a practical depth, the Model 
G sight has a distance of 45 mm from the ret¬ 
icles to the front vertices of the lens elements. 
Since the illuminator is necessarily of the order 
of 50 mm deep, the advantage of further re¬ 
duction in the depth of the collimator is not 
great and is not commensurate with the in¬ 
creased difficulties. 

Although no models of the multiple lens sight 
have been constructed for linkage with a lead¬ 
computing mechanism, it is as easily adapted 
for this use as any of the more conventional 
sights. Either the reticle plate can be moved to 
put in the lead, or the sight as a whole can be 
tilted. Designs for both types were made at the 
Eastman Kodak Company. 


12 3 MODELS OF OSRD REFLEX SIGHTS 

The optical characteristics of all the OSRD 
sights have been collected in tabular form in 
Appendix I to this chapter. Specifications of 
the optical systems are given in Appendix II and 
drawings showing the general forms of the 
optical systems appear in Appendix III to this 
chapter. 

Several sights were designed but not con¬ 
structed. Others were made into table models 
(Class B models, Appendix I) for optical test¬ 
ing but not for Service use. The remaining 
sights were made into production prototypes 
(Class A models, Appendix I) for testing 
under Service conditions. 

All of the Class A models and a few of the 
Class B models are discussed in the remaining 
sections of this report. The pertinent informa¬ 
tion about the others is included in the appen¬ 
dices to this chapter. 


1231 Yerkes L9k (T-95) lb 

The L9k sight was made in response to a re¬ 
quest from the Army Antiaircraft Artillery 
[AAA] Board at Camp Davis for a sight to be 
used with the M45 multiple machine gun 
mount. The sight should be reasonably com¬ 
pact, with speed rings of radii 70, 140, 210, and 
280 mils. Long eye relief was not necessary. 


The optical system of the L9k sight, with an 
aperture of 2.75 in., is shown in Figure 11. It 
is equipped for either daylight or electrical 
illumination. 



Figure 11. Optical system of T-95 (L9k lens). 


The sight was built complete with mounting 
and boresighting attachments. It was sent to 
the AAA Board and successfully tested on the 
M45 mount. 

12 3 2 Rochester T-94 4 

The T-94 sight was designed for the M45 
multiple machine gun turret of the AAA Board 
and the single 50-caliber antiaircraft machine 
gun mounted on half-track trucks of the 
Armored Force for the discouragement of low 
strafing attacks. Long eye relief was not neces¬ 
sary. Two models were made which were essen¬ 
tially alike except for the aperture of the col¬ 
limator and the diameter of field. 

Both are daylight illuminated with half-re¬ 
flecting reflex mirrors and single folded optical 
systems. In both, the collimator lens is a pair of 
identical cemented doublets with a combined 




















MODELS OF OSRD REFLEX SIGHTS 


489 


focal length of 100 mm used with a curved ret¬ 
icle of 31.8-mm radius. The optical characteris¬ 
tics are as follows: 

Model 01: 50-mm aperture, 210-mil field (rings 
only) 

Po — 0.2 P 210 — 3 

Model 02: 60-mm aperture, 280-mil field (rings 
only) 

Po = 0.4 P 280 — 5. 

A section of Model 02 appears in Figure 12 



Figure 12. T-94 sight. 

and a photograph of the sight on its boresight¬ 
ing mount in Figure 13. 

Model 02 is provided with an electric illumi¬ 
nator for night shooting consisting of a light 
aluminum shell with a diffusing surface con¬ 
taining a 3 cp red lamp. The shell bayonets onto 
the sight over the reticle. 

Model 02 was tested on the M45 turret in 
May and June 1944. Its performance was satis¬ 
factory and the sight was adopted. 

Pantagraph Mount for Sight 

In order to use the machine gun sight on the 
single truck-mounted gun, it was necessary to 


mount it with an offset of about 18 in. to clear 
the smoke cloud at the muzzle when the gun is 
fired. After much discussion a flexible mount 
was made. The sight is carried on a parallelo¬ 
gram, two sides of which are parallel to the 
bore of the gun. The corners are pivoted, and 
the sight is moved longitudinally with respect 



G 



Figure 13. T-94 sight. 

to the gun by pressure of the gunner’s head 
against a pad located behind the sight. He can 
thus use it in the most comfortable position, 
which is a function of the altitude at which he 
is firing. The mount also serves to absorb the 
vibration of the gun while it is firing and keeps 
the sight aligned with the bore. A photograph 





























490 


REFLEX SIGHTS 


of the mount with Model 02 machine gun sight 
appears in Figure 13C. 

A wooden model of the pantagraph mount 
was made and tested at Fort Knox. Its per¬ 
formance was excellent. Later a metal mount 



B 


Figure 14. 
sight S-3. 



A. “Figure-4” sight S-l. B. “Figure-4” 


was made, but the war ended before it was 
tested. 


12 ' 3 ' 3 Rochester S-l, S-2, and S-3 4a 

The “Figure-4” sights, S-l, S-2, and S-3. 
were designed in response to a need for a sight 
to replace the Navy Mark 8 which was very 
compact but expensive and optically unsatis¬ 
factory. The //0.85 lens had large parallax and 
chromatic errors and the illumination was both 
insufficient and uneven. 

The principle of the “Figure-4” sights is 
simply to use a lens of moderate focal length 
and fold the optical path with two plane mir¬ 
rors as shown in Figure 14A. The name “Fig¬ 
ure-4” comes from the shape of the optical 
path. The functioning of the various parts will 
be evident from the drawing. Two points de¬ 
serve special mention. 

The reticle fits between the upper edge of 
mirror M 1 and the lower edge of the collimat¬ 
ing lens. An increase in reticle size necessitates 
lowering the position of M 1 and hence an in¬ 
crease in focal length. As a result, the apparent 
field increases more slowly than the reticle size, 
and the overall size of the sight increases very 
rapidly with the size of the field. Hence, the 
“Figure-4” sight is useful for small reticle 
patterns but its advantages disappear when an 
attempt is made to use it as a wide-angle sight. 

The position of the reticle is such that unless 
precautions are taken, two extraneous images 
of it are formed by reflection in the mirror M l 
and the rear surface of the collimating lens. 
They can be suppressed by carefully limiting 
the cone of light from the reticle so that it just 
fills the lens. 


S-l 

The S-l sight, Figure 14A, was designed to 
accommodate a reticle pattern consisting of a 
central dot and a single speed ring of 70 mils 
radius. The lens works at //2.5, with P 0 = 1.3 
and an aperture of 3.5 in. It is a separated 
doublet made of the most easily procured 
glasses available. 

Because of the simple nature of the reticle 
pattern, a very efficient illuminator can be used. 
The source is a 3 cp lamp. The central dot of the 
pattern is a virtual image of the filament of the 
























MODELS OF OSRD REFLEX SIGHTS 


491 


lamp formed in the focal plane of the collimator 
lens by a small negative lens mounted about a 
centimeter inside the reticle. The 70-mil ring is 
illuminated by a spherical mirror with its cen¬ 
ter of curvature at the center of the ring and 
a radius of curvature equal to the radius of the 
ring multiplied by \/2. The distance from cen¬ 
ter of curvature to filament is the radius of the 
ring. The mirror then concentrates a sizable 
fraction of the light from the filament on the 
ring in the reticle. Unfortunately, the tangen¬ 
tial spread of the light passing through a given 
point in the ring is insufficient to fill the colli¬ 
mator lens, and it is necessary to use a diffus¬ 
ing surface in the plane of the ring. 

A working model of the S-l sight with the 
optical parts mounted in wood was made for 
demonstration. The electrical illuminator could 
be removed and daylight used instead. A half- 
reflecting reflex mirror was provided for the 
purpose and worked satisfactorily. 

The model was submitted to the AAF Ma¬ 
teriel Center in August 1942. It was adopted 
and produced in large numbers as the AAF N-9 
sight by the Bell and Howell Corporation. 

S-2 

The S-2 sight was designed for the Navy 
BuOrd to accommodate the standard Navy 
reticle pattern with a radius of 150 mils. The 
focal length was increased to //3, and the re¬ 
sulting sight was rather bulky. Samples were 
made for the Bureau of Ordnance by the Pres¬ 
ton Laboratories. 

S-3 

In order to reduce the physical size of the 
reticle required for the Navy pattern, the S-3 
was designed as a solid sight. It consists of a 
single glass lens and a large prism of plastic 
with one lens surface. The optical path, Figure 
14B, is essentially the same as in the S-l sight, 
and the dimensions are roughly the same. But, 
since, the effective focal length in air is l/n 
times that in plastic, the apparent radius of the 
reticle is n times as great as it would be with an 
air system of the same size. By running the 
radial lines of the Navy reticle pattern into the 
corners, a square reticle slightly smaller than 
that of the S-l sight could be used. The paral¬ 


lactic range for the central dot was 1.3 mils. 

Samples of the S-3 sight with a 3.5-in. aper¬ 
ture were made for the Bureau of Ordnance by 
the Preston Laboratories. The Navy reports 
that they were not satisfactory because it had 
been impossible to obtain thick plastic blocks 
of sufficient optical homogeneity. 

12.3.4 Rochester Flightsight 4b 

In the summer of 1942 the Navy Bureau of 
Aeronautics requested a reflex sight for night 
use in the F4U-2 equipped with radar. The 
sight was to present at infinity the radar oscil¬ 
loscope screen, airspeed indicator, and gyro 
horizon in addition to a reticle for night shoot¬ 
ing. The pilot could then intercept and close 
with an enemy until he made visual contact, 
without having to look away from the position 
where he expected to find him, and without 
changing the accommodation or endangering 
the dark adaptation of his eyes by looking at 
the instrument panel. The instrument con¬ 
structed at the Institute of Optics in response 
to the request was termed the flightsight for 
lack of a better designation. 

The optical system of the flightsight is shown 
in the isometric drawing of Figure 15. Only the 
faces of the instruments are shown. The sight 
is a two-story affair with the radar tube on the 
lower level and the airspeed indicator, gyro 
horizon, and reticle on the upper level. The 
radar screen is seen through the half-reflecting 
mirror M 2 by reflection from the fully alumi¬ 
nized mirror M r . The gyro horizon is seen by re¬ 
flection from M 2 through the half-reflecting 
mirror M 3 . The airspeed indicator, with the 
edge illuminated reticle on its face, is seen by 
reflection from M 2 and M s . 

The faces of the gyro horizon and airspeed 
indicator are masked, leaving only the illumi¬ 
nated tips of the horizon bar and airspeed hand 
visible. Four index marks are ground on the 
glass cover of the gyro horizon and are edge 
illuminated. 

As the pilot looks into the reflex mirror he 
sees the radar screen with an apparent diame¬ 
ter of about 250 mils, around the edge of which 
appear the four fixed indices, the tips of the 



REFLEX SIGHTS 



Figure 15. Optical system of flightsight. 









MODELS OF OSRD REFLEX SIGHTS 


493 


horizon bar, and the tip of the airspeed indi¬ 
cator. At the center is a 25-mil diameter circle 
which serves as an aiming point in shooting 
and an index for the radar indications. The 
layout of the field appears at the upper left of 
Figure 15. For daytime use the mirror M 3 is 
rotated 90 degrees to reflect light from a 



Figure 16. Flightsight. 

brightly illuminated reticle of standard Navy 
pattern into the system. The radar screen can 
be seen directly through an opening in the 
housing between the mirrors M x and M 2 . 

The lens is a separated doublet of 3.5-in. 
aperture and 12-in. focal length with a paral¬ 


lactic range of 0.2 mil. The assembled sight 
with airspeed indicator and gyro horizon in 
place, but without the oscilloscope tube, is 
shown in Figure 16. 

The flightsight was delivered to the Bureau 
of Aeronautics in April 1943 but was not tested 
in flight until August 1944. It was installed in 
a JRB airplane, and after one small alteration 
it performed well except for a slight vibration 
of the reflex mirror which blurred the fine de¬ 
tail of the radar indication. By that time the 
need for the device had abated because of the 
improvement in radar which made it unneces¬ 
sary to make visual contact before shooting. 
The mounting of the reflex mirror was altered 
but was never tried in flight. 

12,3 5 Rochester T-67 4c 

In July 1943 the Army Antiaircraft Artillery 
Board of Camp Davis requested that small re¬ 
flex sights with exit pupils of 35 mm be de¬ 
signed to replace the small-aperture unit power 
telescopes then in use on the M-7 lead computer 
of the 40-mm Bofors antiaircraft gun. The 
T-67 sight was designed and constructed at the 
Institute of Optics for the purpose. Three 
models, 01, 02, and 03, were made, which were 
identical in everything except the illuminators. 

The optical system from reticle to reflex mir¬ 



ror is quite conventional and is shown in the 
drawing of Model 03 in Figure 17. The lens has 
a 40-mm aperture and 120 mm focal length. 
The parallactic range P 0 is 1 mil. The reticle 






















494 


REFLEX SIGHTS 


pattern consists of a single line 100 mils long, 
either horizontal or vertical, for sighting in 
altitude or azimuth respectively. 

In all three models the principal illuminant 
is daylight from the target area, and half-re¬ 
flecting reflex mirrors are used. The reticle is 
placed as low as possible to keep the moment 
of inertia around the mounting point to a mini¬ 
mum—a point of some importance on a vio¬ 
lently recoiling gun when the sight is linked 
to a delicate mechanism. The light from the 
target area is intercepted before reaching the 
reticle directly by parts of the gun. A simple 
system of prisms with cylindrical lens surfaces 
is therefore provided to conduct the light from 
just below the line of sight to the reticle. 

Auxiliary illumination for night conditions 
is provided in Models 01 and 02 by electricity. 
In Model 01, the change-over to electrical oper¬ 
ation is made simply by sliding the last prism 
of the daylight illuminator from between the 
lamp and the reticle. In Model 02 the electric 
illuminator is screwed on in place of the day¬ 
light illuminator when wanted. 

The night illumination of Model 03 (see Fig¬ 
ure 17) is obtained from a fluorescent material 
bombarded by a particles from an external 
source of radium. The phosphor is coated on a 
cylindrical arc located directly behind the 
reticle which is pivoted on its own axis and 
can be rotated by an external knob. The radium 
is rolled in gold foil to a concentration of 0.20 
mg per sq cm. Two strips of the foil 1.5 mm 
wide are mounted on the back of the reticle, 
one on each side of the straight line opening. 
The a particles from the radium excite the 
phosphor directly behind the opening and the 
emitted light illuminates it to a maximum 
brightness of about 1 millilambert. The bright¬ 
ness is controlled by spraying a thin wedge of 
polystyrene on top of the phosphor, the thick¬ 
ness changing as the cylindrical surface is ro¬ 
tated past the reticle slit. As the polystyrene 
becomes thicker the intensity of a-particle 
bombardment diminishes. A range of about 100 
to 1 in reticle brightness is attained in this way 
simply by turning the external knob. When the 
fluorescent illumination is not being used, the 
cylindrical arc with the phosphor is turned 
completely out of the optical path and at the 


same time is removed from the vicinity of in¬ 
tense bombardment. 

The usual type of radium luminous paint 
could not be used here because the concentra¬ 
tion of radium required to produce the neces¬ 
sary brightness would destroy the phosphor in 
a few weeks. With the external source of a 
particles, bombardment of the phosphor takes 
place only while the phosphor is being used. 
Tests indicate that after about 1,400 hr of 
continuous use the bare phosphor, i.e., no plastic 
cover, loses half its brightness. 

The three T-67 sights were successfully 
tested at Camp Davis. Four slightly different 
models with electrical night illumination were 
produced in considerable numbers by the Argus 
Corporation with designations M-21 to M-24. 
M-21 and M-22 are azimuth and elevation 
sights for the M-7 lead computer on the Bofors 
gun. M-23 and M-24 are azimuth and elevation 
sights identical with M-21 and M-22 except for 
the mounting parts, which are altered to fit the 
M-55 double 40-mm antiaircraft gun mounted 
on a tank chassis. 

12 - 36 Polaroid //1.6 Sight 

The Polaroid //1.6 sight was designed with 
a plastic optical system to be interchangeable 
with the Navy Mark 8 sight. It had an aper¬ 
ture of 3.5 in. and the optical system was folded 
once by means of a mirror. The lenses were 
mounted as a unit in a plastic sleeve which 
slipped into the cast magnesium housing, main¬ 
taining the elements in alignment with each 
other. The external appearance of the sight is 
shown in Figure 18. 

The sight was tested by the Navy Bureau of 
Ordnance. The parallax errors were much 
smaller than those of the Mark 8 sight but 
showed a marked temperature dependence. 

12.3.7 Mount Wilson Bowen Sight 2 * 1 

Two models of the Bowen Sight were made 
for testing in the AT-6 and the P-51 B aircraft 
respectively. 

Bowen Sight for AT-6 Aircraft 

The first model of this sight was built by 



MODELS OF OSRD REFLEX SIGHTS 


495 



speed ring of 400 mph (400 mils diameter), 
and with a focal ratio of f/2. The parallax due 
to spherical aberration is held to about 1 mil of 
angle. The exit pupil has a diameter of 4.5 in., 
and is located about 12 in. back of the reflex 
plate, or very nearly at the normal position of 
the pilot’s eye. A view from this position is 
shown in Figure 19B. This was taken with a 
wide-angle lens, and to obtain correct perspec¬ 
tive the photograph should be viewed from a 
position 4 in. above the paper. 

The reticle pattern is cut in a thin metal 
shell which fits over the upper lens of the illu¬ 
minating system. To conserve light, the upper 
surface of this lens is aluminized, leaving clear 


Figure 19. Bowen sight for AT-6. 

bands somewhat wider than the rings of the 
reticle. When Figure 19B was made the reticle 
itself was not available, and the circles shown 


the Observatory independently of Contract 
OEMsr-101. It was designed specifically for the 
AT-6 training plane, since drawings were not 
immediately available for a combat plane. 

The optical parts are mounted on a single 
casting, as shown in Figure 19A. This is de¬ 


Figure 18. Polaroid //1.6 sight. 

signed to follow closely the shape of the frame 
surrounding the windscreen so that when it is 
mounted in the airplane it adds little or no ob¬ 
struction to the pilot’s vision. The optical sys¬ 
tem is inclined at such an angle that the mirror 
is seen edge on from the normal eye position, 
as in Figure 19B. 

The focal length of the spherical mirror is 
9 in. (radius of curvature 18 in.). The mirror 
itself has the shape of a half circle, and was 
cut from a circular mirror 9 in. in diameter. 
This shape gives good visibility in the lower 
corners of the field, and vignettes only those 
parts of the reticle which are not actually used. 

The field is adequate to provide for a reticle 







496 


REFLEX SIGHTS 


are images of the clear bands in the reflecting 
coat. Although they are far too wide, they show 
the correct positions of the various rings of the 
reticle pattern. These have diameters of 400, 
300, 200, and 100 mils. The inner edge of the 
smallest ring shows approximately the position 
of a 70-mil circle. This was for some time the 
largest pattern available in any standard sight. 
A 100-mil circle is now standard for fixed sights. 

This model was tested in January 1944, at 
Matagorda Peninsula, by several experienced 
pilots. Firing tests were carried out on towed 
targets. Once the sight had been properly har¬ 
monized, the scores compared favorably with 
those made with standard sights. 

Although the 400-mph reticle speed ring 
could not be used effectively in an AT-6 ship, 
the pilots were able to form a judgment 
whether so large a circle would be useful in a 
combat plane. Their conclusion was that it 
would lead to confusion, and that the reticle 
should contain only the 100 and 200 mph 
circles. 

The pilots who made the tests spoke very 
highly of the excellent visibility of the reticle 
image, due mainly to the size of the exit pupil 
and its proximity to the pilot’s eye. A lens sys¬ 
tem to give equivalent visibility on a 200-mph 
reticle would require an aperture of about 7 in. 
Another factor improving vision is the use of 
the large reflecting plate free from obstructions 
near the line of sight. The target as well as the 
image is seen through the glass instead of to 
one side of it as in standard sights. 

One criticism of the model was the feeling of 
the pilot that the presence of the mirror and 
reflecting plate so close to his face crowded 
and confined him so that he could not reach the 
instrument panel with sufficient freedom. The 
reduction in size of the instrument due to the 
use of a 200- instead of a 400-mph reticle speed 
ring would aid somewhat in modifying this dif¬ 
ficulty, but the sight would necessarily be con¬ 
siderably larger than those in general use. 

A second objection which was not foreseen 
was the reduction in contrast of the field of view 
caused by sky light reflected into the pilot’s eyes 
by the glass plate. A marked improvement re¬ 
sulted from painting the supporting bracket 
black and removing a zinc sulfide coating from 


the glass. The pilots did not consider either the 
reduction of visibility due to the spherical mir¬ 
ror or the crash hazard of the glass in front of 
the operator as serious. The sight used in the 
P-38 was considered much more dangerous. 

As a result of the tests at Matagorda Penin¬ 
sula it seemed desirable to construct a second 
model designed for a combat plane, eliminating 
so far as possible the objections which have 
been described. 

Bowen Sight for P-51 B Aircraft 

The second model, shown in Figure 20, intro¬ 
duced the following modifications: 

1. Inversion of the optical system, placing 
the spherical mirror below and turning the 
glass reflecting plate roughly parallel to the 
windshield, thus reducing sky reflections. 

2. Reduction of the reticle speed ring size to 
200 mph, instead of 400 mph. 

3. Support of the reflex plate on a separate 
yoke which can be rotated as a unit for bore¬ 
sighting; this replaces individually adjustable 
brackets. 

4. Simplification of the lighting system in 
the interest of greater compactness. 

The first of these changes was successful in 
reducing reflection of general sky light from 
the reflex plate. At the same time it leads to a 
design which fits more compactly into the 
plane. But it has three disadvantages which 
will have a bearing on the design. (1) Light 
from the reticle passes through the reflex plate 
at a steeper angle, causing astigmatism which 
is noticeable under some conditions but prob¬ 
ably not of practical significance; (2) the 
spherical mirror is placed in a more exposed 
position; and (3) in the present instrument it 
is possible at certain angles to see a weak image 
of the sun reflected by the reflex plate, the mir¬ 
ror, and the reflex plate again. 

The modified system for illuminating the ret¬ 
icle retains the principle of limiting the rela¬ 
tive aperture of the mirror, but accomplishes 
this by means of a diaphragm placed a short 
distance behind the reticle. This diaphragm has 
openings similar to the reticle pattern but 
enough wider to admit cones of light of the 
desired angle. This method provides a fairly 
satisfactory substitute for the more elegant de- 



MODELS OF OSRD REFLEX SIGHTS 


497 



Figure 20. Bowen sight for P-51 B. 




498 


REFLEX SIGHTS 


sign described above. It is not clear whether 
the slightly inferior optical performance is jus¬ 
tified by the smaller size of the lighting unit. 

As in the first model, the shape of the main 
frame of the instrument follows closely the 
supports of the enclosure of the aircraft for 
which it is designed. Both models are so 
planned that they can be installed with little 
or no modification of the airplane. 

Upon completion, this model was sent to 
Wright Field where it was mounted in an air¬ 
plane and tested in flight. It was then for¬ 
warded to Matagorda in October 1944 for fur¬ 
ther testing. 

As a result of the latter tests a few modifica¬ 
tions in the design were found to be advisable. 
The forward pair of bolts used for azimuth 
adjustment should be more easily accessible. 
The reticle image was not intense enough under 
all conditions. It is believed that this can be 
improved by minor refinements in the design 
of the lighting system. A cover of some sort 
should be provided to protect the mirror sur¬ 
face from damage. The mirror should also be 
shielded from direct sunlight more completely 
than in the present model. 

The sight was used in target firing with 
4,000 rounds of .50-caliber ammunition. The 
towed targets presented areas of from 64 to 80 
sq ft, and the results, calibrated for full-sized 
targets, showed 28 per cent hits. It was thought 
that better results would undoubtedly have 
been obtained had full-sized targets been avail¬ 
able. Assessment of the film showed much 
smoother attacks than when firing with other 
fixed sights, due to the greater eye freedom 
afforded. No restriction to the forward visi¬ 
bility of the pilot was noted. 

These flight tests indicated that use of the 
Bowen sight would result in substantial in¬ 
creases in scores for aerial gunnery over the 
results obtained with other sights. 

The greatest value of the Bowen sight ap¬ 
pears to lie in the easy visibility of the reticle 
with both eyes and the lack of obstruction in 
the neighborhood of the line of sight. Its re¬ 
markable ability to handle large reticles would 
not seem to be of great advantage for fixed 
sights, since pilots have concluded from firing 
tests that reticles with speed rings larger than 


200 mph are not needed. The Air Forces are 
rapidly converting to computing sights and it 
is probable that further development work on 
fixed sights is not warranted. On the other 
hand, the advantages of the optical system in 
the Bowen sight for use in lead-computing 
sights, in which the reticle is moved over the 
whole range of possible leads, are worth serious 
consideration, and the hope that its advantages 
may be incorporated into computing sights is 
expressed in the Matagorda report. 


12.3.8 Mount Wilson Hayward Solid 
Sights, S-l 2b 

The solid reflector sights developed at the 
Mount Wilson Observatory are simple compact 
units which employ daylight illumination, de¬ 
signed for use where large exit pupils and 
long eye relief are unnecessary. Five different 
models for specific uses were made and will be 
discussed below. 

The optical systems for all were essentially 
identical although two aperture sizes, 1 in. 
square and % in. square, were used, with the 
same focal length of 0.97 in. The %-in. aper¬ 
ture system is shown in Figure 21. The 1-in. 
aperture models were similar except for the 
fact that there was not room to fold the optical 
path, and the reticle was placed on a surface 
parallel to the collimating mirror. A 45-degree 
prism or mirror above the reticle reflected the 
illuminating light from the target area into the 
sight. 

Solid Sights for the M-l Rifle 
(Two Models) 

The standard Service rifle is not suited to 
deflection shooting at rapidly moving targets 
like airplanes because there is no device for 
setting in the correct lead. The solid reflex sight 
for the M-l rifle was developed to fill this need. 

The first model had a clear aperture 1 in. 
square and a reticle in which the largest circle 
had a radius of 160 mils. Two later models had 
an aperture % in* square and an equivalent 
focal length of 0.97 in. A mounting was de¬ 
signed in which the sight was clipped directly 
to a standard Garand rifle and held by spring 



MODELS OF OSRD REFLEX SIGHTS 


499 


hooks, but since this did not appear to be thor¬ 
oughly satisfactory, another mounting was 
built in which the sight was clipped to a short 
track fastened permanently to the rifle. Figure 



22A contains two photographs of a wooden 
mockup of the mounting, showing the method 
of clamping the sight in position with a single 
lever. In the model tested, the track was 
mounted on a saddle which clamped directly to 
the metal of the gun, instead of being screwed 
to the stock as is indicated in the photographs. 

The sight was tested at a military range with 
72 rounds of firing. It proved to be sufficiently 
rigid, and after adjustment of the reticle, a 
typical series of eight shots fell within a circle 
the diameter of which was about 3 mils, or the 
size of the central dot of the reticle pattern. 
This would be quite adequate for use against 
airplanes, but against stationary targets the 
accuracy was less than that obtained with open 
sights. It seems probable that the scatter was 
due rather to imperfect visibility of the reticle 
than to the sight itself. Light for the reticle 


was taken from an area about 10 degrees above 
the target which under the conditions of the 
test formed a dark background. This difficulty 
limits the application of the sight in its present 
form, although against targets in the sky or 
near the horizon the visibility of the reticle is 
satisfactory. 

Solid Sights for AA Guns 

Two samples of solid sights of 1 in. square 
aperture were made for tests with antiaircraft 
guns at Camp Davis for the AAA Board. The 
reticle was illuminated by light from the target 
area which was reflected into the sight by an 
external 45-degree mirror. While the assembly 
was light, it was not as compact as that of the 
%-in. rifle sight. 

Tests of the sights indicated that they per¬ 
formed well, but the light reflected from the 
glass surface nearest the eye tended to reduce 
the contrast of the field. 

Solid Sights for Rockets 

The optical parts for twelve solid sights with 
an aperture of % in. were completed and sent 
to the California Institute of Technology for 
experimental use in aiming a hand-held device 
for launching rockets. No formal report on 
these sights has been made. The devices for 
which they were intended were discontinued. 

Solid Sights for the Navy 
Mark 17 Gunsight 

At the request of the Bureau of Ordnance a 
solid reflector sight was designed and con¬ 
structed for use on the Mark 17 gunsight. This 
is a mechanism which is connected to turret 
guns in a bomber and introduces the necessary 
deflection for an own-speed sight by means of 
the vector principle. Since in this case the gun¬ 
ner keeps the reticle centered on the target, a 
large reticle pattern is not required, but a large 
visible field is still desirable, and a large aper¬ 
ture is valuable in allowing freedom of eye 
position. 

The finished model of this sighting unit is 
shown in Figure 22B. The upper fifth of the 
glass block is a separate prism, cemented to the 
main block with the thin metal reticle between. 
Light from the sky enters this upper prism 


























500 


REFLEX SIGHTS 


from the front and is reflected downward 
through the reticle. This is the most compact 
of the designs developed. The very high relative 
aperture (about f/1) is optically satisfactory, 
but presents serious problems of illumination. 
When viewed from different parts of the aper¬ 
ture the reticle is illuminated by different areas 
of the sky. This increases the chance that some 


obstruction will cut out or reduce the light. It 
also makes less effective the great advantage of 
illuminating the reticle by light from the direc¬ 
tion of aim, the advantage being that the con¬ 
trast between reticle and field is fairly constant 
even when the brightness of the field varies 
widely. 

The extreme lower end of the glass block is 



Figure 22. A. Solid rifle sight, wooden mockup. B. Solid sight for Bureau of Ordnance. 









EASTMAN FLY’S-EYE SIGHT 


501 


cut round in section, so that the spherical mir¬ 
ror fits against it and is held much as a lens is 
ordinarily mounted. The sides of the entire 
glass block are cylindrical in shape, the axis 
of the cylinder being that of the optical sys¬ 
tem. These cylindrical surfaces fit the walls of 
the mounting which is cut from a single piece 
of steel. The glass is cemented in place with 
balsam, giving a solid construction which re¬ 
inforces the cemented 45-degree surface of the 
glass. 

A sight utilizing a half-silvered mirror was 
developed elsewhere at about the same time the 
solid sight was being investigated at Pasadena. 
This sight, which uses a concave half-silvered 
mirror through which the field is viewed, has 
several marked advantages, being light in 
weight, well illuminated, and easy to manufac¬ 
ture. It was found to be better adapted to use 
on the Mark 17 gunsight than the model of the 
solid sight which had been built. The solid 
sight could be made much lighter than at pres¬ 
ent by the use of an aluminum mounting, and 
the illumination could be varied within wide 
limits. Under the circumstances, however, it 
did not seem desirable to continue development 
of the solid sight further without securing 
additional information concerning the require¬ 
ments. 


12 39 Yerkes Solid Sight M-16 lc 

Although no model of the M-16 solid sight 
was built, it should be mentioned here. Inspec¬ 
tion of the drawing in Appendix III to this 
chapter will show the lens which has been added 
to the solid Mangin design. This addition has 
enabled the designer to reduce the off-axis aber¬ 
ration to such an extent that the parallactic 
range for a circle of 210 mils radius is only 0.5 
mil. To reduce the weight to a minimum the 
materials used were CHM and styrene plastics. 
It is regrettable that no samples were con¬ 
structed. 

12.4 EASTMAN FLY’S-EYE SIGHT 3 

Although it was developed under the auspices 
of Division 7 of NDRC, the Fly’s-Eye sight is so 


directly related to the subject of this report 
that a full account of it is included. 

The principle of the s^ght has already been 
described. Several different experimental mod¬ 
els were constructed, culminating in the Mark 
14 illuminated sight, Model G, which is the 
production prototype. Since Model G repre¬ 
sents a combination of the best features of its 
predecessors, the earlier models are not de¬ 
scribed here. However, several problems en¬ 
countered in the course of the development of 
any sight of the multiple lens type will be dis¬ 
cussed. Model G is shown assembled in Figure 
23, and disassembled in Figure 24. 

The lens plate is made up of hexagonal as¬ 
pheric molded lens elements 15.67 mm in diam¬ 
eter (inscribed circle) cemented on a plane- 
parallel glass. The effective aperture is a 
rectangle 118 by 141 mm. An individual reticle 
etched through a curved shell of copper is 
mounted below each lens element on an Invar 
reticle plate. The illuminator consists of four 
50-w ceramic coated lamps at the foci of four 
parabolic mirrors which are carefully fitted to¬ 
gether to fill the whole area behind the reticle 
plate. The heat of the lamps is removed by 
filtered air circulated through the instrument at 
a rate of 15 cu ft per min. The reticle brightness 
B 1 can be adjusted from zero to about 26 
lamberts by varying the resistance in series 
with the illuminator. The armor glass in the 
airplane serves as reflex mirror. 

The reticle is of the standard Navy pattern 
with a central dot and two rings of radii 50 and 
100 mils. Four radial lines extending from 25 
to 150 mils in the 12 o’clock, 3 o’clock, 6 o’clock, 
and 9 o’clock positions complete the pattern. 

The parallax errors are rather complicated 
and cannot be readily specified in any simple 
form. As the eye moves across the exit pupil it 
crosses successively a series of lens elements 
along chords at random distances from their 
centers. The apparent parallax of any one lens 
element depends upon the chord which the eye 
traverses and the field angle, and is different for 
the circles and the lines of the pattern. The 
effect observed is aptly described as an appar¬ 
ent “wriggling” of the reticle pattern. The use 
of aspheric elements gives a negligible wriggle 
to the central dot of the pattern, and a maxi- 



502 


REFLEX SIGHTS 


mum of the order of 3 mils parallactic range at 
the edge of the field. Chromatic aberration is 
uncorrected and results in a spread of about 
4 mils between C and F at the edge of a lens 
element. 

The sight is held in a boresighting mount 



Figure 23. Mark 14 Model G sight. 


and is designed to fit into an opening in the air¬ 
plane fuselage forward of the instrument panel 
and directly under the armor glass. In this posi¬ 
tion it is completely out of the way and offers 
no obstruction whatever to the pilot's field of 
view. This is the great point of superiority of 
the Fly’s-Eye sight and is the principal justifi¬ 
cation for the greater complexity of the design. 
In most airplanes, however, some planning of 
the layout of the instrument panel is required 
to provide space for the sight. 

12.4.1 Ge nera I Problems of Design and 
Construction 

Many of the problems encountered in the de¬ 
velopment of the various models of Fly’s-Eye 
sight at Eastman Kodak are inherent in this 
type of sight and hence are of general interest. 
A few of them are therefore described briefly 
below. 

Lens Plates 

All lens plates (with the exception of the pre¬ 


liminary model to demonstrate the multiple 
lens principle) were made by cementing hexag¬ 
onal lens elements onto a plane-parallel glass 
plate. The division line between neighboring 
elements was kept as fine as possible to avoid 
fluctuations in brightness as the eye crosses 
from one element to the next. The possibility of 
molding the whole plate in one piece is an at- 



Figure 24. Mark 14 Model G sight disassembled, 
tractive one, but will not be practicable in glass 
until methods of molding large pieces have been 
developed. The use of plastics, however, may 
deserve serious consideration. 

Lens elements for use on the lens plate should 
have, as far as possible, the attributes desired 
in the usual reflex sight lens. In addition, they 
must be in one piece with a plane back surface 
for cementing onto the plane-parallel plate. The 
latter requirements impose a serious handicap 
on the designer’s efforts to attain the necessary 













EASTMAN FLY’S-EYE SIGHT 


503 


corrections. Finally, the lens elements must be 
adapted to easy production in large numbers. 

Four types of lenses were used in the various 
models of Kodak sights: 

1 . A cemented achromat, //2.92. 

2. A simple lens, //2.92, of EK-11 glass, 
characterized by a high index and low disper¬ 
sion. n B = 1.69677, v — 56.1, P 0 = 2.4. 

3. A simple lens with a hand-figured as¬ 
pheric surface, //2.92, of the same glass as lens 
No. 2. 

4. A simple lens with a molded aspheric sur¬ 
face, //2.43, of cane glass. n D = 1.51. 

Of these, the hand-figured aspheric lens (No. 
3 ) gave the best performance except for chro¬ 
matic aberration. Unfortunately, the glass, 
which was in part responsible for its superior 
correction off axis, could not be molded. Since 
the hand figuring of the aspheric surface is not 
a practical production method, the somewhat 
less satisfactory lens No. 4, which could be 
molded, was adopted as a production prototype. 
It was considered that the chromatic aberra¬ 
tion was not too serious since it could be con¬ 
siderably mitigated by the use of a light orange 
or yellow filter to eliminate the high-dispersion 
end of the spectrum. 

Once the lenses have been shaped and edged, 
the elements have to be carefully cemented in 
place. Since it would be most undesirable in 
production to have to adjust the reticles in the 
reticle plate individually, the lens elements must 
be positioned with an accuracy of about 0.02 
mm. A jig was designed to accomplish the posi¬ 
tioning with the requisite accuracy, but was not 
made because it would be too expensive for use 
on only a few samples. With its use in manu¬ 
facture, however, it is expected that inter¬ 
changeable lens plates could be produced. 

Reticles 

The original method of making the reticle 
plate for the multiple lens sight was simply to 
photograph a reticle at infinity through the lens 
plate. An image of the reticle pattern was thus 
obtained in its proper position behind each lens 
element. This method was satisfactory for ret¬ 
icles of 50 mils radius or less, but failed when 
used for the larger reticles of the standard 
Navy pattern because the individual reticles 


could not be curved to fit the focal surfaces of 
the lens elements. 

In an effort to obtain better approximation 
to the focal surface, a two-layer photographic 
reticle plate was tried. The outer parts of the 
reticle pattern were photographed on one plate 
with a circular clear area at the center of the 
pattern. The inner parts of the pattern were 
photographed on a second plate below the first 
and viewed through the clear areas. A two-level 
reticle was obtained which was quite satisfac¬ 
tory for the rings in the pattern, but showed a 
wriggling break in the radial lines as the eye 
scanned the aperture. The result was consid¬ 
ered unsatisfactory and, in spite of the ease of 
manufacture, the photographic reticle plate 
was abandoned. 

In its place, reticles etched through a thin 
curved metal shell were used. They were shaped 
to coincide with the surface of best focus for 
radial lines at 150 mils from the center where 
the lines ended, and with the surface of best 
focus for rings from 100 mils in to the center. 
The metal reticles were completely transparent, 
durable, and heat resistant, but, like the lens 
elements, had to be assembled individually on 
the reticle plate with considerable precision. No 
quick method for positioning them in produc¬ 
tion has been worked out. 

Illumination 

The difficulties of electrical illumination are 
encountered in their worst form in the multiple 
lens sight. The total reticle area to be illumi¬ 
nated is large and devices for concentrating 
light on the lines of the reticle pattern are out 
of the question because of the great numbers of 
reticles used. It is regrettable that the position 
of the sight in the airplane makes the use of 
daylight illumination extremely difficult. 

Two methods of illumination were tried. The 
first was to place a small lamp in a white-walled 
cavity behind each reticle. The illumination 
was insufficient because of the low efficiency of 
the small lamps, and in spite of its compactness 
the system had to be abandoned. 

The second form of illuminator consisted of 
several lamps with highly diffusing bulbs at the 
foci of parabolic mirrors. The lamps were of 
such size that any ray through a lens element 








504 


REFLEX SIGHTS 


and through any point in the corresponding 
reticle would terminate on a bulb either directly 
or via reflection from one of the parabolic mir¬ 
rors. By carefully fitting square mirrors to¬ 
gether, the whole reticle plate was provided for, 
without any dark spots. The brightness of the 
reticles was then the brightness of the diffusing 
surfaces of the bulbs. 

As was mentioned, 200 w are consumed by the 
four lamps of the illuminator of the Model G 
sight at full brightness, and some cooling device 
was necessary to prevent the sight from going 
up in smoke. Air circulation was provided by a 
combined centrifuge and blower. The purpose 
of the centrifuge is to filter the air, and in par¬ 
ticular, to remove the particles of salt prevalent 
in ocean air, since it is quite certain that the 
metal parts of the sight, and especially the 
reticles, could not long withstand the corrosive 
action of warm moist salt air. 

Provision was made for varying the reticle 
brightness continuously from zero to 8.6 milli- 
lamberts, and in steps of 0.497 in the logarithm 
of the brightness to a maximum of 26 lamberts. 
Since only about 10 per cent of the light is re¬ 
flected by the reflex mirror, the apparent bright¬ 


ness of the reticle pattern is at most only 2.6 
lamberts greater than that of the target area. 
The brightness should be satisfactory against 
the usual clear blue sky or a dark overcast, but 
seems definitely low for use against a bright 
translucent haze or in the neighborhood of the 
sun. 


125 RECOMMENDATIONS BY NDRC 

1. Further studies of the Lens-Mangin sight, 
the Bowen sight, and the Fly’s-Eye sight should 
be made to determine which is best adapted 
for use in aircraft, taking into account space 
occupied, aperture, diameter of field available, 
brightness of reticle attainable, possibility of 
reflecting the collimated beam from the armor 
glass, and finally, ease of construction and cost. 

2. Solid glass sights should be developed 
further, with full attention to means for in¬ 
creasing the contrast between reticle pattern 
and background when sky illumination is used. 
This should include studies of high-efficiency, 
cemented, partially reflecting films and the use 
of quarter-wave plates to increase brightness. 



Chapter 13 

STADIAMETERS 

By Theodore Dunham , Jr: 


T he range of a distant object may be meas¬ 
ured with a rangefinder, which depends on 
differences in apparent direction at the two 
ends of a known base line within the instru¬ 
ment itself, or it may be determined with a 
stadiameter, which depends on measuring the 
apparent angular size of the object and com¬ 
paring this with known dimensions of the ob¬ 
ject. Rangefinders have the advantage that they 
require no information about the size of the 



Figure 1. Optical system of IX stadiameter. 

target, but they are large, cumbersome and ex¬ 
pensive, and their use is restricted by this fact. 
Stadiameters are small and inexpensive, so that 
they can be used under a great variety of cir¬ 
cumstances, but they are limited in their useful¬ 
ness by the fact that the size of the target must 
be known. Moreover, the target must be ori¬ 
ented with its long axis nearly perpendicular 
to the line of sight, otherwise a cosine factor 
enters which cannot usually be estimated with 
accuracy. 

Stadiameters have one marked practical ad¬ 
vantage over rangefinders, namely that no ac¬ 
curate aiming of the instrument is necessary. 
The target may appear anywhere in the field 
where the two images can be set in contact. 

The marked simplicity of stadiameters, as 
compared with rangefinders, has lead to the 
development of three types at the University of 
a Chief, Section 16.1, NDRC. 


Rochester 1 under Contract OEMsr-160 for a 
variety of applications. 

13 1 UNIT-POWER STADIAMETER 

This stadiameter was orginally designed to 
permit the pilot of one aircraft to fly at a speci¬ 
fied distance behind another aircraft in a cer¬ 
tain tactical maneuver. The wing span of the 
leading plane was of course known. Unit power 
was entirely adequate for this application, since 
it was merely necessary to hold the spacing be¬ 
tween the two aircraft constant within 5 per 
cent, at a range of about 1,000 ft. A precision 
of 1 mil in measuring angles is obviously ade¬ 
quate in this case. 

Figure 1 shows the optical system. The ob¬ 
server looks directly at the target through the 
mirror M, which is coated to transmit and re¬ 
flect approximately 50 per cent (see Section 9.5 



B 

Figure 2. Appearance of target in stadiameter. 

A. Incorrect setting. B. Correct setting. 

for a description of the technique for depositing 
high-efficiency 50-50 coats). The observer also 
sees the target by double reflection from the 
totally reflecting movable mirror M' and the 
mirror M. W is pivoted about an axis perpen¬ 
dicular to the plane of the paper at P. When the 
two mirrors are parallel, the two lines of sight, 


505 













506 


STADIAMETERS 


S and S', are parallel, but when M' is turned 
through an angle a/2, S' is deviated through an 
angle a with respect to S . This causes a doubling 
of the image of the target, by an angle a. In 
general, with any random setting, an airplane 
may appear as in Figure 2A. By adjusting the 
angular rotation of M', and by rotating the 
entire instrument about the line of sight, it is 
easy to bring the two images into contact, so 
that the appearance is that of two planes flying 
with the wing tips touching, as in Figure 2B. 



Figure 3. Photograph of IX stadiameter. 

The angle of deviation a may then be read from 
the calibrated drum and, if the wing span of 
the plane is known, the range can easily be cal¬ 
culated. A predetermined distance can be main¬ 
tained by setting the angle a to the required 
value, and then maintaining the distance so 
that the two images appear to fly with the tips 
of their wings in contact. 

The angular setting of mirror M' is controlled 
by an arm and tangent screw, with an engraved 
dial reading in mils (thousandths of a radian). 
The fixed mirror M is adjustable about an axis 
perpendicular to that on which M' turns, so 
as to make the two images coincide exactly when 
they are superposed. The reflectivity of M 
is such as to make the two images equally 
bright, but the coating introduces a slight color, 
so that the two images can be readily distin¬ 
guished. The instrument is protected by a glass 


window and is made waterproof. Figure 3 
shows the complete instrument with rubber 
eyecup, calibrated drum, and index mark. The 
total weight is 8 ounces. 

The drum on the tangent screw has fifty divi¬ 
sions 2 mils apart. The total range of the in¬ 
strument is 300 mils. The field of view is 18 de¬ 
grees. Settings can be reproduced to somewhat 
better than 0.5 mil. The zero point is set on a 
distant object by adjusting the relation of the 
engraved ring to the knurled knob on the screw. 

This stadiameter has been used in a number 
of experimental applications. In addition to 
measuring distances between airplanes, stadi- 
ameters have been successfully used for check¬ 
ing close radar ranges, for station-keeping be¬ 
tween ships, and for establishing the distances 
between ships and floating targets in experi¬ 
mental tests conducted under Division 6. 

Fifteen units of the instrument were made 
by the U. S. Management and Engineering 
Company. These have been distributed as sam¬ 
ples to the National Bureau of Standards, the 
Armament Laboratory (AAF) at Wright Field, 
the Frankford Arsenal, the Armored Force 
Board at Fort Knox, the Antiaircraft Service 
Test Section, Ground Forces Board No. 1 at 
Fort Bliss, the Bureau of Aeronautics, the 
Bureau of Ships, the Bureau of Ordnance, and 
the Office of Research and Development in the 
Navy. 

13 2 THREE-POWER STADIAMETER 

An application in the Southwest Pacific sug¬ 
gested the need for a stadiameter capable of 
higher precision than that just described. In 
photographic reconnaissance it was necessary 
for F-5 aircraft to fly at high altitude on paral¬ 
lel courses 6 miles apart. At this distance the 
length of the fuselage of the aircraft subtends 
an angle of only a little more than 1 mil. It 
seemed likely that settings could be made with 
sufficient accuracy if the instrument were de¬ 
signed to display an erect image with 3x 
magnification. Angular displacement of such a 
system causes only twice as great an apparent 
displacement of the target. This seemed com¬ 
patible with the requirements of a hand-held 
instrument in a single-seater airplane. 




THREE-POWER STADIAMETER 


507 


The optical system is a 3x mirror-prism 
wide-held system (Model 5201) which was de¬ 
veloped for night telescopes at the University 
of Rochester. 2 The triplet objective is placed 



between a conventional Porro prism and the 
two mirrors. This optical system, which is ex¬ 
tremely compact, is shown schematically in 
Figure 4. The beam-splitting arrangement is 



Figure 5. Beam splitter for 3 X stadiameter. 


shown in Figure 5. A glass rhomb R is cemented 
to one of the reflecting faces of the prism P, 
which is provided with a 50 per cent reflecting 
coat before cementing. The normal field of the 
telescope is unaltered except for the reduction 
in brightness of the beam S. The adjustable 
image is formed by light S', which has been 


reflected from the internal face of the rhomb 
and has been transmitted through the 50 per 
cent interface between -the rhomb and prism. 
The deviation of the beam S' is produced by a 
variable angle prism, made up of a plano-convex 
lens L and a plano-concave lens L'. These have 
the same radii of curvature. The piano face of 
lens L is cemented to the rhomb. The concave 
lens L' is pivoted about the center of curvature 
of its curved face C so that the angle of the 



Figure 6. Photograph of 3X stadiameter. 

prism may be adjusted. The two curved lens 
surfaces are separated enough to permit motion 
without contact. Color is not serious up to about 
50-mils separation of the images with 3X mag¬ 
nification. The field is 23 degrees, of which 18 
degrees can be deviated and is available for 
stadiametric setting. The exit pupil is 7 mm in 
diameter, which is an advantage at dusk. 

Rotation of the concave lens is controlled by 
an arm and screw, with a dial engraved in mils 
and a vernier reading to 0-1 mil. Zero adjust¬ 
ment is made by rotating the engraved head 
relative to the screw. Settings can be repro¬ 
duced to 0.1 mil. 

A photograph of the complete instrument is 
shown in Figure 6. It can be used conveniently 
with one hand, the second finger turning the 
knurled drum for setting while the rest of the 























508 


STADIAMETERS 


same hand supports and aims the instrument. 
Total weight is 40 ounces. 

One unit was completed and will probably be 
kept at the University of Rochester, with the 
understanding that it will be available for Serv¬ 
ice testing at any time. 


13 3 STADIAMETER FOR B-29 

FIRE-CONTROL SYSTEM 

A quick means for estimating range with 
reasonable accuracy and feeding the result into 


this application because it does not require the 
image to be located at any particular position 
in the field. All that is necessary is to have the 
double image included in the field of view so 
that the observer can bring the wing tips into 
coincidence. The adjustment can probably be 
made even if the double image is moving about 
in a moderately rapid and erratic fashion, 
under conditions which make the use of the 
common ranging dots difficult. 

The design was based on the assumption that 
the simplest arrangement would be to mount 
a stadiametric device behind the present ped- 





Figure 7. Optical system of B-29 stadiameter. 


the B-29 fire-control system is of the greatest 
importance for increasing the present accuracy 
of firing, particularly when rapidly approach¬ 
ing or receding targets are involved. 

At the request of the Armament Laboratory 
at Wright Field, experiments were undertaken 
under Project AC-114 to determine whether a 
double-image stadiameter could be incorporated 
in the present pedestal sight. The University of 
Rochester contributed optical engineering and 
some special mirrors. The General Electric 
Company developed a prototype model for 
testing. 

A stadiameter seemed likely to have definite 
advantages over the present ranging method in 


estal sight with a minimum of changes. The 
field should be restricted as little as possible, 
and eye relief should be adequate. It was de¬ 
sirable that the device be adapted for use with 
a camera to check performance of the gunner 
with the stadiameter. It seemed likely that unit 
power would be sufficient, and in any case the 
avoidance of lenses and an erecting system was 
a controlling consideration, at least in a pre¬ 
liminary model. 

The optical system for the stadiameter is 
shown in Figure 7. Two mirrors are mounted, 
close together, in a vertical plane inclined 45 
degrees to the line of sight. A 90-degree glass 
prism receives light reflected from these two 











































































RECOMMENDATION BY NDRC 


509 


mirrors and sends it to the eye of the observer. 
The first mirror has a coating on its first sur¬ 
face which reflects 35 per cent of the light to 
the prism and eye to provide a fixed image of 
the target. The light transmitted by the first 
mirror is reflected back through the same mir¬ 
ror by an aluminum coat on the front of the 
second mirror, and is turned by the prism to 



Figure 8. Photograph of B-29 pedestal sight with 
stadiameter. 

enter the eye in almost the same direction as 
the first beam. The second mirror can be ro¬ 
tated about a vertical axis through a mechanism 
linked to the range knob of the pedestal sight 
so as to control the apparent separation of the 
two images. It can also be adjusted to bring the 
two images into coincidence in the vertical di¬ 
rection. Figure 8 shows a photograph of the 
model mounted on the B-29 pedestal sight. 


The basic design and the mirrors with ap¬ 
propriate high-efficiency coats were supplied to 
the General Electric Company. A prototype 
model was constructed just before the end of 
World War II, but information is not available 
regarding any tests that may have been made. 

The design described above was intended 
merely to permit tests aimed at establishing 
the usefulness of a stadia ranging device. In a 
future model it would be very desirable to ar¬ 
range for bilateral motion of the two images so 
that aiming could be done on the point of con¬ 
tact of their wings where attention must be 
concentrated for making the stadiametric 
setting. 


13 4 RECOMMENDATION BY NDRC 

The stadiametric ranging device for the B-29 
fire-control system should be developed and 
tested, since it offers a promising solution of 
the requirements. Several alternative designs 
are possible. Unity power has marked advan¬ 
tages if it is adequate, since the entire field 
and also a large part of the present aperture 
can be used. Comparisons should be made with 
the present ranging equipment to determine 
the relative accuracy of the two methods, par¬ 
ticularly in the case of fast approaches when it 
is difficult to hold the target steady at the center 
o^ the field. If the present stadiametric device 
appears to be promising, consideration should 
be given to developing a bilateral beam splitter, 
so that the gunner can aim at the point of con¬ 
tact between the two images of the target. 



Chapter 14 

ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 

By Hobert W. French, Jr. a 


T he use of optical instruments in aircraft 
required suitable mounts to reduce the ef¬ 
fects of vibration upon optical performance. 
Since no satisfactory mounts were available, 
their development was undertaken, first by the 
Institute of Optics, University of Rochester, 
and later by the Kodak Research Laboratories 
and the Technicolor Motion Picture Corpora¬ 
tion, all under contracts with the NDRC. The 
mounts developed by the three groups differ in 
design and performance, but both laboratory 
and field tests demonstrate that all make possi¬ 
ble a wider application of optical instruments 
in aircraft and under other conditions of severe 
vibration. 


141 INTRODUCTION 

Early in 1941, an investigation of aids to 
night vision was undertaken by NDRC, at the 
request of the Air Corps under Project AC-26. 
The Institute of Optics, University of Rochester, 
under Contract OEMsr-160, carried out exten¬ 
sive studies and developments in this field be¬ 
tween 1941 and 1945, first under Section D-3 
and later under Section 16.1 of NDRC. A con¬ 
ference with representatives of Section D-3 and 
certain British aircraft experts indicated a need 
for some device to bridge the gap between 
locating enemy aircraft at night with AI equip¬ 
ment and the actual sighting and firing of the 
guns of the pursuit planes. Preliminary studies 
of the problem led to the development of a 
wide-field 6x binocular night sight with illu¬ 
minated reticle, for use by the pilot or gunner 
in the night interceptor planes. One serious ob¬ 
stacle to the practical use of this device, how¬ 
ever, was the detrimental effect of plane vibra¬ 
tion upon its optical performance. Since no 
effective mounting for an optical sight with 

a Institute of Optics, University of Rochester, 
Rochester, New York. (At Argus, Incorporated, Ann 
Arbor, Michigan, since November 1, 1945.) 


this magnification was available, the Rochester 
group was faced with the necessity of develop¬ 
ing a suitable one. After initial experiments 
with modifications of the rubber-in-shear type 
of shock mounting proved disappointing, the 
gimbal type of mount was conceived. Several 
successful mounts embodying this principle 
were built, both for binocular and monocular 
systems. 1 * 2 * 3 

To provide models better adapted to produc¬ 
tion and suitable for installation in specific 
aircraft, the Eastman Kodak Company Camera 
Works was requested by Section 16.1, NDRC 
(Contract OEMsr-1090) to redesign the Roch¬ 
ester gimbal mounts, both binocular and mo¬ 
nocular. 4 This Eastman design was later incor¬ 
porated in the Air Forces specifications when 
procurement was ordered. 

In addition to the gimbal mount, two other 
types of antioscillation mounts were developed 
under Section 16.1, NDRC. The Kodak Re¬ 
search Laboratories entered into a contract 
(OEMsr-392) to improve the definition in aerial 
photography. 5 An important phase of this prob¬ 
lem involved a suitable antivibration mount for 
aerial cameras. Experience gained with camera 
mounts led to a somewhat different solution of 
the vibration problem, but one which was 
equally applicable to the mounting of telescope 
systems. 6 ’ 7 * 8 Models of binocular mounts were 
built, both for aircraft and shipboard use. Con¬ 
fronted with the vibration problem in a peri- 
scopic binocular scanning device for aircraft 
use, the Technicolor Motion Picture Corpora¬ 
tion evolved (Contract OEMsr-617) still an¬ 
other type of antivibration mounting. 9 This 
mounting also was applied to other types of 
optical systems, among them binoculars for air¬ 
craft and shipboard use. 10 

142 VIBRATIONS IN OPTICAL SYSTEMS 

While vibration is generally harmful to any 
delicate instrument, in an optical instrument it 




VIBRATIONS IN OPTICAL SYSTEMS 


511 


may not only cause mechanical damage, but 
it is almost certain to affect the optical per¬ 
formance adversely. Satisfactory antivibration 
or “shock mountings’’ had been developed for 
the protection of aircraft gyroscopic flight in¬ 
struments prior to 1941, 11 but these mountings 
were not directly applicable to optical instru¬ 
ments. The problem of vibration in aerial pho¬ 
tography was recognized in World War I, and 
some progress was made toward its solution at 
that time. 12 In the intervening years to 1941, 
however, very little more appeared to have 
been done on the aerial camera problem, and 
the mounting of other types of optical instru¬ 
ments in aircraft was apparently neglected en¬ 
tirely. 

The purpose of the shock mountings used on 
flight instruments is to protect the delicate 
mechanism from the effects of the linear vibra¬ 
tional forces, thus preventing mechanical dam¬ 
age. While the protection of optical systems 
from mechanical damage by these forces may 
occasionally be necessary, this is not the pri¬ 
mary problem. In viewing a distant object in 
a telescope system such as a pair of binoculars, 
a linear translation of the instrument as a 
whole in any direction causes no apparent mo¬ 
tion of the distant object. This pure linear 
vibration of such a telescope, provided its com¬ 
ponent parts are sufficiently rigid so that no 
relative motion occurs, has no effect upon its 
optical performance. Furthermore, rotation of 
the telescope about its optic axis, or about any 
axis parallel to it, has no optical effect. Only 
components of rotation about axes lying in a 
plane perpendicular to the optic axis cause ap¬ 
parent motion of the distant object. 

The magnitude of the effect of angular vibra¬ 
tion depends upon the magnification of the 
optical system as well as upon the angular am¬ 
plitude of the vibration. In an erect-image 
telescope, such as the ordinary binocular, the 
apparent angular motion of the distant object 
will be (m — 1) times the angular motion of 
the optic axis, where m is the magnification. 
In an inverted image system the apparent mo¬ 
tion will be (m + 1) times the real motion. It 
is evident that erect-image systems are prefer¬ 
able to inverted-image systems when vibration 
is present. For example, a 3-power erect-image 


system is only affected half as much as a 
3-power inverted-image system. Furthermore, 
unity-power erect-image systems are entirely 
unaffected by angular Vibration, that is, they 
are invariant. Even for magnifications up to 
two or three power, very little effect is ob¬ 
served. Hence low-power erect-image systems 
do not generally need a special mounting to 
reduce angular vibration. For greater magni¬ 
fication, some type of mount is usually required, 
depending upon the optical performance de¬ 
manded and the severity of the vibration. 

In any type of antivibration mount, the vi¬ 
bration is never entirely eliminated; it is only 
reduced in amplitude until its effect is neg¬ 
ligible or imperceptible. The measurement of 
effectiveness of a mount is the ratio of the am¬ 
plitude of the instrument within the mount to 
the amplitude of the vibration impressed upon 
the mount. This ratio is called the “transmissi- 
bility” or “magnification factor.” As has been 
mentioned, the optical effects of vibration are 
confined primarily to angular oscillations of 
the optic axis. A satisfactory mount must there¬ 
fore reduce to negligible amplitude all angular 
vibrations about axes lying in any plane per¬ 
pendicular to the optic axis. The amplitude may 
be considered negligible if the optical perform¬ 
ance of the instrument is not perceptibly dif¬ 
ferent in the presence of the vibration than it is 
in the complete absence of any vibration. It may 
sometimes be necessary to modify this rigorous 
requirement, and to consider a mount satisfac¬ 
tory if it permits an instrument to perform 
its useful function, even though some effects of 
vibration are still perceptible. 

The antioscillation mount must not only re¬ 
duce the effects of vibration in the optical sys¬ 
tem, it must also provide a satisfactory support 
and method of attachment so that the instru¬ 
ment is convenient to use. This problem is more 
difficult when the instrument is a gunsight, for 
then the direction of the optical axis must 
accurately maintain a fixed and known orienta¬ 
tion with respect to the gun bore. In telescopic 
instruments for observation only, such accurate 
alignment is not necessary, but a smoothly 
operating swivel is essential for ease of use. 
An antioscillation mount will not function 
properly if any external forces act upon the 



512 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


mounted instrument except through the filtering 
system. Hence, mounts must be provided with 
headrests or other devices which will properly 
position the observer's eyes without allowing 
him to touch the instrument. In some applica¬ 
tions, the mounted system must also be shielded 
against buffeting by the wind. 


14.3 types OF ANTIOSCILLATION 
MOUNTS 

In all of the mounts considered here, the 
optical instrument is, in effect, suspended at its 
center of gravity in such a way that it is free 


tude of the torque and the moment of inertia 
of the system about its center of gravity. To 
limit the amplitude of these oscillations at and 
near the natural frequency, some type of damp¬ 
ing is always necessary. These mounts differ 
from one another in the method of support 
at the center of gravity, in the form of the re¬ 
storing torque, and in the type of damping. 


14,31 Gimbal Mounts 

In the mounts developed at Rochester and 
Eastman the optical system is supported in 
gimbals. 4 ’ 13a Unlike the pendulous gimbal mount 



AZIMUTH BRAKE 
ORUM 

COLLETS 


OUTER FRAME 

INTERMEDIATE 

FRAME 

INTERPUPILLARY 


Figure 1 . Eastman antioscillation-mounted binoc¬ 
ular. 



Figure 2. Eastman antioscillation binocular in 
P-61 aircraft. 


to rotate about it. If an instrument could be 
suspended exactly at its center of gravity, in 
perfectly frictionless bearings which permit 
complete rotational freedom about any axis, it 
would be impossible to exert any torque on the 
system through its support. The orientation of 
the instrument would remain fixed in space, 
regardless of any linear or angular motion of 
the support. Such a mounting is not possible, 
for perfectly frictionless bearings are unattain¬ 
able, nor would it be desirable, for it is neces¬ 
sary to point the instrument by means of its 
support. Consequently, some type of restoring 
torque between the instrument and its mount 
is used to establish a mean or neutral angular 
relationship between the two. Because of this 
restoring torque, the instrument oscillates with 
a natural frequency determined by the magnU 


commonly used for ships' compasses and chro¬ 
nometers, the center of gravity of the optical 
instrument is accurately located at the inter¬ 
section of the gimbal axes, hence gravity exerts 
no torque on the system. The optical axis is per¬ 
pendicular to the plane of the gimbal axes, 
permitting the former 2 degrees of rotational 
freedom. The gimbal axes are supported in 
precision self-aligning ball bearings, providing 
the maximum possible freedom of rotation. 
This mount is illustrated in Figure 1. In Fig¬ 
ure 2 it is shown installed in the P-61 aircraft. 
Figure 3 shows a monocular mount, based on 
a similar design, for the gunner’s station in 
the P-61. 

The restoring torque and the damping are 
combined in a single air dashpot unit for each 
gimbal axis. The displacement of an elastic dia- 




TYPES OF ANTIOSCILLATION MOUNTS 


513 


phragm, either of natural or synthetic rubber, 
provides the restoring torque. This diaphragm 
also forms one side of an air chamber. The dis¬ 
placement of the diaphragm forces the air in 
or out of the chamber through a small hole 
in the chamber wall. The length of this hole is 
large compared with its diameter, so that the 
air flow is essentially laminar. This type of air 



ADAPTER 

CASTING 


MOUNT 
BOLTS 


CLAMP SCREWS 


GUNNER'S SIGHT 
ELEVATION UNIT 


Figure 3. Eastman antioscillation-mounted monoc¬ 
ular. 

flow results in viscous damping, in which the 
resisting force is proportional to the velocity. 


14,3 2 Ball-Cone and Rubber Shell Mounts 

The mounts developed by Kodak Research 
Laboratories and Technicolor as applied to bin¬ 
oculars, support the instrument at its physical 
center of gravity. This is possible in a binoc¬ 
ular, since the center of gravity falls outside 
the case, midway between the two halves, and 
below the hinge. Rotational freedom is obtained 
by a ball and socket type of universal joint, 
with the center of the ball located at the center 
of gravity of the binocular. This type of mount 


has three degrees of freedom of rotation about 
the center of gravity, whereas the gimbal mount 
permits only two. 

While both the Kodak Research Laboratories 



Figure 4. Kodak antioscillation unit. 

and Technicolor mounts use frictional damping 
with this ball mount, they differ in the manner 
in which the damping is obtained, and in the 



HEAD REST 


GUIDE YOKE 

VIBRATION 
FILTER UNIT 


HEAD REST 
ADJUSTMENT S 


Figure 5. Kodak antioscillation mount without 
binocular. 

method of applying the restoring torque. In 
the Kodak mount, 6 - 7 the ball, attached to the 
binocular system, rests in a plastic conical seat 





































514 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


fixed on the top of the vertical supporting pillar 
(see Figure 4). Angular motion of the binoc¬ 
ular, with resultant rotation of the ball in the 
plastic seat, is damped by the frictional forces 
between the ball and seat. The magnitude of 
the damping torque may be controlled by vary¬ 
ing the diameter of the ball, or the taper of 
the cone, or by using different types of plastics 
having different coefficients of friction on steel. 
The restoring torque is applied by a helical 
metal spring concentric with the supporting 



Figure 6. Kodak antioscillation mount with 
binocular. 


pillar, and placed between it and the binocular 
unit. Since both ends of the spring are anchored, 
restoring torque is applied about any direction 
of rotation. The mount for aircraft and for 
shipboard use is illustrated in Figures 5, 6, 
and 7. 

In the Technicolor mount, 10a the ball is also 
attached to the binocular, but is held in a re¬ 
silient rubber socket formed by two rubber 
washers compressed in a metal sphere com¬ 
posed of two hemispherical cups (see Figures 8 
and 9). This metal sphere is firmly attached 
to the top of the vertical supporting pillar. 
Any rotation of the ball about its center sets up 
shearing forces in the rubber which apply the 
restoring torque. Thus the same restoring 
torque is applied about any axis of rotation. 
Frictional damping is obtained by a felt washer 
attached to the binocular part of the system 
and pressing against the outside of the top 
metal hemisphere. 101 " Since the latter is attached 
to the supporting pillar, angular motion of the 


instrument results in relative motion between 
the felt and the metal hemisphere, with fric¬ 
tional damping of the motion. In one version 
of the mount, the damping may be varied by 
changing the pressure of the felt washer upon 
the hemisphere. The restoring torque depends 
upon the elastic properties of the rubber and 
upon the physical dimensions of the rubber 
washers, the hemispheres, and the ball. Fig¬ 
ures 10 and 11 show models of this mount for 
use in aircraft and on shipboard. All of the 
binocular mounts have interpupillary adjust¬ 
ments so designed as to preserve the location 
of the center of gravity with respect to the 
support point. In the University of Rochester 
and Technicolor designs this is accomplished by 
separating the two halves of the binocular and 
pivoting them about axes so located' that the 



Figure 7. Kodak antioscillation alidade mount. 

shift in the center of gravity is a minimum. 
In the Kodak Research Laboratories' design, a 
unique cam arrangement permits the binoculars 
to be used without alteration. In all the models, 
headrests and eye guards attached to the sup¬ 
porting members position the observer’s eyes 
with respect to the exit pupils of the instru- 





TYPES OF ANTIOSCILLATION MOUNTS 


515 


ment, yet prevent contact or interference with 
the suspended instrument. In those Kodak Re¬ 
search Laboratories and Technicolor models 
designed for shipboard use, the entire instru¬ 
ment is enclosed in a shield to prevent transient 
oscillations from gusts of wind. This feature is 
not necessary for mounts used in enclosed air¬ 
plane cockpits. 


14.3.3 Comparison of Designs 

Without any consideration, for the moment, 
of the performance of these different types, 


as the Kodak mount, the Technicolor design 
approaches the Rochester and Eastman units 
in bulk, weight, and complexity, although not 
in the precision required. 

The gimbal principle, as used in the Rochester 
and Eastman mounts may be applied to monoc¬ 
ular telescopes or other optical systems having 
the center of gravity located either inside or 
outside the body of the instrument, 3 * 4 while the 
Kodak and Technicolor mounts are restricted 
in this respect. Because of the separate damper 
units about each gimbal axis, in the Rochester 
and Eastman mounts the restoring torque and 
damping can be independently controlled to 


PRESSURE SPRINGS 
CARDAN HINGE 


FRICTION PLATE 


UPPER HEMISPHERE 


RUBBER WASHER 
(UPPER) 


LOWER HEMISPHERE 


RUBBER WASHER 
(LOWER) 


SUPPORT BRACKET 
SUPPORT STUD 



BINOCULAR CONNECTION 


ADJUSTABLE NUT 
SPRING WASHER 


ROTATION STOP 


BASE CONNECTION 


Figure 8. Technicolor antioscillation unit. 



some comparison may be made of their general 
construction. Of those intended for use in air¬ 
craft, the Rochester and Eastman mounts are 
the largest and the heaviest. They also require 
more precision parts in their manufacture and 
hence are more expensive. The Kodak mount is 
the smallest, lightest, and simplest in construc¬ 
tion. As pointed out above, it requires no modi¬ 
fication of the standard type of binocular, and 
is undoubtedly the least expensive to build. 
Although embodying the same simple principle 


give the same natural frequency about both 
axes, even though the moment of inertia is dif¬ 
ferent about the two. Further, the gimbal sys¬ 
tem provides vibration isolation about only two 
axes—the two which affect optical perform¬ 
ance. The third degree of freedom in the Kodak 
and Technicolor mounts adds almost nothing to 
the optical performance, and may cause greater 
linear motion and uncertainty in the position 
of the eye point. Finally, the performance of 
the balanced gimbal mount is completely inde- 








































































516 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


pendent of its orientation with respect to grav¬ 
ity, while the Kodak type of ball mount is 
limited to orientations within 60 degrees of its 
normal position, and the characteristics of the 
Technicolor mount are altered if the direction 
of gravity is changed materially. While these 
points are of concern in some applications, they 
are of little importance for most requirements. 



Figure 9. Technicolor antioscillation unit dis¬ 
assembled. 

By far the most important characteristic of an 
antioscillation mount is its vibration perform¬ 
ance. 


144 LABORATORY TESTING 

PROCEDURES 

Since the results of any test can be properly 
evaluated only if the conditions under which 
the test is made are known, a careful consider¬ 
ation of the testing procedures is necessary. 
All three groups have conducted laboratory tests 
of their own type of mount. In addition, all 
three types of mount have been tested at the 
University of Rochester. Both the procedure 
and the results differ among the three groups. 

One purpose of a laboratory test is to study 
the performance of a mount under known and 
controlled conditions of vibration. Only by such 
a study can optimum design and performance 
be obtained. A second and equally important 
objective of the laboratory test is to predict 


the performance of a mount under service con¬ 
ditions. To meet this requirement, the labora¬ 
tory test should simulate service conditions as 
far as possible. Since the latter are not easily 
measured, and are known to vary widely,, some 
compromise is obviously necessary. 


14,41 Shake Tables 

The shake table developed at the University 
of Rochester specifically for the testing of anti¬ 
oscillation mounts submits the mount simul- 



Figure 10. Technicolor aircraft antioscillation 
mount—vacuum cup mount. 

taneously to the three components of linear 
vibration and to the two components of angu¬ 
lar vibration which affect optical performance. 14 
The amplitudes of these vibrations may be ad¬ 
justed over a range greater than that observed 
in any service conditions. In any one test, how¬ 
ever, these amplitudes are held constant, and 
the transmissibility is measured as a function 
of frequency. The frequency is varied from 50 
to 2,000 vibrations per minute [vpm], a range 
which includes the fundamental frequencies of 
aircraft as well as marine engines. The fre¬ 
quency of linear and angular vibration are the 



LABORATORY TESTING PROCEDURES 


517 


same in any one test. Great care has been exer¬ 
cised in the design and construction of this 
shake table to insure that only one frequency at 
a time is present. While this condition obviously 
does not exist in actual service use, it is very 
important that it hold true in laboratory tests 
if an intelligent analysis of the performance 
characteristics is to be made. As a further pre¬ 
caution, the amplitudes of both the impressed 



Figure 11. Technicolor shipboard antioscillation 
mount. 

and the resultant vibration are measured at 
each frequency. No assumptions are made as 
to the shake table having a constant amplitude 
over the frequency range. The performance is 
expressed in terms of the transmissibility or 
ratio of these two amplitudes, not in terms of 
the resultant amplitude alone. Although the 
impressed amplitude is held approximately con¬ 
stant over the entire range of frequencies, the 
transmissibility is relatively independent of 
amplitude over a wide range of amplitude, 
hence is a true characteristic of the mount, not 
of the mount and shake table combined. In 


order to insure that only one frequency at a 
time is present, and to provide a positive con¬ 
trol of the amplitude at each frequency, the 
table is exceedingly rigid and is positively 
driven. Unlike shake tables which have a natu¬ 
ral frequency of their own, this table provides 
an amplitude which is almost entirely inde¬ 
pendent of load or frequency. This permits a 
direct quantitative comparison between instru¬ 
ments of widely differing size, shape, and 
weight. 

Although the results of a shake table test are 
contained in the final Kodak Research Labora¬ 
tories report, 7 no description or discussion of 
the testing equipment or procedure is given. 
However, the shake table used in this test is 
described in an earlier report. 5a This shake table 
provides only one angular component about a 
horizontal axis. If the center of gravity of the 
mounted system is displaced from this axis, one 
component of linear vibration is also obtained. 
In the test reported, however, it is stated that 
the axis of rotation is transverse through the 
center of gravity; hence no linear component 
is present. 7a The test was made at constant 
impressed angular amplitude, and the resultant 
amplitude, rather than the transmissibility, is 
measured. The frequency range extends only to 
500 vpm. 

The shake table used by Technicolor was built 
for the testing of a periscope scanning device, 
and is described in a report on that equipment. 98 
Like the Kodak shake table, this unit provides 
only one component of angular vibration, and 
one linear component whose amplitude depends 
upon the distance between the center of gravity 
of the mount and the rotational axes of the 
shake table. By changing the orientation of the 
optical axis with respect to the axis of rotation 
of the table, all three angular components can 
be tested, but not simultaneously. Likewise, the 
three linear components can be checked, again, 
only one at a time. The frequency range of the 
table extends to 900 vpm. 


144,2 Testing Methods 

All three groups measure the angular ampli¬ 
tude of the optical system by the deviation of 



518 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


a beam of light reflected from a small mirror 
rigidly attached to the system. In the Rochester 
tests, the deviation of the beam is obtained by 
measuring the linear displacement of a spot of 
light on a screen. Presumably the Kodak tests 
were similar. The Technicolor technique records 
the motion of the spot on a photographic film 
wrapped around a rotating drum. Although this 
requires considerably more time for a test, it 
has the advantage of providing a permanent 
record which can be studied in detail at leisure. 
The Rochester group on occasion photographed 
a distant object through the optical system of 
a binocular under test. 2 This method gives a 
very good record of the overall performance, 
but does not supply enough detailed information 
to permit a thorough analysis of the perform¬ 
ance unless photographs are made at a large 
number of frequencies. The latter procedure 
was deemed too time consuming to be practical. 

Since none of the groups has reported in any 
detail the actual performance of these testing 
devices, it is difficult to make any real evalu¬ 
ation of their relative merits. The unit was de¬ 
signed at the University of Rochester and many 
of the tests were carried out on it there. From 
this experience, it is possible to point out some 
of the more important factors to be considered 
in such an evaluation. First of all, it would 
seem very desirable to test a mount over the 
entire range of frequencies which it will en¬ 
counter in service use. In aircraft, the normal 
operating engine speeds are around 1,800 rpm, 
although marine engines run considerably 
slower. For this reason, the Rochester shake- 
table frequency range extends from 50 to 2,000 
vpm and tests are always made up to this 
frequency. The limit of the Kodak tests is 
1,000 vpm, and the Technicolor 900 vpm. While 
it may be argued on theoretical grounds that 
for any mount having a natural frequency below 
these values the transmissibility at 2,000 vpm 
will always be lower than at 1,000 or 900 vpm, 
such is not necessarily the case. Secondary 
resonance points may arise from flexures in the 
mount. While it is the object of the designer 
to avoid such resonance points, it is exceedingly 
difficult, if not impossible, to predict whether 
they will be present or not. It is far safer to test 
the mount over the entire frequency range. 


In the Rochester tests, the amplitudes of both 
the impressed and the resultant angular vibra¬ 
tions are measured at each frequency. This is 
done because experience has shown that, in spite 
of all precautions, the impressed amplitude 
varies somewhat with frequency due to flexures 
in the table structure. If great care is not ex¬ 
ercised in both the design and workmanship 
of the table, the amplitude may vary several 
hundred per cent over the frequency range, 
the nature of this variation changing with dif¬ 
ferent loads on the table. Depending on the 
phase relation between the impressed force and 
the flexure, the true amplitude may be either 
greater or less than expected. The Kodak data 
is given in terms of resultant amplitude, with 
a single value of impressed amplitude. While 
it is stated that this impressed amplitude is 
constant, either the tolerance on this “con¬ 
stancy” should be given or the data should be 
in terms of the transmissibility ratio. The 
Technicolor data is likewise reported as result¬ 
ant angular amplitude, with the photographic 
records showing the impressed amplitude at 
only one frequency. The occurrence of nonsinu- 
soidal wave forms is mentioned and attributed 
to vibrations in the bed of the testing equip¬ 
ment. 100 This would seem to emphasize the ne¬ 
cessity for measuring impressed amplitude at 
each frequency. 

While a test which simultaneously impresses 
the components of linear vibration and the two 
important angular components more closely ap¬ 
proaches actual conditions of use, its primary 
advantage is probably that of convenience. 
There is, however, a possibility of interaction 
between the two angular components which 
might lead to a different result if each com¬ 
ponent were tested separately. Both theoretical 
considerations and experience indicate that this 
possibility is slight in a well-behaved mount. 
As has been pointed out in the Kodak Research 
Laboratory report, 8 if linear and angular vi¬ 
brations are impressed simultaneously on a 
system which is not completely balanced, the 
phase relation between the two must be taken 
into consideration. In such a case, it is better 
to test the effects of linear and angular vibra¬ 
tions separately, and to take the pessimistic 
attitude by adding the resultants. In practice, 



LABORATORY PERFORMANCE 


519 


however, the system should be so well balanced 
that only a negligible amount of linear-angular 
coupling exists. The primary reason for testing 
with linear vibrations present is to determine 
whether the linear accelerations, through flex¬ 
ure of the inner components, impair the effec¬ 
tiveness of the mount. Proper balance may be 
determined easily and accurately by a static 
test, but the effects of flexures require a 
dynamic test. 

14 5 LABORATORY PERFORMANCE 

The generally accepted method of expressing 
the shake-table performance of an antioscilla¬ 
tion mount is a plot of transmissibility, or 
ratio of resultant to impressed angular ampli¬ 
tude, against frequency of impressed vibration. 
Resultant angular amplitude may be used in 
place of transmissibility provided the impressed 
amplitude is known to be constant over the 
entire frequency range. It is also customary to 
resolve the angular vibration into components 
about a horizontal axis and a vertical axis, both 
perpendicular to the optical axis, and to give 
plots of the transmissibility for each compo¬ 
nent. The component about an axis parallel to 
the optical axis may also be measured, but is 
usually omitted since it does not affect optical 
performance. In the event that linear vibration 
is also present, the amplitudes of the three 
linear components at the center of gravity 
should also be given. 

Tests Reported 

A Rochester report gives the performance 
curve of a Type II-c binocular antioscillation 
mount as a graph up to 700 vpm with the value 
of transmissibility at 1,600 vpm and the maxi¬ 
mum value between 750 and 2,000 vpm. 2 Photo¬ 
graphs of a distant building taken through the 
binoculars vibrating at 1,600 vpm, with the 
mount in operation and clamped, and also with¬ 
out vibration, are included in the same report. 
In a later report, 14a the results of a number of 
tests on Type Il-b mounts are given in a single 
graph for frequencies up to 2,000 vpm. The 
standardized conditions under which these tests 
were run are also given. 


A Kodak report gives the results of a single 
test on a binocular mount, for one component 
of angular vibration, wit If] no linear component 
at the center of gravity. 7 * The frequency range 
is 0 to 500 vpm. 

A Technicolor report 10 contains a consider¬ 
able number of graphs, showing the perform¬ 
ance of their units under a variety of condi¬ 
tions. These curves show the effect of inter¬ 
pupillary adjustment, the effect of rotation only 
and of rotation plus translation, changes in 
restoring force, and different types of supports. 
The photographic records from which the 
graphs are drawn are also included in some 
cases. The highest frequencies reported in these 
tests range between 750 and 1,000 vpm. Except 
for this limitation on frequency range, the data 
is much more comprehensive than that of the 
other two groups. In addition to the above tests, 
all three types of mounts were tested at Roch¬ 
ester under the standardized conditions estab¬ 
lished for the Rochester mounts. 14 These tests 
cover the gimbal mounts built at Rochester; 
three gimbal mounts of the Eastman Kodak 
Camera Works production design, one each 
made by the Camera Works, the Houston Com¬ 
pany, and the Robinson-Houchin Company; 0 
and a monocular unit made by the Camera 
Works. Of the ball-type mounts, three tests of 
the Technicolor shipboard model and one of the 
Kodak Research Laboratories’ unit are re¬ 
ported. 

14 5 2 Evaluation of Tests 

Any comparison of the various types of 
mounts which is based upon the above informa¬ 
tion must necessarily be limited, since this in¬ 
formation is far from sufficient for a truly 
complete analysis. The results of many tests 
on Rochester mounts have not been reported 
because of obvious limitations of time and 
space. The same is undoubtedly the case for 
both the Kodak and Technicolor tests, although 
the latter are reported in the most detail. Fur¬ 
thermore, the primary objective of each group 
has been to develop its own type of mount; 

b These units were the first production models built 
for the Army Air Forces for installation in P-61 night 
fighters. 




520 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


hence the tests have been made with this in 
mind rather than any comparison between 
types. Only the tests at the University of 
Rochester of the various types provide any 
direct basis for comparison, and these were 
limited to a single test on one type, and three 
tests on the other. 

The performance of any antioscillation mount 
should be judged primarily in terms of its 
effectiveness in reducing the harmful effects of 
vibration upon optical performance. As we have 
pointed out earlier, only angular vibrations of 
the optical system about an axis perpendicular 
to the optical axis are harmful, but it must be 
remembered that such vibrations may result 
from impressed vibrations which are either 
linear or angular, or a combination of both. 
In any of these mounts, the transmissibility, 
which is a quantitative measure of effective¬ 
ness, varies greatly with the frequency of the 
impressed vibration. Hence any statement of 
effectiveness must include this frequency. In 
general, the higher the impressed frequency 
' above the natural or resonance frequency of 
the mount, the lower the transmissibility and 
the more effective the mount. For this reason, 
it is desirable for the mount to have as low a 
natural frequency as possible. Unfortunately, 
practical design considerations and factors con¬ 
cerned with service use impose a lower limit. 
The effectiveness of the mount should be eval¬ 
uated in terms of its transmissibility at that 
frequency to which it will be subjected in actual 
use. In aircraft, the predominant vibration 
arises from the engines, which have a funda¬ 
mental frequency of 1,600 to 2,000 vpm. In 
mounts intended for aircraft use, the transmis¬ 
sibility should then be compared at these fre¬ 
quencies. Since neither the Kodak nor Techni¬ 
color reports contain any data at these fre¬ 
quencies, the sole basis for comparison is the 
Rochester tests. 14 In this range, the gimbal 
mounts built at the University of Rochester 
have transmissibilities between 1 and 5 per 
cent, c the latter value being on the upper limit 
of acceptable performance. Two of the three 
Eastman production design gimbal mounts also 

c Transmissibility may be expressed either as a per¬ 
centage or as a decimal fraction. Technicolor reports 
use the former, and University of Rochester reports 
the latter. 


have transmissibilities of about 5 per cent in 
this range. 14b A Technicolor shipboard mount, 
tested at three values of the damping adjust¬ 
ment, showed transmissibilities of 5 per cent 
or less for low values of damping, but values 
of 10 to 15 per cent for moderate and heavy 
damping. 140 The single test on the Kodak mount 
gave values between 5 and 7 per cent. 14(1 

The engine vibration is the predominant vi¬ 
bration in aircraft, but by no means the only 
one. There are, of course, higher harmonics of 
the fundamental engine frequency, from the 
multiple cylinders, multibladed propellers, and 
natural resonances in the plane structure. The 
amplitude of these higher harmonics is usually 
much smaller than that of the fundamental, 
and the effectiveness of the mounts is much 
greater at higher frequencies; hence higher har¬ 
monics may be neglected. Among the lower 
frequencies, which cannot be neglected, are vi¬ 
brations arising from flutter in the air foil, 
beats between the engines in multiengine 
planes, and most important, the motion of the 
plane as a whole in pitch, yaw, an^l roll. The 
frequency of these latter motions is apt to be 
very close to the resonant frequency of the 
mounts. An inspection of the transmissibility 
curves will show that none of the mounts is 
effective at or near its resonance frequency, for 
the transmissibility is greater than 100 per 
cent. Since 100 per cent represents the per¬ 
formance which would be obtained if no mount 
at all were used, antioscillation mounts are 
actually harmful in this frequency range. For 
this reason, it is necessary to provide consider¬ 
able damping in a mount, so that oscillations 
excited by the motion of the plane will not build 
up in amplitude, but will die out quickly. The 
second point at which mounts should be com¬ 
pared is therefore at the resonant frequency. 

As a result of experience gained in flight 
tests, the Rochester mounts are provided with 
sufficient damping so that the maximum trans¬ 
missibility at resonance does not exceed 120 per 
cent. Both the tests at Kodak 7a and at Roch¬ 
ester 140 show that the Kodak mount has a very 
low transmissibility at resonance, between 120 
and 140 per cent. The Technicolor mounts have 
an adjustment which permits a wide range of 
damping. In their report, 10 values all the way 




FIELD PERFORMANCE 


521 


from 130 to 270 per cent are shown. In the 
Rochester tests on the Technicolor unit, the 
damping was varied to give resonance trans- 
missibilities from 120 per cent to well above 
300 per cent. It was only at low values of damp¬ 
ing that the transmissibility in the 1,600 to 
2,000 vpm range was reduced to 5 per cent. 

There are both theoretical reasons and ex¬ 
perimental evidence to show that low damping 
gives the best performance at high frequencies. 
The selection of the optimum damping is there¬ 
fore a compromise between the highest trans¬ 
missibility which can be tolerated at resonance, 
and the best performance at the predominant 
frequency, which is 1,600 to 2,000 vpm in air¬ 
craft. Obviously, a low transmissibility at both 
points is highly desirable, but theoretical con¬ 
siderations discussed later set a lower limit 
which is dependent upon damping. 

146 BORESIGHTING 

Although the primary aim of an antioscilla¬ 
tion mount is to reduce the effect of vibration 
on optical performance, the use of mounts for 
optical gunsights introduces a further require¬ 
ment. In this application, the optical axis of 
the instrument, or the “line of sight,” must 
accurately maintain an angular relationship 
with the line of fire of the gun, that is, the 
optical sight must “boresight” accurately. In 
aerial gunnery, the boresighting must not have 
an error greater than about 1 or 2 mils (thou¬ 
sandths of a radian). 

This boresighting accuracy, at least in air¬ 
craft, is only required under conditions of vi¬ 
bration, since the guns are used only when the 
plane is in flight. The term “dynamic boresight¬ 
ing” has been used to designate the boresighting 
performance under conditions of vibration, 
while “static boresighting” refers to the per¬ 
formance in the absence of vibration. It might 
appear, at first glance, that static boresighting 
is unimportant in service use, but such is not 
the case. The boresighting is customarily 
checked before each operational flight, or after 
any servicing of the guns. Since a convenient 
and accurate check of boresighting can only be 
made with the plane on the ground, the normal 
flight conditions of vibration are not present. 


In fact, it is difficult to make such a check with 
the plane motors revolving at cruising speed. 
It is therefore desirable, that the boresighting 
be done in the absence 'of vibration, and this 
requires that both the static and dynamic bore¬ 
sighting be accurate. 

Since the Rochester mounts were initially 
designed for aircraft gunsight use, the problem 
of boresighting was given important consider¬ 
ation, along with the vibration performance 
itself. For this reason, the use of antifriction 
bearings and viscous damping was adhered to 
in all of the mounts. As a result, the ball-bear¬ 
ing gimbal mounts meet both the static and 
dynamic boresighting requirements. The ball- 
type mounts, employing dry frictional damping, 
are not as satisfactory in this respect. While 
the dynamic boresighting of these mounts may 
be sufficiently accurate, as in the Kodak mount 
tested at the University of Rochester, 144 the 
static boresighting is inherently inaccurate. In 
the Technicolor mount tested at Rochester, 
neither the static nor dynamic boresighting was 
satisfactory for gunsight applications. 140 It 
should be remembered, however, that imper¬ 
fections in boresighting do not necessarily indi¬ 
cate poor vibrational performance. For appli¬ 
cations not requiring accurate maintenance of 
the line of sight, such as detection and recog¬ 
nition, the ball mount with dry friction damp¬ 
ing is entirely satisfactory. 

147 FIELD PERFORMANCE 

While the ultimate criterion of the usefulness 
of an antioscillation mount is its performance 
under service conditions in an aircraft or on 
shipboard, the difficulties in conducting such 
tests have been formidable. During World War 
II, both equipment and personnel at the various 
Army and Navy bases in this country were 
overburdened, hence the time and facilities 
which could be allotted to the testing of anti¬ 
oscillation mounts were limited. The results of 
such tests were consequently confined to the 
qualitative observations of military and civilian 
personnel, the former, in most instances, en¬ 
tirely new to the problem, and the latter un¬ 
avoidably biased by long familiarity with it. 
Nevertheless, these qualitative tests were val- 



522 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


uable in guiding the development of the mounts 
and in demonstrating their practicability. 

Although the Rochester mounts were sub¬ 
jected to a variety of tests, both by the Army 
Air Forces and the Navy, only the earlier tests 
have been reported in any detail. 18 The results 
of the several months of testing and operational 
use in night-fighter training at the Orlando Air 
Base has not been reported by the contractor. 
This program not only gave valuable aid to the 
improvement of mounts, but clearly proved 
their tactical value. As a result of these tests, 
the Air Forces issued procurement orders for 
these mounts for use in P-61 night fighters. 

On other occasions, Rochester mounts were 
tested in Navy patrol planes of various types, 
for sea search and surface-vessel recognition 
and for submarine detection at night. Although 
the tactical effectiveness seemed to be less 
clearly shown in these applications, the vibra¬ 
tion performance of the mounts was in every 
case satisfactory. Only one shipboard test was 
reported, 13b and the vibration to which the in¬ 
strument was subjected was so slight that no 
far-reaching conclusions could be made. 

Both aircraft and shipboard tests of the Tech¬ 
nicolor mounts have been reported. 10 The air¬ 
craft test, in a PBY patrol bomber, gave val¬ 
uable information as to the optimum damping, 
and indicated the need for certain minor me¬ 
chanical modifications, but the vibration per¬ 
formance was satisfactory. The shipboard tests 
on a destroyer yielded further information on 
the type of vibration encountered in surface 
vessels and on the optimum damping for these 
conditions. Again the vibration performance of 
the antioscillation mount was a great improve¬ 
ment over that of rigid mounts or hand-held 
instruments. 

The Kodak mount was tested on a destroyer 
with encouraging results, and the results of 
these tests were the basis for redesigning the 
alidade mount to provide greater stiffness and 
filtering for linear vibration. 

VISCOUS VERSUS FRICTIONAL 
DAMPING 

The relative merits of viscous and frictional 
damping have been subject to some difference 


of opinion among the groups working on the 
problem of antioscillation mounts. The Roch¬ 
ester group upholds the advantages of viscous 
damping, while the Kodak and Technicolor 
groups adhere to frictional damping. These 
differences include both theoretical perform¬ 
ance and practical applications. 

Viscous damping exerts a resisting force 
proportional to the velocity of the system being 
damped, while frictional damping exerts a 
nearly constant force, almost independent of 
the velocity. Theoretically the two types of 
damping give rise to different characteristic 
curves of transmissibility versus frequency. An 
adequate discussion of the theory of damping 
would require more space than is available in 
this report, hence only an outline of the argu¬ 
ments and conclusions can be given here. 


14.8.1 The Function of Damping 

In the application of the theory of vibration 
isolation to antioscillation mounts for the op¬ 
tical systems, the only case which need be con¬ 
sidered is that in which the source of vibration 
is external to the system, and affects it through 
a harmonic motion of the supports. If the sup¬ 
ports are rigid, the system vibrates with the 
same amplitude as its surroundings, and there 
is no vibration isolation. If the supports are 
resilient, the amplitude of the system may be 
either greater or less than the amplitude of 
the surroundings, depending upon the ratio of 
the natural frequency of the system to the 
frequency of the surrounding or impressed vi¬ 
bration. In either case, the motion of the system 
is the result of a harmonic force transmitted 
through the resilient supports and dampers. 
The theoretical transmissibility versus relative 
frequency of a typical antioscillation mounting 
is shown in Figure 12. (1 The transmissibility is 
the ratio of the amplitude of the suspended sys¬ 
tem to that of its surroundings, while the rela¬ 
tive frequency is the ratio of the frequency of 
the surroundings vibration to the natural fre¬ 
quency of the mount. 

Curve A, Figure 12, represents a system 

d The theoretical derivation of these curves may be 
found in the literature. 15a 




VISCOUS VERSUS FRICTIONAL DAMPING 


523 


without damping. At very low relative frequen¬ 
cies, where the natural frequency of the mount 
is high compared with the impressed frequency, 
the transmissibility is unity. As the impressed 
frequency approaches the natural frequency, 
the transmissibility increases rapidly, becom¬ 
ing infinite at unity relative frequency, the 
resonant point of the system. As the relative 
frequency increases above the resonant point, 
the transmissibility decreases, passing through 
unity at a vibration frequency of 1.41 (\/2) 
and becoming less than unity for still higher 
frequencies. In the relative frequency range 
from zero to 1.41, the antioscillation mount is 



Figure 12. Transmissibility versus relative fre¬ 
quency. Curve A, zero damping, curve B, medium 
viscous damping, and curve C, heavy viscous 
damping. 

valueless, and, in fact, even harmful, for the 
transmissibility is greater than unity. At or 
near the resonant point, an undamped mount 
may cause real damage to the suspended sys¬ 
tem. Only for the relative frequency range 
above 1.41 is the mount useful in reducing the 
effects of vibration. 

Curve B shows the transmissibility of a sys¬ 
tem with a moderate amount of viscous damp¬ 
ing. For relative frequencies below 1.41, the 
effect of damping is seen to be beneficial, since 
the transmissibility no longer goes to infinity 
at the resonant point, but is limited to a rather 
moderate value. Nevertheless, the transmissi¬ 
bility is greater than unity over this range. 
Hence, the mount is still ineffective, although 
less harmful, with damping than it was with¬ 
out. In the range above 1.41, where the un¬ 


damped mount was effective, the damped mount 
is still effective, but less so, since the transmis¬ 
sibility is everywhere greater. Curve C, for 
greater viscous damping, shows a further de¬ 
crease in transmissibility from zero to 1.41, 
and a further increase above that value. 

From these curves, we may conclude that 
damping is in the nature of a necessary evil. 
Some damping is required to keep the trans¬ 
missibility at and near resonance within rea¬ 
sonable limits. The minimum damping which 
will accomplish this is the optimum amount, 
for damping decreases the effectiveness of the 
mount in the range above resonance. Curve A 
for zero damping gives the theoretical mini¬ 
mum transmissibility for relative frequencies 
above 1.41, thus representing the theoretical 
optimum performance in the useful range. 


14.8.2 Theoretical Comparison 

Since damping is necessary in the range 
below 1.41, but undesirable above that value, 
the question naturally arises as to whether some 
other type of damping than viscous might be 
more suitable. Any other type must still limit 
the transmissibility at or near resonance, but 
should increase the transmissibility at high 
frequencies less than does viscous damping. 
The proponents of dry friction, or coulomb 
damping, claim just this property. 50 ’ 8a ’ 9b 

The theory of vibration isolation for systems 
with viscous damping has been well developed 
by a number of workers in the field of vibra¬ 
tion, in particular, Timoshenko 16 and Den Har- 
tog. 15 The rigorous treatment of problems in 
forced vibration for other forms of damping is 
complicated, but approximate solutions are 
available which are sufficiently accurate for 
most practical applications. 15 ’ 16 A brief consid¬ 
eration of the energy relationships involved 
will give an indication of the general behavior 
of such systems under different types of damp¬ 
ing. 

If one considers a vibrating system at reso¬ 
nance, the energy input per cycle is ttP 0 xo, 
where P 0 is the harmonic exciting force, and 
xo the amplitude of the motion. Plotting energy 









524 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


per cycle against amplitude, X o, in Figure 13, 
we may represent the energy input per cycle by 
the straight line OA. The energy dissipated per 
cycle by viscous damping is ttCwxo 2 , where C is 
the damping coefficient and to the angular fre¬ 
quency. The energy dissipated in viscous damp¬ 
ing may be represented by the parabola OB. It 
is evident that the parabola must intersect OA 
at the origin and one other point. At this latter 
point, the energy dissipated per cycle just 



AMPLITUDE X 0 

Figure 13. Energy per cycle versus amplitude. 
OA represents energy input, OB and OB' energy 
dissipated in two values of viscous damping, OC 
and OC' energy dissipated in two values of fric¬ 
tional damping. 

equals the energy input, and the system is in 
equilibrium. The amplitude at which the sys¬ 
tem is in equilibrium is obviously the resonant 
amplitude for that particular value of the 
damping coefficient. 

A different value of damping coefficient may 
be represented by another parabola OB' which 
intersects OA at another value of amplitude. 
Thus the amplitude at resonance depends upon 
the value of the damping coefficient. The im¬ 
portant point, however, is that for all values of 
damping coefficient, the parabola representing 
energy dissipated always intersects the straight 
line representing energy input at a point corre¬ 


sponding to a finite amplitude. There is always 
a finite amplitude at which the system is in 
equilibrium. 

In the case of frictional damping, the energy 
dissipated per cycle is 4 F X o, where F is the 
damping force, which is independent of velocity. 
The energy dissipated here will be represented 
by a straight line such as OC or OC', depending 
upon the magnitude of the damping force. If 
the frictional damping force is small, as it is for 
OC, the energy dissipated is less than the 
energy input for all amplitudes. This is illus¬ 
trated by the fact that OC has a smaller slope 
than OA. Under this condition, no equilibrium 
amplitude is reached, for the energy flowing 
into the system continually exceeds that dissi¬ 
pated, and the amplitude becomes infinite. If 
the frictional damping force is large, repre¬ 
sented by OC', the energy dissipated per cycle 
by the damping exceeds the energy input, and 
true sinusoidal oscillations do not occur. An ex¬ 
act solution of the equation of motion in this 
case is not possible, but an approximate solu¬ 
tion has been worked out by Den Hartog. 15b It 
is too complicated to be included in this discus¬ 
sion, but the results are given in the literature. 
The important point here is that only for large 
values of frictional damping is the amplitude 
finite at resonance. 

Thus from the above theoretical considera¬ 
tions, we may conclude that below a certain 
critical value, frictional damping fails com¬ 
pletely in its most important function, that of 
controlling the amplitude at or near resonance. 
Furthermore, this critical value is not a con¬ 
stant of the system but depends upon the en¬ 
ergy input per cycle, which is nPoxo, as ex¬ 
plained above. Hence, for one set of conditions, 
in which the impressed force is small, the fric¬ 
tional damping in a system may be sufficient to 
prevent the amplitude from becoming infinite. 
In this same system, however, a small increase 
in the magnitude of the impressed force may 
raise the energy input per cycle above the 
energy dissipated by the frictional force, and 
the amplitude will become infinite. While vis¬ 
cous damping will always limit the amplitude 
at resonance, frictional damping may or may 
not, depending upon the external conditions. 

In the region in which the mount is effective, 




VISCOUS VERSUS FRICTIONAL DAMPING 


525 


we have stated that damping is always detri¬ 
mental, as shown by the curves in Figure 12. 
This statement applies both to viscous and to 
frictional damping, but the problem is to deter¬ 
mine which is less harmful. While it is possible 
to calculate the performance of a system with 
viscous damping under any set of conditions 
from the constants of the system alone, such a 
generalized calculation unfortunately is not 
possible for a system with frictional damping. 
In the latter case, specific external conditions 
must be assumed, hence no generalized theo¬ 
retical comparison between the systems them¬ 
selves may be made. Only general trends may 
be considered. 

The exponents of viscous damping have 
argued that it possesses an important advan¬ 
tage over frictional damping from the view¬ 
point of filtering action at high frequen¬ 
cies. 50 ’ 8a> 9b They reason that the rate of energy 
absorption by a damper is equal to the product 
of force and velocity. Since the force for a vis¬ 
cous damper is proportional to the velocity, it 
follows that the rate of energy absorption is 
proportional to the square of the velocity. For 
the dry-friction type of damper, the force is 
substantially independent of velocity, hence the 
rate of energy absorption is proportional to the 
first power of the velocity. Up to this point, the 
two groups are in agreement. The proponents 
of frictional damping go on to compare the two 
types of damping at low and at high frequen¬ 
cies, making the comparison at the same ampli¬ 
tude, and conclude that the frictional damper 
absorbs relatively less energy at high frequen¬ 
cies, hence is more favorable. This comparison 
seems to us to avoid the real issue. The ques¬ 
tion is not which damping absorbs the more 
energy at the same amplitude. The real ques¬ 
tion is what is the resultant amplitude for each 
type of damping under equal impressed forces. 
The absorption of energy by a damper is a de¬ 
sirable, not an undesirable characteristic, hence 
on this line of reasoning we should conclude 
that viscous damping is better than dry-fric¬ 
tion. The detrimental action of a damper arises 
from its ability to transmit part of the exter¬ 
nally applied force through to the suspended 
system. The net effect of damping is the re¬ 
sultant of these two opposing actions. 


Experimental Comparison 

Since it is necessary to calculate the perform¬ 
ance with frictional damping by laborious 
methods for each specific set of conditions in 
order to compare it with viscous damping per¬ 
formance, it would seem both easier and more 
conclusive to resort to direct experimental com¬ 
parison. For viscous damping in the Rochester 
gimbal mounts, the theoretical and experimen¬ 
tal performance are in close agreement. In one 
example reported, 1 the experimental values for 
a Type II-c antioscillation mount correspond 
very closely to the theoretical transmissibility 
for a system with a damping coefficient of 0.3 
critical. It should be noted that this value of 
damping corresponds to a maximum transmis¬ 
sibility at resonance of about two. Similar ex¬ 
perimental performance is also reported for a 
group of tests on Type Il-b mounts. 14a 

The few available shake-table tests of mounts 
with frictional damping show a considerable 
range of performance. The Technicolor mount 
tested at the University of Rochester was pro¬ 
vided with adjustable damping, thus permit¬ 
ting some investigation of the effects of varia¬ 
tion in this characteristic. 140 For low amounts 
of damping, the transmissibility at resonance 
was very high. The amplitude was, of course, 
limited by the mechanical stops, but from the 
theoretical standpoint it might be considered 
infinite. At frequencies from 10 to 20 times the 
resonant frequency, the transmissibility was 
very low, approaching the theoretical value for 
zero damping. A second test, with a moderate 
amount of damping, showed a transmissibility 
at resonance of about 2.6 for one component 
and 1.5 for the other. At the higher frequen¬ 
cies, the transmissibility was appreciably 
higher. The third test, with relatively heavy 
damping, gave transmissibilities at resonance 
between 1.2 and 1.3, and a still greater increase 
at the higher frequencies. This performance is, 
at least qualitatively, in agreement with the 
theoretical predictions for frictional damping 
considered above. 

The single Rochester test of the Kodak 
mount, 14d on the other hand, gave somewhat 
different results. In this test, the damping was 
sufficiently high to limit the transmissibility at 



526 


ANTIOSCILLATION MOUNTS FOR OPTICAL INSTRUMENTS 


resonance to about 1.2. This corresponds to a 
high value of viscous damping. Yet the trans- 
missibility at higher frequencies was relatively 
low, corresponding to only a moderate amount 
of viscous damping. 

1484 Conclusions 

The limited theoretical consideration of the 
problem and the meager experimental results 
made it difficult to draw any decisive conclu¬ 
sions regarding the relative merits of viscous 
and frictional damping. The theory seems to 
favor the former, but the experimental test on 
the Kodak mount cannot be passed over lightly. 
While the performance of the Kodak mount 
does not substantiate the claim that frictional 
damping is superior to viscous damping in com¬ 
petitive tests, 7b it does prove that the trans- 
missibility at resonance may not only be finite, 
but even lower than is normally the case with 
viscous damping, while the transmissibility at 
higher frequencies may be as low as a moder¬ 
ately damped viscous system. It should be noted, 
however, that the transmissibility in the oper¬ 
ating range (1,600 to 2,000 vpm) is on the 
upper limit of acceptability for gimbal 
mounts. 14a It is not as low as the average for 
the latter, and is several times as large as that 
of the Type II-c reported. 1 In the matter of 
boresighting accuracy, the Kodak claims are 
also not borne out. The dynamic boresighting 
was not quite within limits established as ac¬ 
ceptable for gimbal mounts at Rochester. 

Other claims for the Kodak mount are cer¬ 
tainly justified. It possesses simplicity of con¬ 
struction, uses standard binoculars without 
modification, is compact and light in weight, 
and would seem to require a minimum of main¬ 
tenance. In all these respects, its superiority to 
the gimbal mount must be conceded. Only where 
the best possible vibration performance is re¬ 
quired, or in gunsight applications where accu¬ 
rate boresighting is essential, does the more 
elaborate gimbal mount have a real advantage 
which may justify its increased complexity. 

149 DISCUSSION 

The experience gained with the various anti¬ 
oscillation mounts, both in the laboratory and 


under actual service conditions, is ample proof 
that there are definite advantages in their 
use. Undoubtedly, considerable improvement in 
both design and performance can be made. The 
wartime pressure under which these develop¬ 
ments were made does not lead to the ultimate 
in results. Both from the academic and the 
practical viewpoint, much value would be de¬ 
rived from further investigation of the relative 
advantages of viscous versus frictional damp¬ 
ing, particularly based on practical tests under 
service conditions. Certainly improvements in 
both types of damping should be expected. 

The application of antioscillation mounts is 
an urgent problem. The basic principles have 
been firmly established and successfully re¬ 
duced to practice. A search for additional 
profitable applications should be undertaken. 
During World War II, it was exceedingly diffi¬ 
cult to carry out thorough and unhurried field 
tests of this equipment. Some of those leads 
which looked promising, but which had to be 
abandoned for more urgent problems, should 
now be followed up. A careful study of the re¬ 
quirements of each application would probably 
result in a considerable modification of the de¬ 
sign and performance characteristics of the 
present mounts. Obviously, one design cannot 
be optimum for all problems. A further investi¬ 
gation of the theory and general performance, 
combined with the adaptation to specific prob¬ 
lems, should provide more incentive and yield 
more beneficial results than a laboratory re¬ 
search investigation alone. 

1410 RECOMMENDATIONS BY NDRC 

1. The gimbal mounts (University of Roch¬ 
ester and Eastman), the ball-and-cone mount 
(Eastman), and the rubber-shell mount (Tech¬ 
nicolor) should all be tested more completely in 
the laboratory, in aircraft, and on shipboard 
to determine the extent to which each is applic¬ 
able to existing needs. The effectiveness of the 
base filtering unit in the Technicolor mount 
should be determined. Boresighting accuracy 
should be measured, both statically and dynam¬ 
ically, for each mount. 

2. Further theoretical and experimental 
studies are desirable, to settle the relative 
merits of dry-friction and viscous damping. 



RECOMMENDATIONS BY NDRC 


527 


3. Antioscillation mounts should be devel¬ 
oped for a number of special applications, such 
as high-power telescopes for use in aircraft and 
on ships, and for gun cameras. 

4. Comparisons should be made between the 
performance of binoculars in the most effective 
antioscillation mount with hand-held standard 
binoculars and with binoculars to which 


weighted arms have been added, with elbow 
rests and with provision for relieving the ob¬ 
server from carrying the extra weight involved 
(see Chapter 5). These tests should be made 
both in aircraft and on shipboard. A compari¬ 
son should also be made with the British sup¬ 
port for the elbows, which is carried on the 
thighs. 



Chapter 15 

PHOTOTHEODOLITES 

By Leo Goldberg a 


is-* INTRODUCTION 

15 11 Origin of Problem 

A t a conference held at Fort Monroe on 
u December 3, 1941, representatives of 

the Ordnance Department, Aberdeen Proving 
Grounds, Signal Corps, Coast Artillery Board, 
and Naval Proving Ground agreed that there 
was a need for the development of a pair of 
phototheodolites capable of measuring the posi¬ 
tion of a target in space with all possible accu¬ 
racy. NDRC was asked to undertake a compre¬ 
hensive study of the problem. A preliminary in¬ 
vestigation was begun by Section D3 (Instru¬ 
ments) at the request of Section D2 (Fire Con¬ 
trol). On March 1, 1942, project OD-48 was 
established to cover development of these in¬ 
struments. When NDRC was reorganized in 
December 1942, this project was transferred 
to Section 16.1. The Services requested that 
plans be made to modify a pair of existing 
phototheodolites, if sufficient accuracy could be 
obtained. Otherwise the design and construc¬ 
tion of two entirely new instruments was rec¬ 
ommended. The study of existing phototheodo¬ 
lites as well as the design and construction of 
the new instruments was carried out by the 
Eastman Kodak Company [EKC] under Con¬ 
tract OEMsr-503. 


5. Recording of motion of rockets and other 
special projectiles. 

6. Recording of aircraft motions. 

7. Testing the overall accuracy of fire-con¬ 
trol equipment. 

15,1,3 Specifications 

In order to meet the recommendations of the 
Services, the following specifications were in¬ 
dicated. 

Optical Features 

1. Cameras with interchangeable optical sys¬ 
tems, having focal lengths of 12, 24, and 48 in., 
and relative apertures of //5 at least, for night 
photography. 

2. Exposures on 35-mm film, each frame to 
contain, in addition to the photograph, the film 
number, azimuth and altitude angles, time of 
exposure, and an image of a fixed reference 
reticle. 

3. Exposures to be made at intervals of from 
1 to 10 sec, or at motion-picture rates, as de¬ 
sired. 

Tracking Features 

1. Design to provide for two-man aided 
tracking. 

2. Tracking telescopes to be 8X with 8-de¬ 
gree field. 


15,1,2 Applications 

The most important applications for preci¬ 
sion phototheodolites envisaged by the various 
branches of the Services were: 

1. Testing of heightfinders. 

2. Testing of performance of operators of 
heightfinders and rangefinders. 

3. Construction of range and fuze tables for 
use with antiaircraft guns. 

4. Construction of bombing tables. 

a McMath-Hulbert Observatory of the University of 
Michigan, Lake Angelus, Pontiac, Michigan. 


Synchronization 

1. When a target is being tracked by two sta¬ 
tions, the recording of altitude and azimuth 
angles is to be synchronized to within 0.001 sec. 

Mechanical Features 

1. Overall accuracy of angle measurement to 
be 0.1 mil. 

2. Automatic correction for refraction of 
atmosphere, if practical. 

3. Instrument to be equipped with an accu¬ 
rate bubble level. 


528 



DESIGN PROBLEMS 


529 


15 2 STUDIES OF EXISTING 

PHOTOTHEODOLITES 

Akeley Instrument 

Tests were conducted to determine the accu¬ 
racy of the azimuth worm and worm wheel of 
an Akeley phototheodolite especially selected as 
being the best of a lot of 100 instruments. The 
results, as shown in Figure 1 of the EKC final 
report, 1 gave a maximum error of 0.2 mil for 
the worm wheel and 0.1 mil for the worm, or 
a total of 0.3 mil for the combination. Addi¬ 
tional faulty features were found. 

1. The method employed to eliminate back¬ 
lash leads to excessive wear on the worm and 
worm wheel. 

2. The measured backlash in the gear train 
that connects the azimuth and altitude angle 
counters to the worm shaft is approximately 
0.1 mil. 

3. The design of the leveling base is such 
that excessive tightening of the leveling screws 
may distort the azimuth worm wheel. 

4. Two-man tracking was found to be impos¬ 
sible. 


Askania Instrument 

The Askania instrument employs a gradu¬ 
ated circle for azimuth and elevation angle 
readings which, together with a fiducial reticle, 
makes possible readings to 0.5 min of arc. Tests 
of the azimuth circles of two instruments 
showed that the errors in the circle divisions 
were insignificant. 

The eccentricity of one of the circles was 
only 0.00075 in., or 0.03 mil. The measured 
eccentricities of the main azimuth bearings, 
however, were 0.008 in. and 0.00035 in., corre¬ 
sponding to errors of 0.35 and 0.13 mil, respec¬ 
tively. 


152 3 Conclusions 

The Akeley phototheodolite could not be mod¬ 
ified to conform to the desired specifications. 
The Askania phototheodolite could be made to 


yield the desired accuracy by the installation of 
new precision ball bearings for the vertical and 
horizontal axes. Consideration of other neces¬ 
sary changes, however, made it likely that the 
demands of the Services could best be met by 
the development of an entirely new instrument. 


15 3 DESIGN PROBLEMS 

The following theoretical considerations en¬ 
tered into the design of various components of 
the phototheodolites. 

1531 Camera 

Lens 

The decisive factor in determining the re¬ 
quired lens diameter was photographic speed, 
not resolving power, since the minimum diame¬ 
ter required to resolve an angle of 0.1 mil at 
wavelength 5,600 A is only 0.26 in. At Fort 
Monroe good photographs were obtained dur¬ 
ing daylight hours with exposures of % 0 sec 
and focal ratios between //22 and //45. The 
Eastman Research Laboratory conducted tests 
which showed that satisfactory photographs of 
flash from shellbursts at night were obtained 
in y 30 sec at //5 to f/S. 

The maximum focal length of the camera 
lens was determined by the expected accuracy 
of linear measurement on the film. Experience 
showed that measurements could be made con¬ 
sistently with an accuracy of 0.001 in., corre¬ 
sponding to 0.1 mil at a focal length of 10 in. 

Lens Mounting 

In ordinary usage, slight lateral shifts of 
lenses in their mounts are permissible. In the 
phototheodolite, the lens must maintain a line 
of sight defined by its nodal point and the 
fiducial point in the film plane. This line must 
at all times be maintained perpendicular to the 
elevation axis. It follows also that the lens 
mount must be so designed that temperature 
changes or vibrations will not cause lateral 
shifts of the lens. The material chosen for the 
mount must be such that good focus is main¬ 
tained over a range of temperature. 



530 


PHOTOTHEODOLITES 


When a worm and worm wheel are used for 
the determination of angles, the lenses must be 
counterbalanced to avoid undue loading of the 
worm wheel and to minimize the torque neces¬ 
sary for tracking. 

Film Size and Shrinkage 

The longest focal length desired for the pho¬ 
totheodolite is 48 in., which with 35-mm film 
gives a field of 15.6 mils. Since tests at Fort 
Monroe showed that, with fair tracking, the 
maximum error is 1.7 mils, the use of 35-mm 
film provides ample space for the dial readings 
and time counter. 

Under the worst conditions, the film will 
change its dimensions by about 0.5 per cent 
between the time it is exposed and finally proc¬ 
essed. However, since the target is never ex¬ 
pected to be more than about 2 mils from the 
fiducial point, the maximum error introduced 
by film shrinkage is only about 0.01 mil. 

Camera Drive 

To simplify the problem of synchronization 
of target exposure with two phototheodolites, 
and also to simplify the mechanism for produc¬ 
ing variable picture rates, an intermittent or 
“single-frame” mechanism was chosen in pref¬ 
erence to a continuously driven cine-type 
camera. 

Frequency of Picture Taking 

In tracking target airplanes, the number of 
photographs per second required to define the 
flight path is determined by the maximum cur¬ 
vature of the path. On the reasonable assump¬ 
tion that 10 g is the maximum acceleration that 
a plane can stand, it can easily be shown that 
with 3 or 4 frames per second the plane posi¬ 
tion can be located to within 1 yd. In shellburst 
photography, however, a rate of about 18 per 
second is required to record the burst on at 
least two frames. 

Exposure Time 

The maximum exposure time is determined 
by the expected accuracy of tracking, the re¬ 
quirement being that the rate of change of 
tracking error, when multiplied by the expo¬ 
sure time, shall be less than 0.1 mil. If a peri¬ 


odic tracking error is assumed, of amplitude 
1 mil and period 3 sec, the required exposure 
time must be less than 0.05 sec. The same limi¬ 
tation applies to the time difference between 
the centers of the dial and target exposures. 

15 3 2 Angle-Recording Dials 

Exposure Time 

The time of exposure on the angle-recording 
dials is determined by the maximum rate of 
tracking, the requirement being that the blur¬ 
ring shall be less than 0.1 mil. Assuming, for 
example, a target distance of 1,000 yd and 
speed of 150 yd per second, the angle to the 
target changes by 0.1 mil in less than 10~ 3 sec. 
The only satisfactory exposure device is an 
Edgerton lamp, with an exposure time of 10 -4 
sec. When two phototheodolites are employed 
simultaneously on the same target the exposure 
on the respective dials should also be synchro¬ 
nized to within 10~ 3 sec to avoid complications 
in the reduction of the data. 

15.3.3 Leveling and Misleveling 

The main requirements for the phototheodo¬ 
lite levels are high accuracy (error less than 
0.1 mil), high sensitivity, and a means for re¬ 
straining motion of the instrument in the level¬ 
ing plane during the leveling operation. 

Because of the earth’s curvature, provision 
must be made for misleveling one of the two 
phototheodolites, in order that the angles at 
the instruments be measured in two parallel 
planes. Misleveling may be accomplished by 
putting on one leveling screw a dial graduated 
in divisions representing the distance between 
the two phototheodolites. 

15.3.4 Bearing Accuracy 

The bearing design must be such that the 
error contributed by eccentricity is constant 
and only a small fraction of the overall error of 
the instrument. If the graduated circle or the 
worm wheel is 16 in. in diameter, a linear value 
of 0.001 in. for the eccentricity corresponds to 
an angular error of 0.025 mil. 



EASTMAN RECORDING PHOTOTHEODOLITE 


531 


15 3 5 Atmospheric Refraction 

Available information on daytime astro¬ 
nomical “seeing” is not sufficiently accurate to 
determine whether atmospheric conditions 
would interfere with the performance of a 
0.1-mil photo theodolite. Tables of refractive 
errors and a useful bibliography have been 
compiled. 2 

15 4 CONSTRUCTION OF THE EASTMAN 
RECORDING PHOTOTHEODOLITE 

Figures 1 and 2 show respectively a photo¬ 
graph of the completed phototheodolite and a 
layout of the instrument. The mounting of the 


Figure 1 . Eastman phototheodolite. 



camera is of conventional design. A yoke free 
to rotate about the vertical axis supports the 
horizontal axis, about which the optics and 
camera swing in elevation. The adopted design 
permits maximum flexibility and sky coverage. 


Angle Measurement 

Worm and worm-wheel systems were chosen 
in preference to a graduated circle partly be¬ 


cause it is difficult to make a graduated circle 
with the smallest division equal to 0.1 mil 
(equals 0.000785 in.) for direct reading with¬ 
out a vernier on a 16-in.I diameter. The circle 
would need to be imaged on a fiducial reticle, 
and readings would be difficult to make as com¬ 
pared with a simple counter. Moreover, perfect 
tracking requires the use of extremely accurate 
worms and worm wheels, which also can easily 
be equipped with counters. 

Design and Construction of 
Worm and Worm Wheel 

The worm and worm wheel were constructed 
according to methods developed and employed 
by the Gould and Eberhardt Company, of 
Irvington, New Jersey. The worm is of the 
dual-lead type, in which one side of the tooth 
profile is generated with a standard lead, and 
the other side with a lead slightly greater or 
smaller than the standard. The resultant worm 
will then have a thread whose profile increases 
in size from one end of the worm to the other. 
A wheel to fit this worm is made by using a hob 
made exactly like the worm. The corresponding 
section of the worm that was used in the hob 
will then mesh with the worm wheel with zero 
backlash. If more backlash is desired, the worm 
is moved in an axial direction, thus bringing 
into mesh a thinner section of the thread. This 
method of design eliminates backlash without 
disturbing the theoretical center-to-center dis¬ 
tance between worm and worm wheel. 

The diameter of the worm wheel is 16 in., 
this being the minimum diameter that seemed 
likely to allow 0.1 mil accuracy with the ex¬ 
pected eccentricity, backlash, and tooth and 
thread errors. The worm wheel was designed to 
have 128 teeth, and, since one revolution of the 
worm wheel corresponds to 860 degrees or 
6,400 mils, each turn of the worm corresponds 
to 50 mils, a convenient number for a counter. 
The diameter of the worm thread is large 
(4.465 in.) which results in a very small helix 
angle, so that flexure of the worm shaft has a 
negligible effect on the measured angle. 

The construction of the worm and worm 
wheel is the same for azimuth and elevation, 
except for the hub design. The worm is made 
of SAE 4620 steel, seasoned and hardened to 










532 


PHOTOTHEODOLITES 




a Rockwell hardness of C58-63. The wheel is 
made with a No. 15 chilled cast-bronze periph¬ 
ery, Brinell hardness 74-93. The periphery cast¬ 
ing, which was made by the Lumen Bearing 
Company, Buffalo, New York, was fitted to a 
Type B Meehanite cast hub made by the 


American Laundry Machinery Manufacturing 
Company, Rochester, New York. 

In the construction of the worm wheels, a 
proof diameter and a face were turned on the 
wheels while they were rotated in their work¬ 
ing bearings. These surfaces were then used to 






































































































































































































EASTMAN RECORDING PHOTOTHEODOLITE 


533 


orient the wheels on the master hobbing ma¬ 
chine workbench so that they were concentric 
with the table’s axis of rotation. When the 
teeth were cut, they were thus generated about 
the working axis of the wheel. The worm was 
also provided with a proof diameter, which was 
ground with the same setting used for grinding 
the thread, and used to determine the amount 
of eccentricity in the worm bearings. 

The fitting of the worm and wheel was ac¬ 
complished by a trial and error process. The 
worm was ground initially so that the large end 
was larger than the drawing specified by about 
0.0008 in. The worm and wheel were assembled, 
and the center-to-center distance adjusted, by 
means of shims, to the theoretical value, which 
is the distance used in hobbing the teeth on the 
wheel. The theoretical center-to-center dis¬ 
tance was held throughout the fitting process. 
The teeth were then coated with a very thin 
layer of a mixture of red lead and oil, and, with 
the backlash adjustment at the loose end, the 
worm was brought into mesh with the wheel. 
The worm was then oriented until the red lead 
indicated the best bearing between the two 
parts. The worm was then reground to compen¬ 
sate for faulty tooth contact, and the testing 
process was repeated. On the average, about six 
trials were necessary before satisfactory re¬ 
sults were obtained. The worm housing was 
then pinned in place and the worm removed for 
a final grinding to size. 

Bearings 

The bearings selected for the phototheodo¬ 
lites were manufactured by the New Departure 
Corporation. They are Radax Ultraperfex pre- 
loaded ball bearings, each set produced by se¬ 
lective assembly from thousands of parts, in 
which the bore, outside diameter, and eccen¬ 
tricity are held to 0.0001 in. 

Various other types of bearings were investi¬ 
gated. For example, sleeve bearings combined 
with ball thrust bearings are of sufficient accu¬ 
racy, but experience with existing phototheodo¬ 
lites showed that they do not maintain their 
precision. On the other hand, preloaded ball¬ 
bearing designs are used in precision shop 
equipment and hold their accuracy after years 
of continuous operation. 


Vertical Axis. The center of the azimuth 
worm-wheel hub provides the housing for the 
bearings, which are separated in order to give 
vertical rigidity to the axis. The entire weight 
of the instrument is taken by the vertical bear¬ 
ings, through the hub, which is fastened to the 
main base, and thus to the leveling screws. The 
result of eccentricity tests for two vertical axis 
assemblies was 0.000025 in., and for the third, 
0.00005 in. 

Horizontal Axis. Figure 3 is a cross-sectional 
view of the horizontal axis. Two pairs of pre- 
loaded bearings are used, one set on each side 
of the camera. For convenience of assembly, 
the yoke housing is split and the bearings lo¬ 
cated in a thick cartridge that seats in the split 
housing. The worm-wheel side of the horizontal 
axis is anchored to the yoke casting by means 
of an angular key which maintains the worm 
wheel in a fixed position with respect to the 
worm. The other end of the axis is free to move 
should differential expansion take place. The 
recesses for the trunnions were machined with 
high precision and each trunnion was finish- 
ground separately, resulting in an assembly in 
which the two trunnion axes are concentric 
within 0.00015 in. The result of eccentricity 
tests for two assemblies was 0.00005 in. on one 
and 0.00010 in. on the other. 

Worm Axis. Figure 4 is a cross-sectional 
view of the worm assembly. The construction is 
similar to that of the horizontal axis. One end 
of the worm is adjustable for the removal of 
backlash, but once adjusted it remains fixed 
with respect to the housing. Because the car¬ 
tridge that houses the bearings at the adjust¬ 
able end is necessarily long to provide enough 
movement, the ball bearings are separated with 
spacers to bring them close to the ends of the 
cartridge. Eccentricity tests indicated that the 
ball bearings are of the same precision as those 
employed in the vertical and horizontal axes. 


15,4,2 Camera Mechanism 

The camera mechanism is of the single-frame 
type, in which a clutch couples a continuously 
operating driver to the mechanism. The clutch 
is a mechanical, self-energizing type developed 





534 


PHOTOTHEODOLITES 


ie x 4 x i KEY 


-GARLOCK SEAL NO. 1221 
? 0 TAPER PIN | L0NO7 




N0.4-40X^ 



GLEASON PRECISION SPANNER 
NUT NO. 315 G-3 


GLEASON PRECISION SPANNER 
NUT NO. 315 G-l 


II II II 

s x i x i KEY 


'GARLOCK SEAL NO. 1221 


NO. 10- 32 


Figure 3. Horizontal axis assembly. 














































































































































































































































EASTMAN RECORDING PHOTOTHEODOLITE 


535 


especially for the phototheodolite. Magnetic 
types were considered and found suitable, but 
the mechanical version offered better design 
flexibility. 

A schematic view of the clutch is shown in 


the solenoid is energized, the engaging disk is 
freed, and is acted upon by the torsional spring 
through shaft A, resulting in a small counter¬ 
clockwise angular displacement as viewed from 
the spring end. Cam action on the right-hand 



Figure 4. Worm assembly. 


TORSIONAL SPRING 



Figure 5. A continuously running gear is driven 
by a 60-c, 110-v synchronous motor operating 
at 1,800 rpm through a 3/2 gear reduction, the 
time for one picture cycle being thus 0.05 sec. 
The sequence of operation is as follows: When 


side of the engaging disk causes it to move 
axially and to be coupled to the driven gear. 
When the solenoid is de-energized, the arma¬ 
ture stops the engaging disk, and the inertia of 
the system causes cam action on the left-hand 






























































































536 


PHOTOTHEODOLITES 


side of the engaging disk to displace it axially 
against the brake. The ratchet spring prevents 
the torsional spring from unwinding until the 
solenoid is re-energized. 

It was not possible to operate the camera 
mechanism at frequencies much greater than 
10 frames per second. When a higher speed is 
desired, the motor is coupled continuously to 
the mechanism, resulting in a speed of 20 
frames per second. 

An Eyemo movement is used for the pull¬ 
down and a single sprocket supplies and takes 
up the film. The shutter is a sector, mounted on 
its own shaft, and geared one-to-one to the 
clutch shaft. Since the sector opening is 72 de¬ 
grees and the complete picture cycle 0.050 sec, 
the time for one exposure is 0.010 sec. The film 
chamber is a standard type made by the Akeley 
Camera Company, its capacity being 200 ft of 
35-mm film. A footage indicator is built directly 
into each film chamber. The fiducial mark con¬ 
sists of a cross, open in the center, engraved on 
a glass plate. 


Optics 


Lenses 

The design of the lenses was largely depend¬ 
ent on the aperture ratio necessary for night 
photography. The night photography require¬ 
ment was taken as //5 at % 0 sec, or //2.8 at 
the adopted exposure of 0.010 sec. Since a 48-in. 
lens at //2.8 requires a lens diameter of 17 in., 
NDRC agreed to a modification of the original 
specifications to include a 48-in. lens at //5.6, 
a 24-in. lens at //2.8, and a 12-in. lens at //4. 

The choice of optical system for the 24-in. 
and 48-in. lenses was governed mainly by con¬ 
siderations of size and weight. The use of a 
normal-type telephoto lens was ruled out for 
the following reasons: 

1. The overall length, that is, the distance 
from the front vertex to the focal plane, is 
about 80 to 85 per cent of the focal length, or 
about 40 in. for a 48-in. lens. 

2. To correct adequately the spherical aber¬ 
ration in a 48-in. lens more than 8 in. in di¬ 
ameter, requires an excessive number of lens 
components of prohibitive weight and leverage. 


3. An aperture of //2.8 for the 24-in. lens 
would be difficult to achieve with a telephoto 
lens. 

4. A normal telephoto lens suffers consider¬ 
ably from secondary spectrum residuals and 
from sphero-chromatism (chromatic variation 
of spherical aberration). 

5. The temperature coefficient of back focus 
is large in a telephoto lens. 

Many of the foregoing disadvantages are not 
present in a mirror system. Chromatic aberra¬ 
tions are absent and the overall length can be 
made much shorter than with a normal tele¬ 
photo lens. The temperature effect is very small 
and is almost perfectly compensated by the use 
of a steel lens barrel. 

Three types of mirror systems were consid¬ 
ered : 

1. A classical Cassegrain system with conic- 
section mirrors and a Ross corrector lens was 
not adopted mainly because the Eastman group 
had no experience in the generation of conic- 
sections, and the time factor was important. 

2. A conventional Schmidt mirror system 
was not undertaken because its overall length 
is twice the focal length. 

3. A mirror-lens combination, in which a 
pair of weak achromatic lenses, silvered on the 
back surfaces, is mounted as a Cassegrain sys¬ 
tem, was the arrangement actually adopted (see 
Figures 6, 7, and 8 for the lens specifications 
and diagrams). The function of the lenses is to 
remove the spherical aberration of the mirror 
surfaces. Careful balancing of the refractive 
errors between the front and rear lens systems 
made possible the elimination of both spherical 
aberration and coma. Curvature of the 1-in. 
diameter field was negligible. 

The gain in length resulting from the use of 
this type of system is shown in the following 
table: 


Effective Total 
Lens Aperture Length 
24-in. //2.8 13.7 in. 

48-in. //5 20.1 in. 


Total Length 
Divided by 
Focal Length 
57 per cent 
41 per cent 


The only noticeable aberration residual is sec¬ 
ondary spectrum, of the following amount, as 
computed relative to the sodium D line: 




EASTMAN RECORDING PHOTOTHEODOLITE 


537 


Line 

24-Inch 

48-Inch 

C 

—0.43 mm 

0.55 mm 

D 

0.00 mm 

0.00 mm 

F 

+0.13 mm 

+ 0.44 mm 

G' 

—0.08 mm 

+0.05 mm 


The optics for three 24-in. lenses were made 
by Perkin-Elmer Corporation and mounted by 
the Eastman group in cells made in Rochester. 

FORMULA! 4-P-362 SPEED: f/4.0 

12-INCH LENS FOR PHOTOTHEODOLITE 
EOUIV FOCUS: 305.00MM B F = 234.95 

(12") FF = 228.67 

ANGULAR FIELD: 2.386° (1-INCH DIAMETER) 


LENS 

GLASS 

"D 

n G' 

2/ 

l 

DBC - 3 

1.61088 

1.62450 

57.2 

2 

LF- 4 

1.57511 

1.59264 

42.8 

3 

CROWN FLINT 

1.52543 

1.53771 

54.8 

4 

DF- 2 

1.61700 

1.63929 

36.6 

5 

EK- II 

1.69677 

1.71255 

56.1 


ALL TEST GLASS RADII 


DIAPHRAGM 

OPENINGS 



f/4.0 

65.30 



5.6 

8.0 

46.64 

32.65 

RIM RAYS 

2.386° 

11.0 

23.75 

UPPER 

>0.015 

16.0 

16.32 

0.7 UPPER 

- 0.015 

22.0 

11.87 

PRIN 

000 

32.0 

8.16 

0.7 LOWER 

+ 0.012 

45.0 

5.80 

LOWER 

+ 0.024 



\ 


/~ 

| 

| 

-N 

> 



. 





> Vi 


SURFACE 

CL APER 

TRIM 

SPHERICAL ABERRATION COMA 

1 

76.20 

78.20 

f/4.0 (D) -0.165 

+0.016 

3 

72.34 

f/5.6 (D) -0.113 

-0.015 

4 

5 

72.34 

71 .44 

78.20 

AXIAL COLOR ~C q J 

-D -0.012 

-D +0.253 

6 

7 

60.66 

59.44 

62.66 

LATERAL COLOR (2.386°) 



(G'-D) 00 


8 

9 

60.10 

59.32 

62.66 

(C-D) +0.003 


SURFACES 3 

AND 4 MEET AT 

PETZVAL SUM (100) 


A DIAMETER 

OF 72.34 MM 

= +0.005765 


FIELD AY AF" AF' 

2.386° -0.003 -0.125 -0.079 

Figure 6. 12-in lens design. 

The components for three 48-in. lenses were 
made entirely at the Hawk-Eye Works of East¬ 
man Kodak Company. Great pains were taken 
in rotating the four elements of each lens com¬ 
bination relative to each other, in order to 
secure the best possible compensation of sur¬ 
face irregularities, and in laterally aligning the 
front and rear members to eliminate axial 
coma. 


Three 12-in. //4 lenses of the projection 
(Petzval) type were also made at the Hawk- 
Eye Works. These gave, excellent images when 
tested on an ordinary l&ns bench. 

In the event that the 48-in. mirror-type lens 
should not pro^e satisfactory, NDRC requested 
the design of a 48-in. f/11 normal telephoto¬ 
type lens. For this purpose, a 15-in. f/11 tele¬ 
photo lens, already on hand, was modified to 
increase its focal length to 48 in. Specifications 
for this lens are given in Figure 18 of the EKC 
final report. la 


FORMULA*. 28-R- 210 SPEED: f/2.8 

CASSEGRAIN SYSTEM FOR PHOTOTHEODOLITE 


EQUIV FOCUS: 609.6MM 
(24") 

ANGULAR FIELD \ 1.69° 


BACK FOCUS : 313.2 

LENS 

GLASS 

n D 

n 6 ' 

V 

1 

DBC - 3 

1.61088 

1.62450 

57.2 

2 

LF- 3 

1.57500 

1.59317 

41.4 

3 

BSC - 2 

1.51700 

1.52709 

64. 5 

4 

DF- 2 

1.61700 

1.63929 

36. 6 



SURFACE 

CL APER 

TRIM SPHERICAL ABERRATION 

COMA 

1 

3 

154.40 

168.00 

f/2.8 (D) 

168.00 

f/4.52 (D) 

0 

-0.015 

+ 0.154 

+ 0.110 

4 

6 

275.28 

289.00 

291.00 

PETZVAL SUM 

(100) = 

-0.015617 

FIELD 

AY 

AF " AF' 

RATIO = 

-0.64 

1.2 ° 

+ 0.035 - 

0.080 -0.209 



1.69° 

+ 0.060 

0.000 -0.393 




Figure 7. 24-in. lens design. 


Tests of Lens Resolution 

Photographic tests of the resolution of each 
phototheodolite lens were made by EKC, using 
a lens-to-target distance equal to fifty times the 
focal length of each lens. The target material 
was V^-in. thick composition board 35.35 in. 
square, or fifty times the size of the target area 
on the phototheodolite film. Cemented to the 
board was a series of small resolution charts 








































538 


PHOTOTHEODOLITES 


FORMULA: 5-R-212 SPEED *.f/5.0 

CASSEGRAIN SYSTEM FOR PHOTOTHEODOLITE 
EOUIV FOCUS! 1219.20 BACK FOCUS :478.51 

(48") 


ANGULAR FIELD'. 0.597° (1 INCH DIAMETER) 


LENS 

GLASS 

n D 

v 

2/ 

1 

DBG- 3 

1.61088 

1.62450 

57.2 

2 

LF- 3 

1.57500 

1.59317 

41.4 

3 

BSC- 2 

1.51700 

1.52709 

64.5 

4 

DF — 2 

1.61700 

1.63929 

36.6 



SURFACE 

CL APER 

TRIM 

SPHERICAL ABERRATION COMA 

1 

111.70 

124.00 

f/5.0 +0.013 +0.026 

3 

122.00 

VI 1.3 -0.033 -0.013 

4 

275.28 

291.00 


6 

284.00 

PETZVAL SUM (100) * +0.004441 

FIELD 

AY 

AF* AF' 

RATIO = +2.25 


0.597° +0.013 -0.113 -0.386 


Figure 8. 48-in. lens design, 

which formed a cross extending to the corners. 
Each chart consisted of eight line patterns of 
six horizontal and six vertical lines, the spacing 
varying in y/2 steps from 10 lines per mm to 
112 lines per mm, as measured at the film plane. 
The center chart was placed with the lines ver¬ 
tical and horizontal and those in the cross with 
the lines radial and tangential. To facilitate 
focusing, a 35-mm film carrier was mounted 
on a jeweler’s lathe compound. For illumina¬ 
tion of the target, an Edgerton flash lamp was 
employed in order to obtain exposures short 
enough to minimize the effect of building vibra¬ 
tion. 

Three types of film were used. Super-XX 
film was developed in D-19 for 8 min at 68 F, 
and Shellburst Pan and Microfile in D-ll for 
5 min. The use of Super-XX and the means of 
development were recommended by NDRC to 
provide a basis for comparison between the 
phototheodolite lenses and high-grade aerial 
camera lenses. Shellburst Pan was used because 
the phototheodolites will normally operate with 
this type of film. The use of Microfile film, with 


a minus-red filter over the light source, gives 
the maximum possible resolution. 

The results are summarized in the following 
table: 


Shell- 

burst 


Lens 


Super-XX 

Pan Microfile 



(lines per mm) 


12-in. //4 

No. 1 

20 

20 

56 


No. 2 

28 

28 

80 

24-in. //2.8 

No. 1 

14 

14 

40 


No. 2 

14 

14 

56 


No. 3 

14 

14 

28 

48-in. //5 

No. 1 

14 

20 

40 


No. 2 

14 

14 

40 


No. 3 

14 

14 

28 

48-in. //11 

No. 1 

14 

20 

56 


No. 2 

20 

20 

40 


The resolution of all the lenses is consider¬ 
ably less than that of high-quality aerial cam¬ 
era lenses developed by NDRC. The resolution 
of the latter on Super-XX has been recorded 
at 30 lines per mm at //11, and at 60 lines per 
mm at //2.8. 

The testing procedures departed in two re¬ 
spects from those commonly employed in test¬ 
ing aerial cameras. First, the phototheodolite 
tests were conducted with 6-line rather than 
with the customary 3-line targets. The resolv¬ 
ing power of the phototheodolites is accordingly 
favored, although probably by no more than 
about 10 per cent. Second, the target spacing 
varied by \/2 steps. Usually ^2 or ^2 steps 
are employed, which provides a more accurate 
determination of resolving power. 

Lens Mounts 

The mount for the 24-in. lens is shown in 
Figure 9. Since all the mounts were designed to 
produce the same moment about the horizontal 
axis, only one counterweight is necessary. Each 
lens is located in its mount following the prin¬ 
ciples of kinematic design. The lens compo¬ 
nents are supported axially against three sta¬ 
tionary pads spaced 120 degrees apart, with 
pressure applied by spring-loaded pads directly 
in line with the stationary pads. Radial loca¬ 
tion is accomplished by three additional pads, 
two stationary and one spring loaded, and 
spaced 120 degrees apart. The spring loading is 
in line with and at the fixed end of the hori¬ 
zontal axis, which should act to maintain the 

















EASTMAN RECORDING PHOTOTHEODOLITE 


539 


line of sight perpendicular to the horizontal 
axis and intersecting the fiducial reticle of the 
camera despite changes in temperature. In the 
24- and 48-in. assemblies, the secondary lens 



Figure 9. 24-in. //2.8 lens mount. 


plate. Corrugations rolled into the barrel of the 
48-in. lens give added stiffness. The optics for 
each of the 24- and 48-in. lenses weighed about 
40 lb and the entire lens assemblies approxi¬ 
mately 75 lb each. Most of the weight of the 
12-in. lens assembly is in a large casting that 
goes between the lens barrel and the auxiliary 
ring. 

For the 48-in. //11 telephoto lens, the design 
of the mount was greatly complicated by the 
need for thermal compensation. Because of the 
probability of temperature gradients in the 
mount, it was doubtful that successful tempera¬ 
ture compensation could be obtained from a 
mount of practical design. A compromise was 
reached involving a mount made of material 
with a coefficient of thermal expansion approxi¬ 
mately matching that of glass. For a tempera¬ 
ture change of 100 F, and in the absence of 
temperature gradients, the calculated change 
in focus is of the same order of magnitude as 
the difference in focus between distances of 2 
miles and infinity. 


is supported by a thick glass plate which also 
provides a cover glass for the whole lens sys¬ 
tem. 

In the Cassegrain systems, daylight from the 
sky is prevented from reaching the film directly 
by a metal baffle tube, which is attached to the 
hole in the large mirror and projects inwards 
to the point at which the useful rays going and 
coming intersect each other. In the 48-in. sys¬ 
tem, a small additional exterior “sunshade,” 
projecting beyond the front window, is neces¬ 
sary. 

To permit focusing and alignment of the 
lenses with respect to the camera focal plane, 
an auxiliary ring was interposed between the 
camera and lens. On one side of the auxiliary 
ring is fastened the lens assembly, and on the 
other are three ball studs that mate with three 
hardened V blocks with U-shaped clamps inte¬ 
gral to the camera housing. The ball studs are 
so spaced that the auxiliary ring will fit to the 
camera in only one position. Good focus and 
alignment of the lens unit are obtained with 
shims placed at the auxiliary ring. 

To obtain lightweight and strong sections, 
the lens barrels were made of welded steel 


15,44 Exposure Control 

A conventional iris diaphragm for exposure 
control could not be used because the outside 
diameter of the lens mount would have to be 
increased considerably, and the theoretical lo¬ 
cation of the diaphragm in a mirror-lens sys¬ 
tem is between the reflecting surface and the 
refractor elements. 

The adopted plan employs neutral-density 
filters close to the image plane. Space limita¬ 
tions forbade the use of a single disk with suf¬ 
ficient filters to accommodate the entire range 
of exposure conditions. Instead, two sectors, 
containing three filters each, are used in com¬ 
bination to effect nine different exposures. The 
filters are made of glass, with essentially no 
lens power and with parallel faces. Each sector 
contains one section of clear glass and two 
filters, the four filters having transmission fac¬ 
tors of 50, 25, 121/2, and 1% per cent. The filter 
transmissions were designed for use with Shell- 
burst Panchromatic film. Because the filters 
were of different thicknesses, a piece of clear 
glass was cemented to each, and all were then 






540 


PHOTOTHEODOLITES 


ground and polished to the same thickness. Low- 
reflection coatings were applied to both faces of 
each filter. 

The filter sectors are actuated by two levers 
that extend to the outside of the camera hous¬ 
ing. A table of the lever settings appropriate 
to each exposure is attached to the phototheodo¬ 
lite. 

The 12-in. lens is also supplied with an iris 
diaphragm to provide, if necessary, a secon¬ 
dary means for controlling the exposure. 


15 4,5 Camera Viewer 

To facilitate the alignment of the phototheod¬ 
olite and to make possible accurate boresight¬ 
ing, a camera viewer is supplied with each in¬ 
strument. The viewer consists essentially of an 
objective lens, a reticle, and an eyepiece, and 
magnifies the camera lens image three times. 
When the viewer is properly oriented, the ret¬ 
icle is aligned with the camera fiducial marks. 
Focusing of the viewer is accomplished when 
the parallax between the two reticles is reduced 
to zero. The viewer is easily attached to the film 
side of the camera with two thumbscrews, but 
before it can be used the pressure pad in the 
film gate must be removed. 


Angle Recording 
Indicating Drums 

A cutaway view of the azimuth angle indi¬ 
cator is shown in Figure 10. A planetary gear 
system is employed, in preference to a Veeder 
Root type counter connected directly to the 
worm. The latter system suffers from the dis¬ 
advantage that the readings must be altered 
when the instrument is turned through more 
than one revolution. The readings are made 
from two drums, one graduated in steps of 0.1 
mil, which is connected directly to the worm, 
and the other graduated in 100-mil steps and 
connected to the output of the planetary gear. 
Since one revolution of the worm equals 50 
mils, the planetary system is designed so that 
the output gear rotates through y 128 of a revo¬ 
lution per revolution of the worm (6,400 mils 


= 360 degrees). By means of controls external 
to the instrument, the azimuth 0.1-mil drum 
may be released from the worm shaft and ro¬ 
tated to any desired orientation setting. The 



Figure 10. Azimuth angle indicator. 

dials are illuminated by a flashing Edgerton 
lamp. Translucent dials are used in preference 
to reflecting dials because of their higher opti¬ 
cal efficiency and the relatively high degree of 
heat dissipation associated with the use of 
Edgerton lamps. The drums are made of Lu- 
cite, with the figures engraved on a coat of 
black paint on the periphery. Light from the 
flashing lamp is reflected directly by a 45- 
degree conical surface underneath the periph¬ 
ery of the drums, thus producing translucent 
figures on a black background. Black figures on 
a white background are less desirable because 
of a smaller permissible variation in exposure 
for readable images. 

The 0.1-mil drum is divided into 500 divi¬ 
sions, every tenth division being numbered 
from 0 to 49. Adjacent to these figures is a sec¬ 
ond row of numbers from 0 to 49, but with the 
zero displaced angularly by 90 degrees from 
the zero of the former row. Similarly the 100- 
mil drum is numbered from 0 to 63, and a sec¬ 
ond row of numbers is displaced by 90 degrees. 
For photographic purposes an aperture plate 
with a fiducial mark covers up the outer rows 



EASTMAN RECORDING PHOTOTHEODOLITE 


541 


of figures, exposing the graduations and the 
inner rows of figures. For visual reading of the 
dials, a second aperture plate covers the inner 
rows of figures and makes visible the outer 
rows and graduations. A magnifying lens and 
incandescent lamp are provided at the exter¬ 
nal reading point. 

Edgerton Lamps 

Three No. 14 General Electric flash lamps in 
series are used to illuminate the azimuth and 
elevation drums and the time counter (see fol¬ 
lowing section). Preliminary tests showed that 
the Edgerton lamps became exceedingly hot 
when flashed at a frequency of 20 per sec, and 
that about 10 per sec represented a safe limit. 
Accordingly, when photographs are taken at 
the rate of 20 per sec, the data appear on 
every other frame and must be interpolated for 
the intervening frames. 

Time Counter 

Simultaneous photography by two photo¬ 
theodolites requires the use of a time counter. 
For this purpose a Bell Telephone type counter 
developed especially for the AAA board was 
employed. Its characteristics are low-inertia, 
decimal-type reading, and actuation at a fre¬ 
quency of 20 counts per sec by a 50-v pulse. 

A second so-called “course counter/’ which 
is manually operated external to the instru¬ 
ment, is used to designate the number of the 
course or test under investigation. The counters 
are covered by an aperture plate on which is 
printed the serial number of the phototheodo¬ 
lite. The counters may be read directly through 
a small window external to the instrument. 

To increase the optical efficiency of the time 
and course counters, a very thin 10-sided 
aluminum ring, with raised numbers chrome- 
plated and buffed, was pinned to each plastic 
drum. 

Film Reading 

A representation of a strip of film, as it 
would be taken with the phototheodolite, and 
a 3X magnified view of one frame are shown 
in Figure 11. The numbers and graduations to 
the left in the angle data spaces are from the 
100-mil drums, whereas those to the right, 


which form a vernier scale, are from the 0.1- 
mil drums. The fiducial mark is in the center 
of the frame, between t,he two scales. The azi- 



Figure 11. Single frame 35-mm film as proposed 
for phototheodolite viewed through back of film. 


muth reading is 3,274.9 mils and the elevation 
reading 1,325.5 mils. 

Optics 

A rather elaborate optical system is required 
in order to bring all of the data to the small 
area provided on the film. The layout, which is 
shown in Figure 12, is divided into four parts. 
There is a separate optical system for the azi¬ 
muth, elevation, and the time and course read¬ 
ings, each with its own objective lens. The rec¬ 
tangle of data is formed in a field lens by the 
prisms and lenses of each of the three optical 
systems, after which an additional lens and 
prism assembly images the data on the film 
plane. Prisms were used exclusively instead of 
front-surface mirrors, because the latter lose 
their reflectivity in a relatively short time. In 
passing through the final optical assembly, the 
light beam traverses an antirotation prism, 
which rotates at half the speed of rotation of 
the horizontal axis and prevents the data image 
from rotating with respect to the film as the 
camera is moved about the horizontal axis. 


15.4.7 Aided Tracking and Telescopes 

The design of the aided-tracking unit before 
the completion of the instrument presented a 
formidable problem, since only a rough esti¬ 
mate could be made of the exact power require¬ 
ments. The design adopted was evolved by 


















542 


PHOTOTHEODOLITES 


Eastman Kodak Company, from study of the 
aided-tracking unit in a captured German Di¬ 
rector. A sector of a ball is mounted so that it 
can be driven at a constant speed about a dia¬ 
metral axis and is capable also of being rotated 


The load that a ball can withstand varies as 
the second power of the diameter. Therefore, 
with sufficiently large diameter balls, the de¬ 
sired torque output can be obtained without the 
use of an amplifier. The difficulty with the Ger- 




Figure 12. Data recording optics. 


about a second diametral axis at right angles to 
the first. A slice out of the midplane of another 
ball is mounted with its periphery tangent to 
the ball sector. With the constant-speed axis of 
the sector perpendicular to and intersecting the 
axis of the ball disk, no motion is imparted to 
the disk. But, as the sector is rotated about the 
other axis, the disk is driven in one direction 
or the other. Thus, the rate of tracking in 
either azimuth or elevation is made propor¬ 
tional to the displacement of a handwheel. 


man system is that, at low or zero rates of 
tracking, the ball disk tends to wear a hole in 
the sector. To remedy this difficulty, two ball 
disks were placed equidistant from the center 
of the sector and connected by a differential 
gear (see Figure 13). Steel equivalent to that 
used in ball bearings is used for the sector and 
disks, and the moving parts operate in a bath 
of oil. 

For tracking, the phototheodolite is equipped 
with standard M-17 telescopes of 8 power and 





EASTMAN RECORDING PHOTOTHEODOLITE 


543 


8 degree field. They are used with Army fire- 
control equipment and have proven usable with 
good aided-tracking units, where the accuracy 
obtained was within 0.3 mil. Consideration was 



Figure 13. Aided tracking unit. 

given to the use of higher magnification or of 
binocular telescopes to improve tracking accu¬ 
racy, but the latter in particular offered too 
serious a complication to the design. 


1548 Levels 

The level vials were obtained from the firm 
of W. A. Moyer and Son, of Parkers Landing, 
Pennsylvania, which also supplies levels of 
high precision to the U. S. Coast and Geodetic 
Survey Division. The levels made by this firm 
are of the cylindrical type, ground on the inside 
surface, with accuracies as high as 1.9 sec per 
millimeter displacement of the bubble. 

The higher the angular value of each divi¬ 
sion, the greater is the time required to bring 
the bubble to equilibrium. As a compromise be¬ 
tween extreme accuracy and slowness of opera¬ 
tion, a value of 10 sec per millimeter was 
adopted, which should make possible satisfac¬ 
tory leveling to 4 or 5 sec of arc. 


A special mount was designed and con¬ 
structed which decreases the danger of break¬ 
age during adjustment. In the standard-type 
mount, the tube containihg the vial usually has 
solid ends with two “ears,” one extending from 
each end. In the Eastman design, a washer with 
one flat and one spherical side is inserted be¬ 
tween the faces of the ears and adjusting nuts. 
The nuts that bear on the washers are made 
with a concave conical face. The washer thus 
seeks a new position with respect to the nut 
when the latter is adjusted. 

Leveling of the phototheodolite is accom¬ 
plished by three fine-thread screws, having 
large hand grips and spherical ends that rest 
in V blocks on the base. The advantage of this 
system is that if the instrument is removed and 
replaced, the center of the vertical axis will 
always come to rest in the same point on the 
base, regardless of temperature change. 

The three leveling legs are placed 120 degrees 
apart on a 12 l/ 2 -in. radius, and the thread was 
so chosen that one revolution of the screw pro¬ 
duces a 2-mil change in angle. Split bearings, 
with clamping screws, are used for the leveling 
screws. 

To provide for “misleveling” the Number 2 
instrument, a dial is attached to the face of one 
leveling hand grip, with graduations every 
1,000 yd of base line from 0 to 12,000. The dial 
is guided radially but is held by friction to the 
hand-grip surface. In operation, the instrument 
is placed with this leveling leg approximately 
on the base line facing the other station, and 
leveled, after which the dial is set on zero and 
the screw turned the proper amount for mis¬ 
leveling. 


15.4.9 Alignment of the Axes 

To insure perpendicularity of the tracking 
axes, provision has been made for adjusting the 
horizontal axis with respect to the vertical, a 
unique feature of the Eastman design. Consid¬ 
eration was given to applying the necessary 
adjustment to the bearings on one side of the 
horizontal axis. Self-aligning ball bearings of 
the required accuracy could not be obtained, 
however, and an additional drawback was the 












544 


PHOTOTHEODOLITES 


certainty of disturbing the mesh between the 
elevation worm and worm wheel with this type 
of adjustment. 

The solution adopted was to divide the in¬ 
strument into two main sections, the upper 
carrying the horizontal axis and the lower the 
vertical axis. Recesses were milled around the 
periphery on the top of the lower section. The 
bottom of the upper section, which was ma¬ 
chined and used as a datum plane for boring 
the horizontal bearings, was set on shims placed 
in the recesses of the lower section. Clamping 
screws are placed adjacent to each shim. When 
the condition of the two axes has been deter¬ 
mined, the shims are varied in thickness to 
effect proper alignment. Either wedge-shaped 
or uniform-thickness shims may be used, de¬ 
pending upon the amount of correction desired. 

15 - 410 Seals 

Extreme precautions were taken to exclude 
all foreign matter that might interfere with 
the operation of the instrument. A Garlock seal 
consisting of a type L neoprene insert is used 
for every turning member that extends to the 
outside of the instrument. Type L inserts are 
softer than the standard types used in high- 
pressure oil systems and are used especially 
to keep out small particles of foreign solids. 
All covers fit into recessed machine surfaces in 
order to exclude particles that might hinder 
operation of the worm wheel. Flange tubes 
mating with Garlock seals are employed for 
connecting parts between the upper and lower 
sections. The electric power pack is in a re¬ 
cessed cavity that separates it from the inside 
of the instrument. The control panel may be 
removed and repairs made to the power pack 
without contaminating the instrument proper. 

15.4.11 Electric Controls 

Both series and parallel hookups were tried 
for the three Edgerton lamps in each photo¬ 
theodolite. Series operation gave consistent re¬ 
sponse, whereas the behavior of the parallel 
hookup was erratic. In a preliminary test, a 
1 id charge of 3,000 v direct current gave suffi¬ 
cient light for good photographs on Shellburst 


Panchromatic film. Since long 3,000-v lines run¬ 
ning to the instrument were not desirable, the 
power pack was built into the instrument with 
a 110-v line entering at the control panel. Al¬ 
though the flashing of the lamps is to be con¬ 
trolled from a central station, a testing switch 
is included in the control panel of each instru¬ 
ment. 

The camera solenoid is likewise controlled at 
the central station, but a test switch is also 
provided for use in threading the film. 

In the event that the time counters in the 
two phototheodolites are out of synchronization, 
a pulsing switch is provided for actuating the 
counter in each instrument. 

For cine operation of the camera (20 frames 
per second), a 60-c source at the central station 
is held in synchronism with the 10 pulses per 
second that go to the flash lamp. The 60-c line 
is provided with a rheostat and voltmeter, since 
the distances from the central station to each 
phototheodolite station will probably be dif¬ 
ferent. 

For night photography, suitable lamps are 
provided to illuminate the telescope cross hairs, 
and by a weak exposure to record the fiducial 
marks on the films. The night lamps are con¬ 
trolled by a switch on the panel. 


15.4.12 Alignment of Instrument 
Camera Reticle 

If the vertical cross hair is made perpendic¬ 
ular to the horizontal camera axis, the hori¬ 
zontal cross hair will by construction be paral¬ 
lel to the horizontal axis. With the camera 
viewer in place, a distant point is sighted and 
the camera traversed in elevation. The reticle 
is then oriented until the vertical cross hair 
is parallel to the direction of motion of the 
sighting point. Since the reticle is fixed to the 
camera, the entire camera must be moved and 
then pinned into place after the adjusting op¬ 
eration. 

Axes 

The method employed for adjusting the per¬ 
pendicularity of the horizontal and vertical 
axes is outlined in Section 15.4.9. Two con- 



CENTRAL CONTROL STATION 


545 


venient methods may be used to determine the 
amount of misalignment. The first is similar 
to the well-known surveyor's “high-low” test 
and differs only in that the error is actually 
evaluated. The instrument is first leveled on a 
solid foundation and azimuth and elevation 
readings are taken on two distant points, one 
at about 0-degree elevation and the other at 
about 45 degrees. The readings are then re¬ 
peated with the instrument “dumped,” or re¬ 
versed. The following notation is employed: 

a-i = azimuth reading of low target with line 
of sight normal. 

di = azimuth reading of low target with in¬ 
strument reversed. 

e x = elevation reading of low target with line 
of sight normal. 

ei = elevation reading of low target with line 
of sight reversed. 

The subscript 2 denotes corresponding read¬ 
ings made on the high target. 

Also, let 

8 = angular misalignment of line of sight in 
a plane formed by the line of sight and the hori¬ 
zontal axis. 

y = angular misalignment of the horizontal 
axis with respect to the vertical axis. 

From the geometry of the problem, the errors 
8 and y are shown to be expressible in terms of 
the following equations. 


0i cos Ei = 
0 2 cos E 2 = 

where 

ad — ai — 3,200 
0i = - 2 -’ 

_ a 2 ' — a 2 — 3,200 


5+7 sin E h (1) 

5 + y sin E 2 , (2) 

jp Ci — ed + 3,200 
E1 = - 2 -’ 

j-, _ c 2 — e 2 + 3,200 


Equations (1) and (2), which are to be solved 
simultaneously for 8 and y, are valid only for 
small values of 8, y, and 0. All the angles are 
expressed in mils. 

A second method for determining the mis¬ 
alignment of the axes can be performed in the 
laboratory. The phototheodolite is placed on a 
solid foundation and brought into approximate 
level. An adjustable reticle is attached at each 
end of the horizontal axis and a filar microscope 
or cathetometer placed solidly on the same 


foundation and focused on each reticle. The 
center of each reticle is first adjusted until it 
coincides with the horizontal axis by observing 
its movement with the microscope as the hori¬ 
zontal axis is rotated through 3,200 mils (180 
degrees). The microscope is adjusted vertically 
to the center of the second reticle, the amount 
of adjustment being a measure of the misalign¬ 
ment of the horizontal axis with respect to the 
vertical axis. 

Lenses and Telescopes 

The lenses must be oriented to the auxiliary 
ring so that the line of sight, which is formed 
by the nodal point of the lens and the intersec¬ 
tion of the camera fiducial marks, is perpendic¬ 
ular to the horizontal axis. If the horizontal 
and vertical axes have been made perpendic¬ 
ular, sin Ex in equation (1) above is zero and 
the misalignment 8 of the line of sight follows 
directly from observation of a low target. 

With the above adjustments accomplished, a 
distant target is sighted with the camera viewer 
and target lens, and the reticles in the tracking 
telescopes adjusted until both are centered on 
the target. 

Elevation Zero 

With the instrument leveled, a distant target 
just above the horizon is sighted and the eleva¬ 
tion angle read. The instrument is then reversed 
and turned in azimuth until the target is again 
sighted. The elevation angles should be identical 
in both cases. If the readings differ, the eleva¬ 
tion 0.1-mil drum is moved on its worm shaft, 
by means of a tangent screw, by half the differ¬ 
ence between the two readings. 


15 5 CENTRAL CONTROL STATION 

The purpose of a central control station is 
to control the operations of the phototheodolites 
and auxiliary testing equipment, the latter to 
be used for collecting data from fire-control 
instruments under test with the phototheodo¬ 
lites. 

As a result of meetings held at Aberdeen and 
Fort Bliss early in 1945, attended by repre¬ 
sentatives of the Services, NDRC, Eastman 







546 


PHOTOTHEODOLITES 


Kodak Company, and Bell Telephone Labora¬ 
tories, the following decisions were reached: 

1. In view of the required testing of high¬ 
speed projectiles, the Edgerton flash illumina¬ 
tion of the angle dials is to occur at the mid¬ 
point of the target exposure. 

2. A controlling pulse generator, of an accu¬ 
racy of 1 part in 100,000, is to send pulses to 
the camera solenoid, and an equally precise time 
delay is to send pulses to the Edgerton lamps 
after a predetermined time delay. 

3. Data from equipment under test are to be 
collected by data recorders. These instruments, 
which are built by Bell Telephone Laboratories, 
translate selsyn data into printed numbers on 
tape. 

15.5.1 Principles of Operation 

Figure 14 is a schematic diagram of the con¬ 
trol equipment. The station is controlled by a 
master oscillator, consisting of a crystal vibrat¬ 
ing at 100,000 c. The oscillator is fed to six 
electronic decade counters, each of which counts 
the cycles, and at a preset number of beats 
sends out a pulse for activating other circuits. 
Thus, the combination of oscillator and elec¬ 
tronic counters is capable of sending pulses 
through six separate circuits at any one of four 
desired frequencies, 1, 2.5, 5, or 10 c, with an 
accuracy of 0.00001 sec. 

The pulse from counter 1 operates the sole¬ 
noid circuit for the phototheodolite camera 
mechanism, and at the same time actuates a 
precision time delay that sends a pulse to flash 
the Edgerton lamps of each phototheodolite. 
Since the flashing of the lamp provides the pri¬ 
mary time base, this time delay is supplied by 
an additional precision electronic counter. 

The remaining five circuits operate the data 
recorders and other test equipment. The pulses 
to these circuits are controlled by frequency 
selector switches, which select pulses in phase 
with those sent to the phototheodolites. In each 
circuit a pulse is sent to a time-delay electronic 
counter, which is adjustable over a range of 
1 sec, with a precision of 0.001 sec. After a 
preset delay, the pulse is passed on to a second 
time delay and then to the test equipment, and 
waits for another pulse. When four data re¬ 


corders are being used, the selector switches 
for circuits 2, 3, 4, and 5 operate at 1 pulse per 
sec, and the time delays are set to operate the 
data recorders at interlocking intervals of 
sec. Every fifth pulse to the data recorders 
is used to produce an identification mark on the 
paper tape. 

The outputs of circuits 2, 3, 4, 5, and 6 and 
of the fifth-pulse selectors are fed into a cross¬ 
connecting grid board which makes a flexible 
electric coupling between the various units. The 
output of the grid board feeds into a bank of 
twenty wet-reed mercury relays, precise to 
±0.001 sec, which distributes the load to the 
test equipment. These relays feed into a second 
cross-connecting grid board, whose output goes 
to four 19-plug connectors and subsequently to 
the phototheodolites and test equipment. 

An additional unit supplies controlled 60-c 
current to run the phototheodolite camera mo¬ 
tors in synchronism with the master oscillator. 
Thus, when the camera mechanism is operating 
at the motion-picture rate (20 frames per sec) 
it is in phase with the flashing of the Edgerton 
lamps. 

15 5 2 Construction and Installation 

The control equipment is installed in an 
Army maintenance truck containing a work¬ 
bench and a motor-generator set with a 10-kw 
power output. 

The master oscillator, time-delay units, fifth- 
pulse selector, and 60-c control unit for the 
camera mechanism were made by the Potter 
Instrument Company, Flushing, Long Island, 
New York. The cross-connecting grid boards, 
the relay rack, and installation of the entire 
unit were made by the Eastman Kodak Com¬ 
pany. 

15 6 INSTALLATION OF 

PHOTOTHEODOLITES 

15,6,1 Location 

The phototheodolites have been transferred 
to the Army Ground Forces Board No. 1, Anti¬ 
aircraft Service Test Section, and at this writ¬ 
ing are being permanently installed at Fort 



INSTALLATION OF PHOTOTHEODOLITES 


547 



MOLE RECEPTACLES 

Figure 14. Central control unit for phototheodolite. 




























































































































548 


PHOTOTHEODOLITES 


Bliss, Texas, under a direct Ordnance contract 
with Eastman. 


1562 Housing 

A photograph of the general appearance of 
one of the two proposed phototheodolite instal- 



Figure 15. Photograph of phototheodolite instal¬ 
lation. 


lations at Fort Bliss is shown in Figure 15 and 
an elevation in Figure 16. The phototheodolite 
rests on a reinforced concrete pier, which is 
isolated from the rest of the structure. A large, 
grillwork steel floor is supported by rollers 
120 degrees apart. Attached to the floor is a 
radome, built of sponge rubber reinforced by 
hardwood ribs and sprayed with Fiberglas both 
inside and out. The dome slit is adjustable from 
the top, and will not open beyond the required 
elevation angle. 

15.6.3 Tracking for Housings 

Provision has been made for automatic fol¬ 
lowing of the dome. Attached to the photothe¬ 
odolite is a movable arm that carries a carbon 


brush contact. The contact mates with a vari¬ 
able-resistance sector, which is attached to the 
base of the housing and which is in turn con¬ 
nected to a motor that drives a selsyn trans¬ 
mitter. The selsyn receiver drives an oil gear 
connected to one of the three wheels that sup¬ 
port the superstructure. 

1564 Electrical 

The electric connections between each photo¬ 
theodolite station and the central control sta¬ 
tion will be provided by bare copper wires 



ELEVATION 

Figure 16. Diagram of phototheodolite instal¬ 
lation. 


strung on telephone poles between the two sta¬ 
tions. Terminal boxes will be constructed at 
various points on the base line for the purpose 
of running the controls from the central station 
to the phototheodolite stations. 

i5.7 TESTING OF THE 

PHOTOTHEODOLITES 

i5.7.i Preliminary Test of Azimuth 
Worm Wheel 

A preliminary test of the two azimuth worm 
wheels was conducted at Gould and Eberhardt 
after assembly of the worms and worm wheels. 13 

































RECOMMENDATIONS BY NDRC 


549 


A Zeiss theodolite was employed, with a col¬ 
limator placed at one end of the test room 
serving as a target. Readings were made for 
every 400 mils rotation of the phototheodolite, 
in both clockwise and counterclockwise direc¬ 
tions, but with only one orientation of the pho¬ 
totheodolite with respect to the theodolite. The 
worm-wheel errors were found to be periodic, 
the amplitudes averaging about 0.030 mil for 
the No. 1 instrument and about 0.020 mil for the 
No. 2 instrument. 

15 8 RECOMMENDATIONS BY NDRC 

15,81 Proposed Tests 

Performance tests of the phototheodolite had 
not been carried out when the contract was 
terminated. NDRC has proposed the following 
series of tests to be made by the U. S. Army. 

Test of Photographic Resolving Power 
of Each Lens 

Exposures of three-parallel-line targets 
(white on black) are to be made at close inter¬ 
vals running through focus on Super-XX film 
processed in D-19 for 8 min at 68 F. The targets 
can be located either (1) at finite distance equal 
to at least fifty times the focal length of each 
lens or (2) at the focus of a 30-ft focal length, 
22-in. diameter concave mirror loaned by Mount 
Wilson Observatory to Eastman Kodak Com¬ 
pany. 

Static Tests 

This test is to determine if the phototheodo¬ 
lite indicates the same azimuth and elevation 
bearings of a target when the target is ap¬ 
proached from four directions. 

Targets may be (1) black-on-white cross at 
11,000 yd in daytime, (2) electric lamp at 
11,000 yd at night, (3) the moon in daytime, 
or (4) stars at night (at least 20 degrees ele¬ 
vation). Photographic tests have shown that 
sixth magnitude stars give adequate density on 
Shellburst Panchromatic film, with a 48-in. 
//5 lens, with 1-sec exposure made by remov¬ 
ing a cardboard shutter by hand. Best results 
will be obtained by using third or fourth 
magnitude stars. Exposures on the moon or 


stars must be timed by Bureau of Standards 
time signals on a loudspeaker or by a good 
watch or chronograph. 

Approach target from each of four directions 
(U, D, R, L), with phototheodolite stationary. 
Make exposures of target. Compare apparent 
azimuth and elevation for U, D, R, L ap¬ 
proaches. 

To calculate azimuth and elevation of stars 
and moon, use the following formulas : b 

elevation 

hav (90° — H) =- hav (L ~ d) 

+ cos L cos d hav t , 

azimuth 

sin Z = sin t cos d sec H, 

where 

H = elevation of star, 

L = latitude of station, 
d = declination of star, 
t = meridian angle of star, 

Z — azimuth of star. 

The above formula for azimuth is based on 
the assumption that the elevation has already 
been computed. Near the zenith this formula is 
indeterminate. 

If it is desired to compute azimuth, without 
having previously computed elevation, the fol¬ 
lowing formulas may be used: 

tan y 2 (Z — A) = cot y 2 t sin y 2 (L — d) 

sec i/ 2 (L + d), 

tan V2 (Z + A) = cot V2 t cos V2 (L — d) 

cosec y 2 (L + d), 
where A is the angle subtended, at the star, 
between the zenith and the pole. After comput¬ 
ing Z — A and Z + A, A is eliminated by adding 
the two equations. 

Dynamic Tests 

This test is to determine whether the photo¬ 
theodolite is sufficiently free from deflection to 
indicate the same azimuth and elevation bear¬ 
ings of a target when the target is approached 

b 1. For sign convention (L ^ d) , see Dutton, Navi¬ 
gation and Nautical Astronomy, p. 266. 

2. For table of haversines, see Bowditch, American 
Practical Navigation Table 34, 1938. 

3. Elevations should be corrected for refraction. 

4. The formula for azimuth will be satisfied by two 
values of Z, differing by 180 degrees, but the correct 
value will be obvious. 








550 


PHOTOTHEODOLITES 


from four directions at slow, medium, and fast 
speeds. 

Targets may be the same as those used in 
the static tests except that, in the case of stars, 
0.0 to 1.0 magnitude stars should be used in 
order to give adequate density for cine opera¬ 
tion which is an exposure time of 0.01 sec. 
Bureau of Standards time signals can be used to 
start cine operation. 

Track at slow, medium, and fast speeds past 
target, using cine operation of camera. Com¬ 
pare apparent azimuth and elevation for each 
speed. 

Overall Static Tests 

This is to determine the overall accuracy of 
the phototheodolite in a static condition at vari¬ 
ous points evenly spaced around the azimuth 
and elevation scales. 

Stars are probably the most convenient tar¬ 
gets, ten or twelve stars preferably about third 
or fourth magnitude, distributed reasonably 
uniformly over the sky and all at more than 
20 degrees elevation, would be sufficient. 

Track phototheodolite onto each star and take 
an exposure of 0.5 to 1 sec by manual removal 
of cardboard shutter. Compare apparent azi¬ 
muth and elevation bearings with calculated 
values of the stars. 

Overall Dynamic Tests 

This test is to indicate the overall accuracy 
of the phototheodolite as in the static tests ex¬ 
cept under dynamic conditions of slow, medium, 
arid fast tracking speeds past the stars. 

Ten or twelve stars of 0.0 to 1.0 magnitude, 
distributed reasonably uniformly over the sky 
all above 20 degrees elevation would be suffi¬ 
cient. 

Track phototheodolite onto each star at slow, 
medium, and fast speed using cine operation of 
camera. Compare observed azimuth and eleva¬ 
tion with calculated values of the stars. 


Test for Periodic Error in Worms 

Track a third or fourth magnitude star 
(above 20 degrees elevation) at night, and the 
moon in daytime, at considerable distance from 
the meridian (so that both azimuth and eleva¬ 
tion are changing). 

Make 0.5- to 1.0-sec exposures on a star at 
night at intervals of 30 sec, during a period of 
30 min. In the daytime make short cine runs 
on the moon at 30-sec intervals, to permit single 
frame exposures (about 0.5 sec) by removing 
cardboard by hand. 

Time the exposures with Bureau of Stand¬ 
ards time signals or with chronograph. 

Plot errors to detect any periodic error in 
azimuth or elevation worms. 

15 . 8.2 Recommended Design Changes 

1. Three differential screws may be used to 
align the horizontal and vertical axes, provided 
the base of the upper casting is strengthened 
so that the uprights do not toe in or out. 

2. Because it is desirable to grind the bear¬ 
ing seats on the horizontal axis after the axis 
assembly is complete, the camera housing 
should be strengthened so that it has the same 
stiffness in all radial planes. 

3. For convenience in assembly, the upper 
casting should be so constructed that the ele¬ 
vation worm and worm wheel can be removed 
without raising the horizontal axis out of its 
bearing seats. 

4. The instrument should be balanced about 
the vertical axis to reduce strain on the base 
casting. The base casting should be further 
strengthened, and steel inserts with an acme 
thread provided for the leveling screws. 

5. The design of oil-drip pans for the worm 
wheels should be improved. 

6. The large castings should be redesigned to 
make more accessible the dials, chain drives, 
optics, etc. 



Chapter 16 

OPTICAL SCANNING DEVICES 

By Theodore Dunham, Jr. a 


T he efficiency of lookouts in detecting air¬ 
craft from land stations and in detecting 
both aircraft and submarines at sea is of para¬ 
mount importance. The loss of even a few sec¬ 
onds in detecting the approach of enemy air¬ 
craft or submarines may have a disastrous 
effect on the outcome of an engagement. What¬ 
ever optical aids improve the performance of 
lookouts, by even a small percentage, justify 
full study and development. 

The use of hand-held binoculars for increas¬ 
ing the range at which targets can be detected, 
both by day and by night, has become universal. 
Studies aimed at establishing the characteris¬ 
tics of night binoculars which will increase 
their efficiency to the maximum that is possible 
with hand-held instruments are described in 
Chapter 5. 

There are two obvious drawbacks associated 
with the use of hand-held binoculars. In the 
first place the field is necessarily limited (7.3 de¬ 
grees in the case of the standard 7x50 binoc¬ 
ular) when a magnifying system is used. This 
means that the observer must sweep in one co¬ 
ordinate when searching for submarines and 
in two coordinates when covering a sector of 
the sky while searching for aircraft. In the 
second place, the observer rapidly becomes 
tired, even under the best conditions, so that 
his efficiency as a lookout inevitably falls off 
after the first few minutes. Under adverse con¬ 
ditions, such as wind and cold, the efficiency 
of any lookout is reduced much further. Any¬ 
thing that can be done to lessen the fatigue 
and increase the comfort of a lookout will mark¬ 
edly better his performance in detecting distant 
targets at the limit of visibility and will extend 
the period of time during which he operates 
efficiently. This is an important consideration 
and should be given serious attention. 

The development of optical and mechanical 
aids for scanning the horizon and the sky was 
given attention by Section D-3 of NDRC, be- 
a Chief, Section 16.1, NDRC. 


ginning early in 1941. The work was continued 
under Section 16.1 (after 1942, Project 16.1-4) 
and although it was never given high priority 
and never aroused much interest in the Serv¬ 
ices, several experimental devices were devel¬ 
oped and tested in a preliminary fashion. 


161 REQUIREMENTS OF A 

SATISFACTORY SCANNING DEVICE 

Although the requirements of a scanning de¬ 
vice depend to a considerable extent on the 
particular application for which it is intended, 
there are several requirements which apply in 
general. 

1. The observer should look directly forward at all 
times and should if possible be comfortably seated and 
protected from the wind. It should be unnecessary for 
him to turn his head to follow the changing direction of 
the field which he is examining. The change in direction 
should be accomplished, whenever it amounts to more 
than a small angle, by the rotation of one or more 
reflecting devices ahead of the eyepiece. When the angle 
to be covered is small, it may be satisfactory to turn 
the whole instrument, including the eyepieces, but if 
this is done the center of rotation should preferably be 
close to the eye so that the observer can follow the de¬ 
vice almost entirely by turning his head, with a mini¬ 
mum of side motion. A third alternative is to mount the 
observer and his instrument on a platform which turns 
as a whole, and which can be stabilized if necessary. 
This is to be avoided, whenever possible, since it re¬ 
quires more space than will usually be available. 

2. The image must be maintained erect, and should 
not be turned right-for-left. Even when scanning for 
aircraft, this is important because if a target is spotted 
it is essential to get the correct interpretation of what 
the observer sees as quickly as possible. 

3. Vibration must be eliminated as far as possible. 
Linear vibration is not particularly serious if it is not 
excessive, but oscillation in angle must be reduced to a 
minimum if the full advantage of magnification is to 
be realized. The human frame is remarkably efficient as 
a vibration and oscillation eliminator and if it is to be 
replaced by a mechanical mounting for the scanning 
device then special care must be taken with the design. 

4. Provision should be made, particularly in the case 
of devices intended for use in scanning for aircraft, for 


551 



552 


OPTICAL SCANNING DEVICES 


scanning the area under observation either manually or 
automatically. A careful comparison will be required to 
determine which method is most efficient. 

5. When automatic scanning is employed, the change 
from one field to another may be made either gradually, 
by a uniform motion of the whole device or of the re¬ 
flectors within it, or it may be made by sudden jumps 
from one field to another, with pauses lasting long 
enough to permit examination of each field. Experi¬ 
ments, under conditions simulating those encountered in 
practice, will be required to choose between these two 
types of scanning programs. If intermittent scanning is 
employed, it may be necessary to incorporate a shutter 
in the system to prevent the observer from being dis¬ 
turbed by seeing the field jump between successive 
periods of rest. 

6. There may be important practical advantages in 
scanning devices which employ standard binoculars, 
with external equipment to produce the scanning effect. 
Such a design will usually be simpler and will involve a 
less serious problem in production than will a device 
which is special throughout. On the other hand, a com¬ 
posite instrument, consisting of standard binoculars 
and auxiliary scanning equipment, is almost certain to 
absorb more light than would a device specially de¬ 
signed to combine the optical and the mechanical func¬ 
tions. At night this consideration is more important 
than in the daytime. 


162 PRINCIPLES OF OPTICAL AND 
MECHANICAL DESIGN 

The purpose of the scanning devices devel¬ 
oped under NDRC was to permit the observer 
to cover the sector assigned to him in system¬ 
atic fashion by employing mechanical and op¬ 
tical means to change the direction of his view 
while he looks always in a fixed direction into 
monocular or binocular eyepieces. It was hoped 
that such a device would lead not only to a more 
systematic coverage of the assigned sector, but 
would also appreciably increase the comfort 
and hence the efficiency of the lookout. 

All the scanning devices described depend on 
the rotation of one or more plane mirrors or 
prisms which are incorporated in the optical 
path. The usual erecting system used with tele¬ 
scopes requires four reflections—two in each of 
two planes at right angles to one another. Sev¬ 
eral scanning devices are based on rotating one 
or more of these mirrors. In some devices addi¬ 
tional mirrors are added expressly for the pur¬ 
pose of scanning, for example, when a standard 
binocular with its built-in erecting system is 


used. The optical design of the panoramic tele¬ 
scope, which is widely used for controlling the 
fire of field guns, has been used in one scanner. 
Several more complex systems have been de¬ 
vised and models of some have been made. 
Obviously, there must be an even number of 
reflections, usually four or six, to preserve the 
right-left orientation of the field when the 
image is erect. 

The present designation of individual scan¬ 
ning devices is by no means satisfactory. The 
names used most commonly by those who de¬ 
veloped the devices and which are in OSRD 
reports have been retained with the full real¬ 
ization that a more logical nomenclature is 
needed. 


16.3 INDIVIDUAL scanning devices 

A systematic study of possible designs for 
scanning devices was made at the Mount Wilson 
Observatory 1 * under Contract OEMsr-101. The 
Yerkes Observatory 2 under Contract OEMsr- 
1078 developed a panoramic scanner and built 
a model for testing. 


16.3.1 Double Dove Prism Scanner 

The first scanning device to be constructed 
and tested at Mount Wilson 3 employed a pair of 
double Dove prisms mounted in front of the 
objectives of an 8x56 binocular, linked together 
and driven by an intermittent mechanism 
which served to bring into the observer’s view 
successive fields of view along the horizon, 
spaced 5 degrees apart. A device of this kind 
might be entirely practical for horizon scan¬ 
ning, but since it has no provision for vertical 
scanning it would be necessary to stabilize the 
Dove prisms and the binocular. The device was 
developed primarily for experimental purposes 
to determine the optimum period of rest and 
the optimum angular spacing of successive fields 
for horizon scanning. A model was made, but 
time did not permit completing the program of 
testing. 





INDIVIDUAL SCANNING DEVICES 


553 


16 ‘ 3 ’ 2 Panoramic Scanner 

A model of this device was made in monoc¬ 
ular form at the Yerkes Observatory. 2 It dis¬ 
plays an erect image of a target and covers the 
entire sky. It is based on the standard military 
panoramic telescope. A double Dove prism di¬ 



rects light from a target at any elevation ver¬ 
tically downward, through the objective and 
through a Schmidt rotating prism in the con¬ 
verging beam, to a roof prism which turns the 
beam horizontally to the eyepiece. The Schmidt 
prism is geared to turn at one-half the rate at 
which the double Dove prism turns in azimuth, 
and serves to compensate the rotation of the 
field which would otherwise occur. The telescope 


system is a 10x50 wide-field 7-degree design 
developed at the University of Rochester. 4 Fig¬ 
ure 1 shows the optical layout of the reflecting 
elements and Figure 2 ishows a photograph of 
the complete instrument. False reflections were 
eliminated by cementing plane-parallel plates 
with beveled and blackened edges to the faces 
of the double Dove prism and by mounting 
curtains at the sides (for details see the Yerkes 
Observatory Report 2 ). Scanning in elevation is 
controlled by two hand wheels on a horizontal 
axis, while scanning in azimuth is accomplished 



Figure 2. Panoramic scanner. 

by rotating the entire upper assembly about a 
vertical axis by means of the same handles or 
by turning a large knurled nut surrounding the 
base of this assembly. 

The Dove prism must be made to close speci¬ 
fications to avoid doubling of the image, and it 
has the further disadvantage that it absorbs 


























554 


OPTICAL SCANNING DEVICES 


a considerable amount of light. A rear-surfaced 
mirror might well be substituted for the Dove 
prism. Such a mirror need not be inconveniently 
long, since it may not be necessary to scan the 
sky up to the zenith, and in any case it is 
probably unnecessary to fill more than about 
half the aperture at the maximum elevation. 
If a mirror were used, it would probably be 



Figure 3. Altazimuth two-mirror scanner. 

practical to design a binocular device with a 
single mirror large enough to cover the two 
objectives. The principal problem would be that 
of holding the dimensions of the two image- 
rotating prisms and their cells within small 
enough limits of size so that they would not 
interfere with one another when their centers 
are separated by only about 65 mm. 

The panoramic scanner is one of the most 
compact and generally satisfactory designs that 
have been investigated. Limited tests have 
shown that the device is capable of convenient 
and efficient operation. If provided with detents, 
for setting at specified intervals in elevation, 
it can be used to cover systematically any de¬ 
sired portion of the sky by sweeping in azimuth 


at each setting in elevation. Circles can be added 
for reading the coordinates of a target. 


16.3.3 Altazimuth Two-Mirror Scanner 

The design of this device 111 resembles that of 
the panoramic scanner in many respects, except 
that the rotating Schmidt prism is omitted and 
that two plane mirrors, one fixed and one ad¬ 
justable, replace the double Dove prism. The 
objective is placed between the two mirrors. 
The entire sky can be covered, but the field ro¬ 
tates when the setting in azimuth is changed. 

The arrangement of optical parts and the 
mechanical controls are shown in the phantom 
sketch in Figure 3. The first mirror is located 
above the second mirror, separated by an 
amount which is sufficient to permit reaching 
the horizon without interference. Some reduc¬ 
tion in aperture is tolerated at the zenith in 
order to avoid excessive length of the first 
mirror. The setting in elevation is controlled 
by a knob turned by the left hand which changes 
the setting of the first mirror. Scanning in 
azimuth is accomplished by turning a crank 
handle, operated with the right hand, which 
rotates the upper assembly consisting of the 
two mirrors and the objective. The two con¬ 
trols are linked together through a cam, so that 
when desired the setting in elevation starts at 
90 degrees and falls continuously to 0 degrees 
after approximately sixteen complete revolu¬ 
tions in azimuth. The setting then returns to 
the zenith through the action of a spring re¬ 
leased by the cam. Thus the entire sky is cov¬ 
ered systematically. A motor could be used to 
provide continuous drive in azimuth. This 
might well be arranged to run at a faster rate 
near the zenith where the circular paths to be 
covered are much smaller than near the horizon. 

In accordance with usual definitions, the 
altazimuth two-mirror scanner causes rotation 
of the field of view, that is, the apparent direc¬ 
tion of the horizon line rotates steadily in the 
field as the instrument scans in azimuth. Al¬ 
though this would undoubtedly be a serious 
disadvantage in an instrument intended for 
searching for submarines close to the horizon, 
it may not be at all disadvantageous when 





INDIVIDUAL SCANNING DEVICES 


555 


searching the sky for aircraft, since efficiency 
in detecting targets in an otherwise clear sky 
field is not likely to depend on the orientation 
of the field. The presentation with this scanner 
is actually exactly as it would be if the observer 
were lying on his back looking up at the whole 
sky and were to imagine it flattened into a 
large disk, except that the roof prism turns this 
presentation through 90 degrees, so that it is 
seen by looking horizontally instead of verti¬ 
cally. The first mirror brings areas in any part 
of the sky into the line of sight (actually into 
the zenith, apparently to the horizon due to the 
roof prism) without rotating these areas. It 
may be that this presentation would be as sat¬ 
isfactory in practice as any other that could be 
devised, particularly if a device were added to 
indicate not only the part of the field of view 
toward which the device is directed, but also 
the direction within the field of a line parallel 
to the horizon. This could easily be done in the 
case of the present monocular device by allow¬ 
ing the inactive eye to watch at will a disk 
representing the entire flattened sky, upon 
which a short straight line would be located 
at a point corresponding to the elevation and 
azimuth of the center of the field of the instru¬ 
ment at the moment. The reference line would 
be mounted perpendicular to a radial arm on 
the sky disk, and so would always indicate the 
direction in the field of a line parallel to the 
horizon. This information would facilitate re¬ 
porting the direction of motion of the targets 
detected with the scanner. If it should prove 
desirable to prevent rotation of the field, this 
could of course be done by adding a Schmidt 
prism, linked at half speed, to the rotation in 
azimuth, similar to that used in the panoramic 
scanner. 

As in the case of other monocular scanners, 
a binocular rubber eyecup should be added to 
aid the observer in locating and maintaining 
his eye in the proper position and to reduce 
fatigue. The view of the inactive eye should be 
blocked, either with an opaque diaphragm or, 
perhaps better, with a ground glass illuminated 
with reflected sky light to present to the in¬ 
active eye an area having approximately the 
same brightness as the sky. Experiments might 
well be carried out to determine whether a com¬ 


fortable headrest and eye guard would improve 
performance and reduce fatigue. 

The altazimuth two-mirror scanner was orig¬ 
inally intended for use in scanning for aircraft 
at shore stations, but it could be used almost 
equally well in scanning for aircraft on ship¬ 
board. It could probably be stabilized more 
easily than most of the other scanners described 
in this chapter by linking the vertical axis to 
a damped gyro pendulum and by providing the 
roof prism with a half-speed motion, about a 
transverse axis, with relation to the same 
pendulum. 

Only limited field tests have been made with 
the present model. Observers found the con¬ 
trols somewhat inconvenient to operate, largely 
because the rate of response in elevation was 
too slow. They regarded monocular instruments 
as inferior to binocular instruments, and criti¬ 
cized the rotation of the field. It seems likely 
that most, if not all, of these criticisms can be 
overcome or shown not to be serious. 

The comparative simplicity of the altazimuth 
scanner and the fact that it covers the entire 
sky makes it seem desirable that further de¬ 
velopment be carried out to investigate fully 
its capabilities after indicated improvements 
have been made. 


16 3 4 Porro System Scanner 

The changes in direction of the line of sight 
which are required for scanning may be 
achieved by rotating one or more of the re¬ 
flectors that are used for erecting the image 
in a telescope system. It is well known that 
four 90-degree reflections from plane mirrors 
properly oriented can invert an optical image 
while maintaining the original direction of the 
beam. No two reflectors may be parallel to one 
another; if they are, their effects cancel. 

At each reflection the incident and reflected 
rays define a plane of reflection. In an erecting 
system two different arrangements are possi¬ 
ble. In one the first and second planes of reflec¬ 
tion may coincide, while the third and fourth 
also coincide. These two planes must be per¬ 
pendicular to one another and they must inter¬ 
sect along the ray which passes between the 




556 


OPTICAL SCANNING DEVICES 


second and third reflectors. This arrangement 
is similar to Porro prisms of Type I. Alterna¬ 
tively, the second and third planes of reflection 
may coincide. In that case this plane must be 
perpendicular both to the first and to the fourth 
plane of reflection. The two lines of intersec¬ 
tion formed by these planes are the rays which 
pass between the first and second reflectors and 
between the third and fourth reflectors respec¬ 
tively. This arrangement is similar to Porro 
prisms of Type II. 

Scanning devices may be based on any of the 
numerous systems which fulfill these conditions, 
provided one or more of the reflectors are 
mounted on axes around which they can be 
rotated. To prevent the image from rotating in 
the field of view, it is a necessary and sufficient 
condition that the axis on which each reflector 
turns shall lie in the plane of that reflector, 
and perpendicular to the plane defined by inci¬ 
dent and reflected rays at the center of the field. 

Reflections in the various directions may fol¬ 
low one another in any sequence which is con¬ 
venient for a given application. 

In a simple optical system which is intended 
for scanning, and in which the final beam is to 
lie parallel to the incident beam, we may, for 
convenience in description, assume that the ob¬ 
ject is north (N) of the observer. Then the 
beam travels south (S) and will first be re¬ 
flected 90 degrees, either upward (U), to the 
east (E), downward (D), or to the west (W). 
The second reflection may turn it 90 degrees in 
any one of the three standard directions (in¬ 
cluding N and S, as well as U, E, D, and W) 
which do not involve placing the second reflec¬ 
tor parallel to the first. The third reflection 
may turn the beam 90 degrees in either one of 
the two directions still permitted. The direction 
of the fourth reflection is uniquely defined, since 
it must turn the beam in the same direction (S) 
as the incident beam. 

Table 1 lists the sixteen possible sequences 
in which the four reflections may be arranged. 

Each of these arrangements corresponds to 
a Porro prism system of Type I or Type II. The 
Porro type is indicated in the last column of 
the table. In Type I the first and second reflec¬ 
tions lie in the same plane, while the third and 
fourth reflections lie in a common plane at 


right angles to the first. These two planes are 
at right angles to one another and both are 
parallel to the N-S line. In Type II the second 
and third reflections lie in a common plane 
perpendicular to the N-S line, while the first 


Table 1 


Type 

Sequence 

Porro prism type 

1 

S—U-^N-^W^S 

I 

2 

S—►U—»-N—»E-*S 

I 

3 

S-*U—>W-*D-+S 

II 

4 

s—►u—* e—ed—> s 

II 

5 

S—► W—*N—»-D—*-S 

I 

6 

S—►W-EN— 

I 

7 

S—► W—*D—»E—>S 

II 

8 

S—► W—»-U—*E—*S 

II 

9 

S—*D—*N—*E—*S 

I 

10 

S—*-D—*N—► W—*S 

I 

11 

S-*D—E—U-*S 

II 

12 

S^D—►W—►U—*S 

II 

13 

S—*E—►N—►U-*S 

I 

14 

% S—*E—»-N—»-D—*S 

I 

15 

S—►E->U-^W-^S 

II 

16 

S—*E—*D—»-W—*-S 

II 


and second reflections lie in separate planes, 
parallel to one another and to the N-S line. The 
eight variations of each type are merely the 
result of changing the order in which the dif¬ 
ferent reflections follow one another and of the 
fact that there is a right-handed and a left- 
handed version of each arrangement. 

These sixteen optical arrangements are il¬ 
lustrated in Figure 4, which may be useful in 
selecting the most suitable system for a specific 
application. 

The objective may be located anywhere in 
the system—ahead of the first mirror, between 
any two mirrors, or following the last mirror. 
The eyepiece must, of course, follow the last 
mirror. 

In order to change the line of sight for scan¬ 
ning, any one of the reflectors may be rotated 
around an axis which lies in its own plane and 
perpendicular to the plane defined by the inci¬ 
dent and reflected rays. When this is done, all 
parts of the system which lie between the swing¬ 
ing reflector and the object must rotate as a 
unit on the same axis and at twice the speed of 
rotation of the reflector. Rotation may be pro¬ 
vided for one, two, three, or for all four of the 
reflectors. Each reflector may be either a totally 







INDIVIDUAL SCANNING DEVICES 


557 



Figure 4. Porro erecting systems. 













































































558 


OPTICAL SCANNING DEVICES 


reflecting prism or a plane mirror. Two or even 
three reflections may be combined in a single 
glass prism. 

Prisms have certain advantages, as com¬ 
pared with mirrors. They are more efficient and, 
when rotated for scanning purposes, they need 
not be nearly so long on their hypotenuse face 
as a mirror must be in order to give the same 
angular displacement of the beam. Also, they 



Figure 5. Mockup of Porro system scanner based 
on Type 12 of Figure 4. 

present no questions such as tarnishing in salt 
air. But they cannot easily be obtained in large 
sizes. Above 2 or 3 in. of aperture mirrors 
must ordinarily be used. 

The greatest angular sweep in two coordi¬ 
nates can be achieved by making all four angles 
flexible in the system. Type 6 for the right eye 
and type 13 for the left eye are probably the 
most suitable systems for such a design. The 
objective, in each case, would probably be 
placed between the second and third reflectors. 
All reflectors would presumably be prisms. It 
would be essential to maintain parallel, within 
close tolerances, corresponding arms in the two 
systems. This would not be as difficult as it 
might at first appear. The first and second re¬ 
flectors need not be held in register with ex¬ 
treme accuracy, since the eye has appreciable 
lateral accommodation. The third and fourth 
reflectors, which control vertical register, must 
be coordinated more accurately, but this might 
not be at all difficult because corresponding 


arms in the right and left units move strictly 
together, and they could be rigidly tied to one 
another by means of right- and left-threaded 
rods which would also serve to adjust the inter¬ 
ocular distance and to mount the whole device 
on its base. The possibilities of such a design 
should be studied in detail. A fundamental re¬ 
quirement is the design of an accurate, but 
simple, optical elbow with half-speed rotation 
of its self-contained prism. 

The other optical arrangement which may be 
of considerable interest is type 12 for the right 
eye and type 11 for the left eye. Considerable 
angles can be covered in two coordinates by the 
first and second mirrors, both of whose axes can 
be mounted on the base of the instrument. The 
third and fourth reflectors can be prisms, 
which might be combined in a single piece of 
glass. The objective would be located between 
the second and third reflectors. It is necessary 
to make the first mirror very wide if more than 
a 60-degree total angle is to be covered in azi¬ 
muth, but for moderate angles its size will not 
be excessive. A reasonably compact scanning 
device intended for covering moderate angles 
can almost certainly be made, based on types 
12 and 11. 

A preliminary model lb of a monocular scan¬ 
ner based on type 12 is shown in Figure 5. Ver¬ 
tical scanning is accomplished entirely with the 
first mirror, and horizontal scanning entirely 
with the second. About 115 degrees can be cov¬ 
ered vertically and 90 degrees in azimuth. The 
addition of a similar system for the other eye 
would provide binocular vision without great 
additional complication. 

When the instrument is in the position 
shown, the center of the useful range is some¬ 
what below the horizon. Provision is made, 
however, for turning the first three mirrors as 
a unit about a horizontal axis through the 
fourth mirror. Readjustment of the fourth mir¬ 
ror and of the eyepiece then permits the verti¬ 
cal range to be changed so that it includes both 
the horizon and the zenith. The axis of the eye¬ 
piece can remain horizontal. These motions, 
which are incorporated in the present model 
only as adjustments, might be linked together 
with the proper relative speeds so that they 
would be available for actual scanning. This 










INDIVIDUAL SCANNING DEVICES 


559 


would permit vertical scanning over a much 
greater range. 

With this instrument there is no rotation of 
field in the usual sense, but the perspective is 
handled by the device in a way which may 
prove somewhat disturbing. This is much im¬ 
proved if the design is based on a Porro type 
which permits the first mirror to turn on a 


B 



Figure 6. Altazimuth four-mirror scanner. 


vertical axis, and the second mirror on a hori¬ 
zontal axis, but such an arrangement is less 
well adapted to binocular use. 


16.3.5 Altazimuth Four-Mirror Scanner 

This device 10 is illustrated schematically in 
Figure 6. Four reflectors are employed and 
arranged in the neutral scanning position, in 


the same way as in the Porro system scanner 
(Figure 5). Referring to Figure 4, Type 12 is 
the system used for the right eye and Type 11 
for the left eye. The first three mirrors rotate 
as a unit about a vertical axis, indicated by A-A 
in Figure 6, which passes through the centers 
of the third and fourth mirrors. In addition, 
the first mirror rotates about a vertical axis 
joining the centers of the first and second mir¬ 
rors (B-B), at twice the rate at which the first 
three mirrors rotate as a unit. Simultaneous 
rotation, at 2/1 speed, about these two vertical 
axes, provides horizontal scanning, and elimi¬ 
nates rotation of the field of view. The first mir¬ 
ror is mounted on a horizontal axis (C-C) to 
provide vertical scanning. 

If the first reflector is a mirror, it will prob¬ 
ably be difficult to carry vertical scanning more 
than about 60 to 70 degrees above the horizon, 
even if some reduction in aperture is allowed, 
since near the zenith the length of this mirror 
would become excessive. Actually, it may de¬ 
velop that it is unnecessary to carry scanning 
close to the zenith when the primary purpose is 
to detect approaching aircraft. 

The second and third reflectors in this device 
can probably be combined in a single Porro 
prism (light entering and leaving through the 
hypotenuse face). For the fourth reflector, a 
right-angle prism will probably be used next 
to the eyepiece. The objective can be located 
between the first and second reflectors, thus 
keeping the first mirror as short as possible 
and permitting the combining of the second and 
third reflectors into a single Porro prism. This 
design involves less glass between the objective 
and eyepiece than does a standard binocular 
and it can therefore be employed effectively 
with wide-angle systems. 

Preliminary studies indicate that this design 
is practical, both as a monocular and as a 
binocular. In a binocular design, the necessary 
linking of motions for the two eyes is not likely 
to present serious difficulty. 

A monocular scanner of this type might be 
well adapted for use by the rear gunner in a 
bomber to detect the approach of fighter air¬ 
craft. The device can probably cover a range of 
about 180 degrees horizontally and about 120 
degrees vertically (60 degrees above and be- 


■■mp* 












560 


OPTICAL SCANNING DEVICES 


low the horizon) without exceeding the limits 
of a reasonably compact unit. Moreover, since 
a monocular device designed for the right eye 
would occupy space principally to the right of 
and below this eye, it would be easy to get an 
unobstructed direct view by merely displacing 
the head 2 or 3 in. to the left. 

On ships, this scanner (in either monocular 
or binocular form) would be particularly satis¬ 
factory for scanning the horizon, because it 
does not rotate the field and because its con¬ 
struction is relatively simple. It can also be 
used for sky scanning whenever it is not neces¬ 
sary to cover a region more than 60 or 70 
degrees above the horizon. Double Dove prisms, 
although somewhat objectionable, permit scan¬ 
ning to the zenith. 


16.3.6 Aircraft Scanning Chair 

This is one of the simplest designs 1(1 and is 
intended for quick production to meet the need 
for a device to aid lookouts on naval and mer¬ 
chant ships in searching the sky. The optical 
system consists of a pair of standard 7x50 
binoculars mounted so that the observer looks 
steadily into them along a line inclined upward 
by about 10 degrees. Light is brought to the 
objectives of the binoculars by two plane mir¬ 
rors. The first mirror is carried on an arm 
which is pivoted at the second mirror (see 
Figure 7). Both mirrors rotate about axes 
perpendicular to the plane defined by the in¬ 
cident and reflected beams of light and are con¬ 
trolled by a suitable linkage. In this way, the 
dimensions of the mirrors are entirely reason¬ 
able. The mirrors provide vertical scanning. 
The entire optical system is mounted on the 
chair in which the observer is seated. The latter 
is carried on a vertical axis on ball bearings, 
which provides horizontal scanning when the 
observer causes it to rotate by pressure with his 
feet on the floor or deck. There is of course no 
rotation of the field of view. 

Figure 7 shows a model of this scanning de¬ 
vice. All optical parts are enclosed in a metal 
housing with two selected plate-glass windows, 
beveled at the edges, where they are in contact, 


and treated with low-reflection coating to re¬ 
duce the intensity of reflections from the 
ground which may otherwise be troublesome 
under certain conditions of lighting. The binoc¬ 
ular is mounted on one of the antioscillation 
mounts developed by the Eastman Kodak Com¬ 
pany under Contract OEMsr-392 (see Chapter 



Figure 7. Aircraft scanning chair. 


14). The cutaway drawing in Figure 8 shows 
the arrangement of optical and mechanical 
parts. 

Tests of this device, both on land and on a 
small vessel, show that it is entirely practical. 
Observers tend to favor this scanner above all 
of the other models so far developed, principally 
because of its simplicity, ease of operation, the 
comfort of the operator, and the fact that it 
provides binocular vision. Although developed 
primarily to meet the need for scanning at sea, 
it is equally well adapted for scanning for air¬ 
craft at land stations. 





INDIVIDUAL SCANNING DEVICES 


561 


Rolling Cylinder Scanner 

This device le was developed at Mount Wilson 
to provide coverage over a wide area of sky 
without rotation of the image. While it is prob¬ 
ably too complicated and the requirements re¬ 
lating to adjustment of mirrors may be too ex¬ 
acting to make it entirely practical, a brief de¬ 
scription is included here, partly because the 



Figure 8. Constructional details of aircraft scan¬ 
ning chair. 


design is decidedly ingenious and partly be¬ 
cause it is needed to complete the description 
of all designs for which models were actually 
constructed. 

The optical layout is shown in Figure 9. 
Light from the target falls on a flat mirror 
which directs the beam horizontally through 
the objective to an image-erecting system con¬ 
sisting of three mirrors, and then to a prism 
in which two 90-degree reflections occur. The 
beam emerges horizontally in a direction at 
right angles to the axis of the objective and 
finally passes to the eyepiece. There are six re¬ 
flections in all. Two independent symmetrical 
systems provide binocular vision. 


The means by which scanning in elevation is 
accomplished, without introducing rotation of 
the field, is shown in Figures 10 and 11. The 
first mirrors of the right and left units are 
mounted at the ends of a single frame which 
rotates around the horizontal axis B (Figures 9 
and 10), which passes through the center of 
the first of the three erecting mirrors, and 
allows the first mirrors at the ends of the frame 
to turn relative to the erecting systems. The 
three mirrors of the erecting system are rigidly 
connected to one another and turn at the same 
rate on a second horizontal axis, A in Figures 
9 and 10, which passes through the centers of 
the third erecting mirror and of the first face 
of the prism. Rotation of the mirror-erecting 
system about the axis A is controlled by a belt 


BACK SURFACE PLANE- 
PARALLEL MIRROR 


RAYS FROM AN OBJECT ON THE HORIZON 


IMAGE ERECTORS 
THE FRONT SURFACE 
MIRRORS EACH 



BACK SURFACE 
PLANE-PARALLEL 
MIRROR 


TO EYEPIECES 
(NOT SHOWN ) 



Figure 9. Optical layout of rolling cylinder 
scanner. 


so that its rate is the same as that of rotation 
of the frame carrying the first mirrors around 
the axis B. 

Scanning in azimuth is accomplished by ro¬ 
tating both first mirrors at the ends of the 
frames which carry them around the axes Ci 
and C 2 . They are linked together by means of 















562 


OPTICAL SCANNING DEVICES 



-AXIS C 1 


THESE WIRES RUNNING ON THIS SURFACE AND THE 
SURFACE D BELOW,WHICH HAS THE SAME DIAMETER, 
CAUSE THE FRAME CARRYING AXES C, AND C 2 
TO ROTATE ABOUT AXIS B AS AXIS B ROTATES 


AXIS C 




LINK 
CONNECTING 
AXES 
Cj AND C 2 

TOGETHER 



ROTATION OF THE END MIRRORS 
ABOUT AXES Cj AND C 2 

ACCOMPLISHES TRANSVERSE 
SCANNING 


AXIS A 


Figure 10. Diagrammatic views of parts of rolling cylinder scanner. 
















































































RECOMMENDATIONS BY NDRC 


563 


a bar which insures that they will always bear 
the correct relation to one another. 

Careful adjustment of all six reflectors is 
necessary to insure that the two images will re¬ 
main superposed when rotation is carried out 
about the axes A, B, and C. Such adjustment 
was actually achieved in the model built at 
Mount Wilson, but only by using special auxili¬ 
ary mirrors previously oriented with the re¬ 
quired accuracy. 

Adjustment of interocular distance is pro¬ 
vided by moving the prism which is attached to 
each eyepiece, along the axis A. A correspond- 



Figure 11. Rolling cylinder scanner. 


ing change in focal setting of the eyepiece is re¬ 
quired. 

A photograph of the complete device is 
shown in Figure 11. A pistol grip handle, shown 
in Figure 10, replaced the knob shown in Fig¬ 
ure 11 after the photograph was made. Rota¬ 
tion of the handle on its own axis causes scan¬ 
ning in azimuth by rotating the end mirrors. 
The same handle is used to turn the two assem¬ 
blies about the axes A and B to scan in elevation. 

Marked simplification would result if this 
scanning device were made for monocular vi¬ 
sion only. 

Preliminary tests in searching for aircraft 
show that this device is entirely satisfactory 
from the observer’s point of view. However, it 
should be noted that scanning is carried out 
around poles which lie in the horizon to the 
right and left of the observer, rather than 
around a pole at the zenith, as in the case of 
most of the other scanners described in this 
chapter. When the azimuth setting is not 
straight ahead, vertical scanning does not pass 
through the zenith but follows a minor circle 
centered at the poles in the horizon at the right 


and left. Thus it is not possible to add circles to 
the device which will show directly the eleva¬ 
tion and azimuth of the center of the field. 


164 DISCUSSION 

A considerable variety of scanning devices 
have been designed and several models have 
been actually constructed. Time has not been 
sufficient to permit tests which are adequate 
for evaluating the usefulness of scanning de¬ 
vices in general in searching either for sub¬ 
marines or for aircraft. Indications are, how¬ 
ever, that such devices may be distinctly use¬ 
ful, not only in searching for targets, but also 
in identifying enemy aircraft within or above 
a group of friendly aircraft, under conditions 
where radar is not effective. 

Further tests will be necessary to determine 
which types of scanning devices are most satis¬ 
factory for various applications. It seems likely 
that for horizon scanning, where rotation of 
the image is definitely objectionable, the air¬ 
craft scanning chair, the panoramic scanner, 
or the altazimuth two-mirror scanner are 
likely to be most effective. For both horizon and 
sky scanning the aircraft scanning chair and 
the panoramic scanner have marked advantages 
on account of their inherent simplicity and gen¬ 
eral reliability. 


16 5 RECOMMENDATIONS BY NDRC 

1. In view of the great importance of the 
lookout problem, the study and development of 
scanning devices should be actively pursued. 

2. Each of the promising types of scanning 
devices should be tested, with whatever im¬ 
provements can be made in present models, on 
both submarine and aircraft targets. Every 
effort should be made to shield the observer 
from wind and cold and to increase his com¬ 
fort. The score for detection should be com¬ 
pared directly with that obtained with hand¬ 
held binoculars. The subjective reactions of 
observers should be recorded in detail. 

3. Experiments should be carried out to de¬ 
termine whether, when using a monocular, it is 








564 


OPTICAL SCANNING DEVICES 


advantageous to present the inactive eye with 
an illuminated field, and if so, what level of 
illumination is best. 

4. A comparison should be made between 
monocular and binocular scanning devices of 
the same type, the monocular being provided 
with a comfortable headrest and eye guard, and 
with an illuminated field for the inactive eye 
if that has been found to be advantageous. 

5. The most promising type of scanning de¬ 


vice should be stabilized for use on shipboard, 
with the center of rotation near the eye of the 
observer. Experiments should be made to com¬ 
pare its effectiveness with that of hand-held 
binoculars. 

6. The most successful scanning device 
should be provided with elevation and azimuth 
indicators in the field of view. Experiments 
should be carried out with selsyn linkage to re¬ 
mote indicators and to the fire-control system. 



Chapter 17 

ANTIGLARE SHUTTER FOR NIGHT BINOCULARS 

By Sidney W. McCuskey a 


171 INTRODUCTION 

O ptimum performance of the personnel 
making night observations in the opera¬ 
tion of air or surface craft may be secured only 
if the eyes of the individuals concerned are 


for adequate protection against sudden flashes 
of illumination is even more acute. 

The device to be described in the following 
pages 1 consists of a high-speed shutter to close 
the aperture of binoculars in a very small frac¬ 
tion of a second. Under Project AC-26 night 



Figure 1. The antioscillation mount binoculars in the P-61 aircraft. 


dark adapted and remain so. The sensitivity of 
the eye to low light levels can be largely de¬ 
stroyed by even a short exposure to high bright¬ 
ness levels. When binoculars are used to in¬ 
crease the effective range of the eye the need 

a Warner and Swasey Observatory of the Case School 
of Applied Science. 


binoculars with antioscillation mounts have 
been developed at the University of Rochester 2 
and at the Eastman Kodak Company. 3 - 4 These 
units were designed for use by the pilot of a 
P-61 aircraft. Figure 1 shows the assembly 
mounted in the cockpit of this aircraft. The 
pilot can easily swing the binoculars into posi- 


565 









566 


ANTIGLARE SHUTTER FOR NIGHT BINOCULARS 


tion as he needs them. Protection for the pilot’s 
eyes, should the enemy employ flares or flashes 
while the binoculars are in use, is afforded by 
the antiglare shutter. The shutter closes the 
aperture of the binoculars in about 0.0015 sec 
and remains closed for approximately 0.3 sec. 
A photoelectric trigger circuit to operate the 
device automatically has been developed at 



Figure 2. Front view of production model of the 

antiglare shutter. 

Stanford University and is described in detail 
elsewhere. 5 Its essential features are given in 
the following pages. 

172 THE HIGH-SPEED SHUTTER 

Preliminary experiments with various meth¬ 
ods of obtaining fast closure of the 2-in. diame¬ 
ter entrance window of the night binoculars in¬ 
dicated that electromagnetic or explosive pro¬ 
pulsion of the shutter mechanism offered the 
greatest chances of success. It was desired to 
close this aperture in 0.001 to 0.0015 sec. Upon 
further investigation the electromagnetic pro¬ 
pulsion was rejected. It was found impossible 
to drive a shutter the requisite 2 in. in less than 
0.004 sec by this means. Further development, 
therefore, was centered on an explosively pro¬ 
pelled shutter. 


The framework and case of the shutter is an 
aluminum casting which may be rigidly con¬ 
nected to the aircraft night binocular. In the 
front surface of the casting are two apertures 
which are closed by the shutter curtains. Fig¬ 
ure 2 gives a front view and Figure 3 a rear 
view of the device as a whole. All moving parts 
of the shutter are mounted on this casting. 

It is evident that very high accelerations 
must be imparted to the shutter curtain by the 
explosive propellant. These are imparted to the 
curtains which close the apertures by means of 
a piston mechanism, within the “cylinder” 
shown in Figure 3. Experiment had indicated 
that the shock wave due to the explosion was 
insufficient to provide the required accelera¬ 
tion. Hence it was necessary to arrange the 
piston and cylinder so as to utilize as effectively 
as possible the expanding gases of combustion. 

The cylinder, or barrel, which was finally 
adopted is a straight bored tube of mild steel 
rigidly fastened to the case. The piston moves 
freely inside the cylinder and has attached to 
it the shutter curtain yoke which passes from 
the piston to the blades through slots cut longi¬ 
tudinally in opposite sides of the barrel. The 
slots are made narrow to reduce as far as pos¬ 
sible deceleration due to loss of gas pressure. 
This type of barrel has an added advantage in 
furnishing axial and rotational guides required 
by the piston and shutter blades during move¬ 
ment. A rubber bumper in the cylinder cap 
serves to dissipate the energy remaining in the 
piston at the end of its travel. 

The piston itself is of carbon steel, hardened 
and drawn to 58 to 60 Rockwell C. It is essen¬ 
tially a cylinder whose central section has been 
reduced in diameter in order to conserve on 
weight. The ends have a bore diameter of 0.468 
in., less the piston to bore clearance of 0.010 in. 
on the diameter, and serve as guide sections in 
the barrel. The lower section serves as the 
piston face. 

Satisfactory materials for shutter curtains, 
capable of withstanding the high accelerations 
involved and being at the same time light and 
dependable in action, were found after consid¬ 
erable experimentation. Although a bellows- 
type curtain of cemented cloth and leather was 
found quite satisfactory, still better results 










THE HIGH-SPEED SHUTTER 


567 


were obtained with a lightweight black nylon 
cloth, along both edges of which pure gum rub¬ 
ber cords were cemented. The cords are fas¬ 
tened so that in the relaxed state the curtain 
is pleated. These cords act as the return springs 
in withdrawing the curtains and also as sup¬ 
ports for the nylon. The yoke, which carries the 
curtains, and the piston are shown in Figure 4. 


will remain in either the locked or unlocked posi¬ 
tion. A linkage to the solenoid which indexes 
the propellant charges unlocks the catch after 
an appropriate time has 'elapsed. 

Figure 4 gives a clear view of the magazine 
used to hold eight propellant units and Figure 3 
shows it in position in the casting. The propel¬ 
lant units are recessed and held in place by a 



Figure 3. Rear view of antiglare shutter. 


It is made of 0.040-in. steel, not beaded, and is 
of the uniform-stress cantilever design. To¬ 
gether with the piston it weighs 8.8 g. 

In order that the shutter be held closed long 
enough for the observer’s eye to be protected, 
a delay mechanism, retarding the reopening of 
the binocular apertures, has been provided. A 
catch, which will hold the piston in the “shut¬ 
ter closed” position (see Figure 3) for 0.3 sec, 
is the delaying mechanism. It is provided with 
a cam surface which is operated by the leading 
edge of the piston, while the locking portion of 
the catch is entered behind the trailing section. 
By a spring and detent mechanism the catch 


hinged cover which does not interfere with the 
expanding products of combustion. 

Attached to one side of the magazine is a 
ratchet whose pitch equals one-half the spacing 
between the charge units and whose alternate 
teeth lie on opposite sides of the ratchet bar. 
The magazine is placed in its chamber by push¬ 
ing it against a spring until the last ratchet 
tooth engages the pawl and locks. Oscillating 
motion of the pawl allows the magazine to be 
moved by the spring. It is only necessary to trip 
the pawl and return it to its neutral position to 
bring a new propellant unit into place beneath 
the barrel. The pawl is actuated by a solenoid 






568 


ANTIGLARE SHUTTER FOR NIGHT BINOCULARS 


which operates on 24 v direct current and re¬ 
quires about 8 amp operating current. At the 
same time that the pawl is moved by the solen¬ 
oid, the linkage to the delay catch holding the 
curtain closed releases the piston and allows 
the curtain to fall. 

The delayed pulse needed to actuate the sole¬ 
noid, thereby reopening the shutter and index¬ 
ing the magazine, comes from the photoelectric 
circuit which initially tripped the shutter. It 
is described in Section 17.4. In case of electrical 



Figure 4. Piston and yoke assembly for the anti¬ 
glare shutter are*shown in the upper figure; the 
magazine is in the lower figure. 

failure, a manually operated indexing lever 
mounted beneath the unit may be used to trip 
the indexing pawl and the delay catch lock. 

A red signal light, mounted in such a way as 
to cast a dim diffuse glow, indicates when the 
magazine is empty. 

17 3 THE EXPLOSIVE PROPELLANT 

A low-voltage firing system (250 v) was 
found to be preferable to a high-voltage spark 
detonation system, both because of greater re¬ 
liability in firing and because of the smaller 
amount of dielectric needed in the units. The 
propellant charge for this type of firing must 
be thoroughly mixed with a conducting ma¬ 
terial, the most suitable discovered being the 
carbon residue from the incomplete combustion 
of acetylene gas. A mixture producing about 
0.25 megohm resistance gives satisfactory re¬ 
sults at 250 v firing potential. 


Many tests of priming materials were made 
in order to select a combination which would 
burn rapidly and leave no residue in the barrel 
after repeated firing. Tetrazene was finally se¬ 
lected. The end products of its combustion are 
various hydrocarbons and ammonia which con¬ 
dense to form a viscous tar. After a few firings 
the piston and barrel became gummed with this 
tarry residue. However, it was found that the 
residue could be solidified and be made in a 
sense self-removing by adding an oxidizing 
agent, potassium perchlorate. 

The explosive charge which was found to 
work satisfactorily contained 65 per cent 
potassium perchlorate, and an additional 1.5 
per cent acetylene black to act as a firing cur¬ 
rent conductor. A higher percentage of tetra¬ 
zene results in a gummy residue; a lower per¬ 
centage results in a sandy residue. The total 
charge weight found to be satisfactory for the 
purpose was 50 to 66 mg. This charge pro¬ 
duced on the average a shutter closing time of 
0.0013 sec. In the experimental stages the firing 
pulse was derived from a 0.5 ^f condenser 
charged to 250 v. 

A standard Remington Arms .60-caliber elec¬ 
tric primer cup was used to case the firing unit. 
These cups are 0.220 in. long and have an out¬ 
side diameter of 0.322 in. In the cups the 
cylindrical wall is electrically insulated from 
the bottom by a washer of acetate sheeting. The 
wall and bottom serve as the electrodes in the 
firing circuit. In order to increase the uni¬ 
formity in explosion when such small charges 
were used the diameter of the primer bore was 
decreased to 0.187 in. A coating of nitrostarch 
lacquer protects the powder charge and de¬ 
creases the chance that one unit will fire the 
adjacent one in the magazine. 

17.4 the photoelectric control 
CIRCUIT 

The electronic circuit 5 which controls the 
action of the shutter performs the following 
functions: 

1. It provides the impulse needed to fire the 
cap which closes the shutter curtain. 

2. It actuates the relay which delays the re¬ 
opening of the shutter curtain. 





THE PHOTOELECTRIC CONTROL CIRCUIT 


569 


3. It operates the magazine indexing magnet 
which controls the flow of caps to the firing 
position. 

Of the two hundred or more circuits and 
parts of circuits investigated, the one finally 
adopted is shown schematically in Figure 5. A 
brief description of its action is best followed 
by reference to the diagram. 

When a light pulse of intensity 0.01 foot- 
candle (about that of the full moon directly 


which in turn actuates the time-delay Relay II. 
Relay II operates the reopening release of the 
shutter. As explained in/ preceding paragraphs, 
the solenoid which releases the curtain catch 
also indexes the magazine containing the pro¬ 
pellant caps. The completion of this cycle leaves 
the shutter ready for reclosing on a succeeding 
light flash. 

While the preceding' description gives the 
essentials of the circuit action, it does not make 



Figure 5. The circuit of the photoelectric control unit. 


observed) falls on the photoelectric cell T1 the 
voltage drop across R1 controls the grid of the 
coupling tube T2. The electric pulse is amplified 
by the succeeding stages of the amplifier and 
finally triggers a thyratron (T5) which becomes 
conducting and serves as a discharge path for 
the storage condenser C16. The current pulse 
thus released through the primary of the trans¬ 
former (Tr) in turn creates a surge in the sec¬ 
ondary which fires the cap, closing the shutter 
curtain. 

The circuit required for reopening the shut¬ 
ter is connected to the firing circuit at the 
cathode of T5. When the cathode of the thyra¬ 
tron (T5) goes positive during the firing ac¬ 
tion, a positive pulse is sent to the grid of T6. 
This tube, which has been biased beyond cut¬ 
off, begins to conduct and energize Relay I, 


clear the particular features required for the 
present device. They will now be discussed. 

The Photocell Unit. The tube T1 is a 929 
photocell, selected in this case because of its 
small size. It must be mounted in front of the 
pilot, and hence the entire unit was designed 
to restrict his view as little as possible. A cylin¬ 
drical lens of Lucite placed before the photo¬ 
cell approximately doubles its light-gathering 
power. The photocell unit was designed to be 
mounted at the base of the gunsight. Consider¬ 
able experimental work was required to deter¬ 
mine a value for R1 which would provide suffi¬ 
cient sensitivity of the unit to light and at the 
same time not be too sensitive to the micro¬ 
phonics set up by vibration of the plane. 

The circuit of the coupling tube T2 is some¬ 
what unorthodox. Limitations of space pre- 





































































































570 


ANTIGLARE SHUTTER FOR NIGHT BINOCULARS 


eluded mounting the unit on springs to remove 
vibration due to motors, gunfire, and the like. 
To keep the voltage pulses due to physical 
vibration of the circuit elements at a minimum, 
therefore, they are mounted extremely solidly, 
leads are kept short and are of heavy wire, low 
electrode voltages are used, the suppressor grid 
of T2 is used as control, and everything is well 
by-passed to ground. An important feature of 
the circuit is the low impedances used. R3, for 
example, is only 5,000 ohms and it is this low 
value which prevents tube T3 from being sensi¬ 
tive to microphonics. 

The tube T2 converts the signal in a resist¬ 
ance of 20 megohms to a signal of approxi¬ 
mately the same voltage in a resistance of 5,000 
ohms. There is little degeneration in the circuit 
because of the un-by-passed cathode resistor, 
as the current changes in the screen circuit act 
to compensate for changes in plate current. 

An interesting feature of the circuit is the 
blocking action of T2 due to a large voltage 
developed across R1 when the light intensity 
incident on the photocell exceeds 1 footcandle. 
Irrespective of what the flare of light does after 
its initial burst, the entire circuit is blocked 
until the light intensity diminishes to 1 foot- 
candle or less. No explosive caps will be wasted 
due to a flickering of the light source after the 
circuit has been triggered initially. 

The Amplifier. The photocell unit connects to 
the main amplifier chassis by a 5-wire cable 
whose shield is grounded. In the amplifier, R5 
and Cl form a decoupling filter in the plate 
supply of T2. This filter reduces the effects of 
transient surges in the power line due to cur¬ 
rent drain when the guns and cannon of the 
aircraft are fired or when the propellers are 
feathered. 

T3 and T4 form a voltage amplifier in which 
the gain per stage is about 90 v. These stages 
are well decoupled to reduce the effects of 
transients. 

The firing tube, T5, is an 884 thyratron 
which gives more reproducible firing character¬ 
istics than a strobotron. R21 and R22 form a 
voltage divider across the B supply to provide 
cathode bias for the 884. Even though the po¬ 
tential of the B supply changes due to current 
drain from the dynamotor supply voltage, the 


ratio of R21 to R22 is adequate to bias properly 
the thyratron. 

When the grid of T5 is made more positive 
than the critical triggering value, capacitor C16 
discharges through the thyratron. C16 thus 
shares its charge with C15, but since the latter 
has a very high capacitance relative to C16, 
most of the charge goes through the primary 
of transformer Tr, whose secondary connects 
to the explosive cap. C15 must be a paper con¬ 
denser, for no leakage can be tolerated because 
of the necessarily high values of resistance in 
the voltage divider R21, R22. 

Although the transformer is unnecessary for 
the firing of the cap, it does serve to isolate the 
cap so that one side of it may be at ground 
potential. Otherwise it would have been neces¬ 
sary to insulate the shutter from the frame of 
the plane. The characteristics of the trans¬ 
former are not critical. The one used is a simple 
audio transformer. 

A second reason for not connecting the cap 
directly in the discharge circuit of the thyra¬ 
tron is that, upon firing, the potential distribu¬ 
tion on the capacitors might vary from cap to 
cap, since the latter show a variation in initial 
resistance of about 40 to 1, while the final re¬ 
sistance is virtually infinite. In such a case the 
operation of the reopening release circuit, 
which depends on the firing circuit, would be 
subject to erratic behavior. 

When the cathode of T5 becomes positive and 
so makes T6 conducting, the Relay I is ar¬ 
ranged to remain closed for 0.5 sec. This relay 
(operating at a current of 7 ma) energizes 
Relay II which takes 0.67 amp and which in¬ 
troduces a delay of 0.25 sec by means of an 
attached cylindrical disk of appreciable moment 
of inertia. About 0.5 sec after the cap is fired 
Relay I opens, Relay II opens, and the opera¬ 
tion is complete. , 

The main chassis connects to the shutter by 
a 5-wire shielded cable. The large capacitor 
C22 helps to prevent surges on the 24-v power 
line from influencing the amplifier by induc¬ 
tion through the long 10-ft cable that connects 
the main amplifier to the photocell unit. C22 
should be as large as possible. Although 20 ^f 
is used here, it is recommended that the size 
be increased, if space is available, up to 200 /xf. 




RECOMMENDATIONS BY NDRC 


571 


The bias for the thyratron cannot be ob¬ 
tained from tapping onto the heater leads as is 
done for T6 because, if that were done, the 
thyratron would fire when the unit is turned 
off, thereby wasting a cap. It is important to 
consider the time constants of the plate and 
cathode circuits together, for if they are not 
the same, or nearly so, the unit will fire a cap 
on either the make or break. 


17 5 SUMMARY AND CONCLUSIONS 

The antiglare shutter and its associated con¬ 
trol circuit unfortunately were not completed 
until World War II was over. No flying tests of 
the device were made to determine its perform¬ 
ance in an aircraft. In the laboratory, however, 
a few deleterious features of the production 
model became evident. They were principally 
mechanical sticking of the cross arm, which 
carries the curtains, at the top of its stroke, 
and breaking of the shutter stop cam repeat¬ 
edly at a point where the cross section is small. 
These defects could easily be eliminated. In 
further design of such a device, simplification 
for servicing should be kept in mind. In its 
essentials the present shutter performs an ade¬ 
quate closure of the 2-in. binocular aperture in 
about 0.0015 sec. 

If a new design of the antiglare shutter were 
to be made, it would be well to design the binoc¬ 
ular as well as the shutter so as to place the 
latter at the smallest cross section of the light 
path. The aperture to be closed could then ap¬ 
proach the dimension of the exit pupil. With 


this smaller distance of closure a mechanically 
operated shutter might prove adequate. Pre¬ 
liminary experiments carried out at Stanford 
University 5a have already indicated that a me¬ 
chanical shuttey can be made to close the 2-in. 
aperture of the present device in 0.002 sec. 


17 6 RECOMMENDATIONS BY NDRC 

1. The shutter and photoelectric control cir¬ 
cuit should be tested as a unit in flight, pro¬ 
vided laboratory tests prove satisfactory. These 
tests should be planned to determine whether 
the sensitivity of the photoelectric pickup is 
adequate, whether microphonic effects have 
been adequately eliminated, whether the explo¬ 
sive drive is objectionable to the pilot, whether 
the closure of the shutter is fast enough to safe¬ 
guard the dark adaptation of the pilot, and 
finally whether the mechanical performance of 
the shutter is dependable under service condi¬ 
tions. 

2. Several minor mechanical changes should 
be made to improve the reliability of the shut¬ 
ter. These are described in the Stanford re¬ 
port. 3 

3. Experiments at Stanford indicate that it 
would probably be entirely feasible to make a 
mechanical shutter for a 2-in. aperture, which 
would be closed by release of steel springs in 
0.002 sec. The photoelectric amplifier could 
probably supply about 15 ma at 200 v from a 
pentode to energize a laminated magnet which 
would hold the shutter open. When the pentode 
turns off the current, the magnet would let go 
the shutter with a very short delay. 



Chapter 18 

RAPID PROCESSING EQUIPMENT FOR PERISCOPE PHOTOGRAPHY 

By James G. Baker a 


181 INTRODUCTION 

T he bureau of ships requested that NDRC 
develop, under Project NS-242, an attach¬ 
ment for periscope cameras for submarines to 
give quickly developed film within a few sec¬ 
onds. It is frequently desirable to take a photo¬ 
graph while exposing the periscope above the 
surface of the water for the minimum possible 
time and then to study the photograph after the 
periscope has been withdrawn. This procedure 
can only be effective from a tactical point of 
view if the photographs can be examined with¬ 
in a minute of the instant of exposure and if 
the resolution is good. 

Under Contract OEMsr-622 the Eastman 
Kodak Company was asked to investigate peri¬ 
scope photography and to provide rapid proces¬ 
sing equipment for use at the periscope under 
conditions of warfare. 1 The project was initi¬ 
ated for the primary purpose of rapid process¬ 
ing, but led naturally to the subject of improv¬ 
ing resolution. 


182 RAPID PROCESSING EQUIPMENT 

Tactically, it was desired to make photo¬ 
graphs through the periscope of a submarine 
and to submerge for a short time for study of 
the situation under conditions of safety. Use of 
normal processing equipment meant lapse of 
time of many minutes which might alter the 
situation and render the photographs worth¬ 
less. 

The problem of securing a usable photograph 
approximately 1 min after exposure was 
brought to the attention of Eastman and solved 
in a short time. The standard camera used on 
periscopes was the Mark 1 35-mm periscope 
camera. It was thought desirable to adapt all 
equipment to this unit. 

a Harvard College Observatory. 


A special back was fitted to the 35-mm cam¬ 
era permitting the removal of short lengths of 
exposed 35-mm film for transfer without fog¬ 
ging to a processing tank. The light-tight proc¬ 
essing cassette contains a knife for cutting the 
film to the correct length. The cassette is then 
immersed in the processing liquids which are 
able to pass through light locks to the film. 
Inside the cassette is a metal frame into which 
the film is automatically fed. This frame is re¬ 
moved from the cassette after processing and 
is placed in a special viewer under 3x magni¬ 
fication. 

Camera Back 

The Mark 1 periscope camera back was re¬ 
placed exchangeably by a special back with slot 
at one end permitting the exposed end of the 
film to pass over into the special developing 
cassette. Film winding is accomplished by the 
normal frame counting recorder replaced by a 
winder. Guides are furnished to prevent curl¬ 
ing of the film and misloading into the cassette. 
The cassette is provided with light traps with 
easy entry for liquids. 

Cassette 

The cassette is a small hard-rubber box, 
holding an internal metal frame into which the 
film is fed. An open side fitted with a light¬ 
locking cover permits assembling or removing 
the frame. The upper end of the cassette car¬ 
ries a knife and slotted anvil. With the knife 
retracted, film can pass over the anvil into 
the metal frame. Actuating the knife cuts the 
film and provides a light lock. Three frames are 
contained at once. The entire cassette is placed 
in the processing solutions. 

A carrying case is provided with stainless 
steel tanks for the processing solutions. Spring- 
loaded covers prevent splashing of the chemi¬ 
cals from the tanks. 

The processed negatives are viewed in a con¬ 
cave mirror-type viewer at threefold magnifi- 


572 



PERISCOPE PHOTOGRAPHY 


573 




Figure 1 

cation. The image may be viewed with both 
eyes from a relatively uncritical position. 

Special film and special developer are used. 
Developing time is only 30 sec. The film is 
hardened beforehand to permit high processing 
temperatures, and to eliminate need for icing 
hot solutions. The complete periscope equip¬ 
ment consists of: 

1. 1 Mark 1 periscope camera with special 
back 

2. 3 Processing cassettes 

3. 18 Film holders 

4. 4 Processing tanks 


18 3 PERISCOPE PHOTOGRAPHY 

Following construction and test of the rapid 
processing equipment, a study was undertaken 
to find some means of improving the quality of 
submarine periscope photography. 

A number of pictures were made aboard the 


5. 1 Viewer 

6. 2 Extra lamps for the viewer 

7. 1 Extra starter for the lamps 

8. 1 Carrying case 

See Figures 1-9 for views of the rapid proc¬ 
essing equipment. 


Figure 3 


Figure 2 


Figure 4 







574 


RAPID PROCESSING EQUIPMENT FOR PERISCOPE PHOTOGRAPHY 




Figure 5 


Figure 7 


USS Pilotfish, but with poor results because of 
weather conditions. Another series of photo¬ 
graphs were made through a Kollmorgen peri¬ 
scope, permanently mounted in the New Con¬ 
struction Training School at New London. A 
third series of photographs was made using a 
new Kollmorgen periscope with a 1.414-in. en¬ 
trance window, mounted vertically in the Opti¬ 
cal Shop and aimed across the Thames River. 



In this latter work a resolution chart prepared 
by NDRC was used for quantitative judgment 
of the negatives. The following are the princi¬ 
pal factors determining the quality of periscope 
photography. 

1. Type of target. 

2. Weather conditions. 

3. Conditions at periscope entrance window. 

4. The motion of the ship. 

5. The optical system of the periscope. 

6. The camera and taking lens. 

7. The film and its treatment. 

Of these seven points, the most important 



Figure 6 


Figure 8 






PERISCOPE PHOTOGRAPHY 


575 


was found to be the optical system of the peri¬ 
scope. Observed resolutions in the first tests 
amounted to only 16 lines per mm, compared 
to the 45 or 50 lines per mm obtainable with a 
variety of fast films. Efforts were made to trace 
the contributions made by each factor, although 
all work was near the optical axis. 

A study of the optical performance of the 
periscope indicated that its visual correction 



I 


Figure 9 

for color aberrations was a predominating 
source of trouble photographically. The films 
used in the tests had been of B and C sensitiz¬ 
ing and were specially sensitive to blue light. 
Moreover, the work was carried out near the 
surface of water, a notable scatterer of blue 
light. The curvature of field present in the 
periscope was also a source of trouble. Whereas 
the eye overcame the curvature by accommoda¬ 
tion, the attached camera had to retain a flat 
image plane. It was considered that a special 
lens with overcorrected curvature might be 
used, but it was not felt to be a practicable 
solution. 

A number of filter combinations were tried 
in special tests. Table 1 gives the chief results. 
From these tests the No. 12 Wratten filter 
seemed to give the best overall compromise. 
Thereafter, tests were made to determine the 
best exposure and the best camera lens to use 
with the periscope. The longer focal lengths 


gave longer exposures, because of the fixed exit 
pupil diameter. The following Ektar camera 
lenses were used. 


50 

mm 

//12 

00 

mm 

f/22 

153 

mm 

f/38 


Thus, the 90-mm lens requires about four 
times and the 153-mm lens about nine times 
the exposure required for the 50-mm lens. On 
the basis of exposure time, which because of 
vibration and movement had to be shorter than 


Table 1 . Resolution measures with submarine 
periscope. 


Filter 

Exposure 

factor 

Resolving 
power 
(lines 
per mm) 

Best focal 
setting 
(ocular 
diopter) 

C-5 

5 

16 

1 

61 

7 

23 

0 

16 

3 

19 

0 

4 

1.5 

16 


12 

2 

19 


16 

3 

19 


58 

6 

19 



V 5 o sec, the longer focal lengths are undesirable 
and even unusable under average conditions. 
For general purposes it is believed that the 
50-mm lens is the most satisfactory, at least 
until a more light-efficient periscope is avail¬ 
able. 

There is a definite advantage to be gained 
from the longer focal lengths in respect of defi¬ 
nition. The following table shows the observed 


resolution. 

50-mm No. 12 

19 lines per mm 

4 targets resolved 

61 

23 “ “ “ 

5 

90-mm No. 12 

18 “ “ 

6 

61 

18 “ 

6 “ “ 

153-mm No. 12 

20 “ “ “ 

rj u a 

61 

20 “ “ 

7 “ 


At the time these exposures were made, visual 
observations were able to resolve only target 8, 
but with great difficulty. The nearly constant 
linear resolution should be noted. 

Entrance Window Condition 

Tests were made to determine the effect of 
the condition of the periscope entrance win¬ 
dow. A 50-mm Ektar lens in an Ektar camera 










576 


RAPID PROCESSING EQUIPMENT FOR PERISCOPE PHOTOGRAPHY 


was exposed on a resolution chart in the lab¬ 
oratory through a piece of grade B polished 
plate. Through a wet glass the resolution was 
variable, but generally very poor. Through the 
same piece of glass treated with an antiwetting 
agent, and then wetted, the resolution observed 
amounted to 50 lines per mm. A General Elec¬ 
tric product called Dri-film was used and, in¬ 
deed, seems a suitable treatment for periscope 
windows. 

Visual Examination of the 
Periscope Color Characteristics 

Visual tests were made by attaching an aux¬ 
iliary 3X telescope to the periscope. The color 
curve of the auxiliary telescope was judged to 
be negligible in separate tests. 

With the telescope ocular set at zero diopters, 
the periscope was focused without filters by 
examination of the line targets in the daytime 
and of a distant light source at night. All suc¬ 
ceeding changes of focus were made by vary¬ 
ing the ocular setting of the telescope. 

Mean diopter settings of the telescope appear 
in Table 2. 

Table 2. Visual measures of focus of periscope at 

several wavelengths. 


U.S. Periscope 92KA40T/1.4HA No. 1341 
4,400 A 5,300 A 6,200 A No filter 


daylight 


—0.25 

—0.40 

—0.21 



—0.14 

—0.24 

—0.19 

night 

—0.64 

0.06 

—0.07 



—0.54 

0.07 

—0.07 



The report 1 states that when the exit pupil of 
the periscope was reduced to 2.2 mm and 1.6 
mm by diaphragms, the image quality was im¬ 
proved markedly. It is concluded that the de¬ 
parture of the blue focus from the red and 
green foci undoubtedly contributes to the poor 


photographic results obtained with this peri¬ 
scope without filter. 


18 4 RECOMMENDATIONS BY NDRC 

1. Resolution tests should be made on distant 
targets, with the periscope mounted in an opti¬ 
cal shop on shore, to determine the resolving 
power at various focal settings, using the 
standard camera and also cameras of several 
different focal lengths, with and without filters. 
The resolving power when photographs are 
taken through the periscope should be compared 
with the resolving power of the same camera 
and film, on the same targets, when the camera 
is used alone. The resolving power should be 
measured at various angular distances from 
the center of the field. 

2. The glass-fluorite folded collimator (see 
Chapter 7) should be used for these tests with 
test targets at the focal plane if it can be made 
available. It was lost, at least temporarily, in 
the course of shipment from the Massachusetts 
Institute of Technology to the National Bureau 
of Standards. This collimator was made spe¬ 
cifically for testing the photographic and visual 
performance of Navy periscopes. 

3. The suggestions for improvements in peri¬ 
scope and camera design, which are outlined in 
Chapter 10, should be carried out. 

4. Resolution tests should be made by photo¬ 
graphing through a periscope with a submarine 
under way, under various conditions of roll and 
pitch, using resolution targets on a ship at a 
distance of about one-half mile. 

5. The effect of wetting the periscope window 
with water should be determined by photo¬ 
graphing through the instrument with the win¬ 
dow wet and dry. If necessary, steps should be 
taken to remedy whatever loss of resolution is 
found to exist by using antiwetting agents. 









Chapter 19 

TWO-STAR NAVIGATING DEVICE 

1 

By Theodore Dunham , Jr. a 


U NTIL THE DEVELOPMENT of Loran and other 
radio methods of navigation, the quick de¬ 
termination of longitude and latitude of an 
aircraft from observations of stars was of the 
utmost importance. Navigation tables and the 
Astrograph have greatly reduced the time and 
effort required to find the observer’s position 
from observations of two stars. Nevertheless, an 
instrument which would display directly the 
position of the observer was much to be desired. 

Previous attempts have been made to develop 
a two-star navigating device which will display 
position directly, but in all such instruments it 
has been necessary to hold the images of two 
stars simultaneously on a reticle mark. This has 
required guiding in three coordinates simul¬ 
taneously, which is more than any observer can 
expect to do in an airplane which is always 
undergoing erratic excursions in roll, pitch, and 
yaw. 


191 PRINCIPLE OF OPERATION 

The Mount Wilson Observatory 1 developed a 
two-star navigating device which eliminates to 
a large degree the need for guiding in one co¬ 
ordinate. This was accomplished by providing 
one degree of freedom as a result of astigmatiz- 
ing the two star images horizontally through 
the use of cylindrical lenses in front of each of 
the two telescope objectives. 

A direct display of direction and distance to 
a selected target is achieved by bringing into 
the focal plane of the eyepiece of the instru¬ 
ment an image of a spherical bubble as well as 
the two star images. The instrument is set in 
advance of take-off so that, if it were at the 
target, the two star images would coincide on 
the reticle mark, with the bubble also on the 
reticle mark. A clock drive maintains the setting 
for the target by counteracting the apparent 
motion of the stars. For any location of the air- 

a Chief, Section 16.1, NDRC. 


craft other than over the target, the direction 
and distance of the bubble from the center of 
the reticle indicates the course and distance to 
the target, provided the two stars are held co¬ 
incident on the center of the reticle. 

19 2 DESIGN AND CONSTRUCTION 

Figures 1 and 2 show a photograph of the 
complete instrument and a diagrammatic view 
of the optical and mechanical parts. The axis 
on which the instrument is turned by clockwork 
is set parallel to the earth’s axis. Light from 
the two stars is combined and carried down the 
axis to an objective, right-angle prism, and eye¬ 
piece. Light from star A enters the instrument 
through an astigmatizing cylindrical lens and 
is directed down the polar axis by a pentareflec- 
tor which is designed to produce a deviation 
equal to 90 degrees plus the declination of the 
star. Light from star B also enters through an 
astigmatizing cylindrical lens and is directed 
down the polar axis by a pentareflector designed 
for its declination. The pentareflector for star B 
has a hole cut in it which allows a beam from 
star A to pass through it which has a diameter 
equal to half the aperture of the objective. Thus 
the beams from the two stars are combined 
and focused by the objective on the reticle. 
Pentareflectors are used to avoid the need for 
precise setting of the reflector units. The pen¬ 
tareflectors are set on the polar axis with an 
included angle between them equal to the differ¬ 
ence of the right ascension of the two stars. A 
separate pentareflector is made for each star. 
In a final model, each pentareflector would be 
located with dowel pins. 

A Dove prism has been included in the proto¬ 
type model, to rotate the astigmatized image 
of star A for an alternative method of using the 
instrument, but this is not needed. 

A clock rotates the entire unit about the 
polar axis once in 24 hours through a system of 
steel belts. 


577 



578 


TWO-STAR NAVIGATING DEVICE 


The level bubble is illuminated at the focus 
of a telephoto system which sends a parallel 
beam into the instrument through the trans¬ 
verse axis by means of two right-angle prisms. 
The bubble and optical system are mounted as 
a unit and rotate about the transverse axis. The 


provides leveling (guiding) about the N-S axis. 
The entire instrument rotates in a ring within 
the base for setting in azimuth. 

The cylindrical lenses are set with their axes 
nearly horizontal, with the aid of a fiducial 
mark which is set opposite a small steel ball 



Figure 1 . Photograph of two-star navigating instrument. 


polar axis is rotated about this transverse axis 
until it makes an angle relative to the plane de¬ 
fined by the bubble which is equal to the latitude 
of the target. A vernier sector permits setting 
to 1 min of arc. A fine screw attached to this 
sector provides leveling (guiding) about the 
E-W axis. A fine screw attached to the base 


which always rolls to the bottom of a circular 
track. The two star images are, therefore, astig- 
matized horizontally, with the result that the 
point of intersection of the two lines, into which 
the point images are converted, is extremely 
insensitive to the setting of the instrument in 
azimuth. It is this feature of the instrument 




DESIGN AND CONSTRUCTION 


579 


which makes it likely that it can be operated by noting the position angle of the bubble and 
satisfactorily by an observer, since he would its distance from the center of the reticle, 
be required to maintain the setting of the in- Obviously, the reticle could be replaced by a 


LIGHT PROM 
STAR "A" 


MIRRORS 

LATITUDE SECTOR 
LATITUDE VERNIER 


ARM OF LEVEL 
BRACKET 


RIGHT ASCENSION 
SLOW MOTION 


LONGITUDE CIRCLE 

FLOATING VERNIER CIRCLE SET TO 
READ RIGHT ASCENSION OF STAR"b" 
ON RIGHT ASCENSION CIRCLE 


CIRCLE FOR SETTING DOVE PRISM 



RETICLE 


POLAR AXIS 
ALTITUDE 
ADJUSTMENT 
(NORTH-SOUTH LEVELING) 


MAIN FRAME 


CLOCK 

AZIMUTH ADJUSTMENT 


ROTATING 
BASE PLATE 


Figure 2. Arrangement of optical and mechanical parts. 


strument in only two coordinates by guiding 
with the leveling screws. 

Figure 3 shows the display in the field. The 
intersection of the two star line images is held 
at the center of the reticle (method A). The 
course and distance to the target are observed 


transparent map, illuminated with faint red 
light, on which the bubble would indicate di¬ 
rectly the position of the aircraft. 

The same instrument may be used in a differ¬ 
ent way, namely, guiding in level to hold the 
spherical bubble at the center of the reticle and 



























580 


TWO-STAR NAVIGATING DEVICE 


noting the location on the reticle of the inter¬ 
section of the star image lines (Figure 3, method 
B). In either case some averaging will be re¬ 
quired because linear accelerations of the air¬ 
craft will displace the bubble. For this reason 
its period should be as long as is consistent with 
free motion. 

The most attractive method for operation 




_ ASSUMED CORRECT POSITIONS OF STAR IMAGES 

-RESULT OF TURNING DOVE PRISM 

Figure 3. Display of reticle, star image lines, and 
bubble. 

would obviously be to stabilize the base of the 
instrument so that it would be held always level. 
The bubble could be dispensed with if this were 
done, and it would then merely be necessary to 
observe the position where the star lines inter¬ 
sect on the reticle. This would involve much 
more mechanism, however, and would eliminate 
the advantage of fundamental simplicity and in¬ 
dependence of other equipment which the pres¬ 
ent instrument possesses. 


193 TESTS 

Qualitative tests show that stars of moderate 
brightness can be picked up and that the astig- 
matized image lines are bright enough for easy 
setting on the reticle. When the images are 
astigmatized horizontally, the desired result is 
achieved, namely, the intersection of the two 
images is not displaced on the reticle when the 
instrument is rotated through small angles in 
azimuth. The observer soon learns how to set 
this intersection at the center of the reticle by 
manipulating the adjusting screws, although 
the direction in which motion is required is 
often quite unexpected. 

It was not possible to conduct tests in an air¬ 
plane, but it seems clear that the device would 
operate as expected. 

The prototype model has been transferred to 
the Bureau of Aeronautics. 


194 CONCLUSIONS 

The present model demonstrates the feasibil¬ 
ity of constructing a two-star navigating in¬ 
strument with astigmatized images. The parts 
of any future model should be made stronger to 
reduce flexure. The design could probably be 
considerably simplified by eliminating some of 
the adjustments on individual optical units 
which are included in the present instrument. 

For the quickest and most convenient opera¬ 
tion, the instrument should be stabilized so as 
to hold its base level within the desired accuracy 
for navigation. The observer would then be re¬ 
quired to guide only very approximately in 
azimuth and to note his position by merely 
noting where the two star image lines intersect 
on a transparent map illuminated with red light. 
Some averaging would be required due to linear 
accelerations of the aircraft, but probably less 
than when a relatively short period bubble must 
be averaged. 

19 5 RECOMMENDATIONS BY NDRC 

1. The design of the navigating instrument 
should be simplified and greater rigidity should 
be provided. 













RECOMMENDATIONS BY NDRC 


581 


2. Tests of the present instrument should be 
made in an airplane, using the method in which 
the star image lines are held on the reticle while 
the position of the spherical bubble is read to 
give the position of the aircraft. 


3. The instrument should also be tested on a 
stabilized platform, so that the observer can 
read his position directly from the position of 
the intersection of the st^r lines on the reticle, 
without the use of a bubble. 




APPENDIX 


I. Chapter 1 


Table 1. List of lenses tested at Wright Field. 


Focal 

Length 

Relative 

Aperture 

Manufacturer 

Lens Name 

Remarks 

3 in. 

//6.3 

Bausch and Lomb 

Metrogon 

For T-7 camera. Not standard. 

4 in. 

//2.8 

Harvard College Obs. 

Spherical Wide- 
Angle 

120° Wide-angle. Experimental. 

5-6 in. 

//2.5 

Harvard College Obs. 


Ophthalmic glass lens. Small scale prototype 
of 7-in. //2.5 for K-24 camera. Experimental. 

6 in. 

//2.8 

Eastman 

Aero-Ektar 

For K-24 camera. Experimental. 

6 in. 

//6.3 

Bausch and Lomb 

Metrogon 

For K-17, K-22, T-5, S-7. Standard. 

6% in. 

//4.5 

Wollensak 


For K-24, K-20, K-25 cameras. Experi¬ 
mental. 

6% in. 

//4.5 

Eastman 

Anastigmat 

For K-20, K-24, and K-25 cameras. Standard. 

6% in. 

//4.5 

Bausch and Lomb 


For K-20, K-24, and K-25 cameras. Experi¬ 
mental. 

6.7 in. 

//2.6 

Harvard College Obs. 


Ophthalmic glass lens. For K-21, K-24 cam¬ 
eras. Experimental. 

7 in. 

// 2.3 

Harvard College Obs. 


Ophthalmic glass lens. For K-21, K-24 cam¬ 
eras. Experimental. 

7 in. 

// 2.5 

Harvard College Obs. 


For K-21, K-24 cameras. Experimental. 

7 in. 

// 2.5 

Polaroid 


Plastic lens for K-21, K-24 cameras. Ex¬ 
perimental. 

7 in. 

//2.5 

Eastman 

Aero-Ektar 

Aerostigmat 

For K-21 and K-24 cameras. Standard. 

7.1 in. 

//2.8 

Curtis 


Plastic lens for K-21 and K-24 cameras. Ex¬ 
perimental. 

7.6 in. 

//4.5 

Bausch and Lomb 


For K-21 and K-24 cameras. Experimental. 

8 in. 

//1.5 

Eastman 

Aerostigmat 

For K-24 camera. Experimental. 

8 in. 

//2.7 

Bausch and Lomb 

Anastigmat 

For K-21, K-24 cameras. Experimental. 

8 in. 

// 2.8 

Bausch and Lomb 

Anastigmat 

For K-21, K-24 cameras. Experimental. 

8 in. 

//3.2 

Curtis 


Plastic lens for K-21, K-24 cameras. Experi¬ 
mental. 

12 in. 

//2.5 

Eastman 

Aero-Ektar 

For K-19 camera. Standard. 

12 in. 

//2.5 

Bausch and Lomb 

Anastigmat 

For K-19 camera. Experimental. 

12 in. 

//4 

Bausch and Lomb 


For K-22 camera. Experimental. 

12 in. 

//4.3 

Curtis 


Plastic lens. Experimental. 

12 in. 

//5 

Eastman 

Aerostigmat 

Anastigmat 

For K-17, K-22 and K-24 cameras. Standard. 

12 in. 

//6.3 

Bausch and Lomb 

Metrogon 

For K-27 camera. Standard. 

13 V 2 in. 

//3.5 

Eastman 

Aero-Ektar 

For K-19 camera. Not standard. 

15 in. 

//5.6 

Eastman 

Telephoto 

For K-21, K-24 cameras. Experimental. 

20 in. 

//5.6 

Bausch and Lomb 

Telephoto 

Anastigmat 

For K-24 camera. Standard. 

24 in. 

//5.6 

Eastman 

Telephoto 

For K-24 camera. Experimental. 

24 in. 

//6 

Bell and Howell 

Aerotar 

For K-17, K-22 cameras. Experimental. 

24 in. 

//6 

General Scientific 

Aero-Scienar 

For K-17, K-22 cameras. Experimental. 

24 in. 

//6 

Perfex Corp. 


For K-17, K-22 cameras. Experimental. 

24 in. 

//6 

Bausch and Lomb 

Aero Tessar 

For K-17, K-18, K-22 cameras. Standard. 

24 in. 

//6 

Eastman 

Aero-Ektar 

For K-17, K-18, K-22 cameras. Standard. 

36 in. 

//8 

Harvard College Obs. 

Fluorite 

For K-22 camera. Experimental. 

36 in. 

//8 

Harvard College Obs. 

Telephoto 

For K-17, K-18, K-22 cameras. Experi¬ 
mental. 

36 in. 

//8 

Eastman 

Telephoto 

For K-17, K-18, K-22 cameras. Experi¬ 
mental. 

40 in. 

// 5 

Harvard College Obs. 

Telephoto 

For K-22 camera. Standard. Pressure com¬ 
pensated and temperature controlled. 

40 in. 

//5.6 

Eastman 

Telephoto 

For K-22 camera. Experimental. 

40 in. 

// 5.6 

Bausch and Lomb 


For K-22 camera. Standard. 

40 in. 

//8 

Bausch and Lomb 

Telestigmat 

For K-15, K-17, K-22 cameras. Standard. 

48 in. 

//6.3 

Bausch and Lomb 

Telephoto 

For K-22, K-32 cameras. Experimental. 

48 in. 

//6.3 

Eastman 

Telephoto 

For K-22, K-32 cameras. Standard. 

60 in. 

//6 

Harvard College Obs. 


For K-22 camera. Experimental. 


582 









APPENDIX 


583 


Table 2. List of lenses tested at R.A.E. 


Focal length //No. Name Remarks 

0.5 in. //3.5* Dallmeyer Reversed Telephoto 


0.54 in. 

//6 

Dallmeyer Anastigmat 

1 in. 

//2.5 

Wray Lustrar 

1 in. 

// 2.5 

Ross 

1 in. 

// 2.5 

Dallmeyer 

1 in. 

//2.8 

Taylor, Taylor, and Hobson 

1 in. 

// 3.5+ 

Ross 

1 in. 

// 3.5+ 

Dallmeyer 

1 in. 

// 3.5+ 

Wray 

iy 2 in. 

//1.9 

TTH 

35 mm 

//3.5 

Wray 

2 in. 

f/1* 

Wray 

2 in. 

//3.5f 

Dallmeyer 

2 in. 

//3.5f 

Kodak 

2 in. 

//3.5+ 

N.O.C. 

3 in. 

// 3.5 

Wray 

3 in. (75 mm) 

// 3.5 

Ross Tessar 

3% in. 

//5.5* 

Ross W. A. Survey 

3.7 in. (93 mm) 

//4.5 

Ensign Ensar 

4 in. 

f/ 4.5 

Dallmeyer 

4 in. 

if/ 1.5 

Kodak (J. L. Houghton) 

5 in. 

// 2.9 

Dallmeyer Pentac 

5 in. 

//4+ 

Ross W. A. Xpres 

5 in. 

ft 5.6 

Ross W. A. Xpres E.M.I. 

5 in. 

// 5.5 

Ross W. A. Xpres E.M.I. 

5 in. 

//4.5 

Ross W. A. Xpres 

5 in. 

// 4.5f 

Wray 

5 in. 

//4f 

Ross 

5 in. 

//3.5 

TTH Series Ha 

5% in. 

// 2.5 

TTH Series X 

5% in. 

//6.3 

Thompson-Courtauld 

5% in. 

//6.3 

Dallmeyer 

6 in. 

// 2 

TTH (5 component) 

6 in. 

// 2 

TTH (4 component) 

6 in. 

//5.5 

Ross W. A. Survey 

7 in. 

//4 

Ross Xpres 

8 in. 

//1.5 

Kodak 

8 in. 

//5.6f 

TTH Aviar 

8 in. 

f/ 1.9 

Dallmeyer 

8 in. 

f/ 2.9 

Dallmeyer Pentac 

8 in. 

//4 

Dallmeyer Pentac 

8 in. 

// 2.9 

Dallmeyer Pentac 

8% in. 

//4 

Ross Xpres 

9 in. 

//2.5 

TTH 

9% in. 

//6.3 

Wray Lustrar 

10 in. 

//4 

Ross Xpres 

10 in. 

f/8 

Aldis Anastigmat 

10 in. 

// 6.3 

Ross W. A. Xpres 

10 in. 

//1.6 

TTH 

IOV2 in. 

//6 

TTH Aviar 

12 in. 

//7.7 

Dallmeyer Dallon 

12% in. 

//1.5 

TTH 

12% in. 

//2 

TTH 

14 in. 

//1.8 

TTH 

14 in. 

//4.5f 

Dallmeyer Serrac 

14 in. 

// 5.6 

Dallmeyer Serrac 

14 in. 

//5.6f 

Dallmeyer Anastigmat 

14 in. 

//5.6f 

TTH Aviar 

20 in. 

// 5.6 

Aldis Triplet 


Instrument recording and cihe work. British Patent 
572086. 

Recording of instruments. 

a u u 

a a ii 

ii ii ii 

U ii H 

a a a 

a a a 

a a a 

C. R. T. recording. 

Instrumentation. 

C. R. T. recording. 

For G.45 camera gun (Ph. 303, 246, and 228). 

« a y a «« u « << << 

u a a .< « « a (( «< 

Instrument and C. R. T. recording. 

Instrument recording. 

Air Survey lens (5"x5") 190/H1237. 

For enlarger type D. 

For enlarger type D. 

Experimental for night work 190/H1237. 

Hand-held and twin-mirror cameras. 

For F.24 camera. 

Experimental for F.24. Not adopted (Ph. 137). 
Experimental for F.24. Not adopted (Ph. 137). 
Experimental for F.46. Not adopted (Ph. 250). 

For F.46 camera (Ph. 250). 

Special design for F.46 camera. 

Experimental for hand-held camera. Not accepted. 

ii ii ii ii ii ii 

Experimental for F.24. Not accepted. (Ph. 125). 
Process lens. 

Experimental lenses for night photography. Not 
adopted (PRC.37/44). 

Experimental lenses for night photography. Not 
adopted (PRC.37/44). 

Air Survey lens for 9x9 in. (various NPL reports). 
For 7x7 in. 

For night photography (114/H1055 and Ph.341). 

For 5x5 in. (F.24) (38/H881). 

Experimental for night use. Not adopted. 

F.24 camera (day and night) 2/H697 and A2/H882. 
Experimental. Produced in small quantities. 

With rare-earth glass components (Ph.257). 

For 7x7-in. air camera and process work. 
Experimental for night use. Not adopted (PRC.37/44). 
Experimental for F.24. Not adopted. 

For 7x7-in. (F.8) camera and process work. 

Process work. 

Experimental. Not adopted (Ph. 127). (For 7x7 in.) 
Experimental for night photography. Not adopted. 
Experimental for F.24 and F.8. Not accepted (Ph.136). 
For ground use (telephoto). 

Experimental for night use. Not adopted. 
Experimental for night use. Not adopted. 
Experimental for night use. Not adopted. 

For F.24 camera. 

For F.24 (experimental). Produced in small numbers. 
For F.8 (7x7 in.). 

For 9x9 in. and F.52. 38/H881. 

39/H.884 (for 9x9 in., but used on 5x5 in.). 









584 


APPENDIX 


Table 2. ( Continued) 


Focal length 


//No. 


Name 


Remarks 


20 in. 


//6.3 Dallmeyer Anastigmat 


20 in. 

//6.3f 

Ross Survey E.M.I. 

20 in. 

//6.3 

Cooke Aviar 

20 in. 

f/e. 3 

Ross Survey E.M.I. 

20 in. 

//5.6f 

Cooke Telephoto 

20 in. 

//5.6+ 

Dallmeyer Dallon 

20 in. 

//6.3f 

Ross Telephoto 

20 in. 

f/e. 3* 

Ross Xpres 

20 in. 

f/e. 3 

Ross Astro Aero 

20 in. 

f/e. 3 

Kodak Triplet 

20 in. 

//5.6f 

TTH Aviar (1918) 

20 in. 

// 5.6+ 

TTH Aviar 

20 in. 

//5.6f 

TTH Aviar 

20 in. 

f/e 

TTH Design 287:903:11 

20 in. 

// 8 

TTH Design 327:112 

20 in. 

f/e 

TTH Design 327:323 

20 in. 

//9.5 

TTH No. 284084 

20 in. 

// 5.6 

Ross Aero 

20 in. 

//4.5 

TTH Aviar 

25 in. 

f/e. 3* 

Ross Xpres 

30 in. 

f/e. 3 

Ross Astro Aero 

36 in. 

//6.3+ 

Booth Telephoto (Dallmeyer) 

36 in. 

//6.3+ 

Booth Telephoto (TTH) 

36 in. 

f/e. 3+ 

Booth Telephoto (Ross) 

36 in. 

f/e. 3f 

Booth Telephoto (Canadian) 

36 in. 

f/e. 3 

Ceilar Telephoto (Australian) 

36 in. 

f/e. 3* 

Wray Telephoto 

36 in. 

f/e. 3 

Ross Telephoto 

36 in. 

f/e. 3 

Booth Telephoto (figured) 

40 in. 

f/s 

Dallmeyer Dallon 

50 in. 

f/e* 

Ross Telephoto 

56 in. 

f/s 

Ross Telephoto 

56 in. 

f/s 

Cooke Telephoto 

32 cm 

f/S.5 

Xenar 

7 in. 

// 2.5 

Kodak Ektar 

20 cm 

f/e. 3 

Zeiss Topogon 

75 cm 

f/e. 3 

Zeiss Telikon 

40 in. 

f/s 

Bausch and Lomb Telephoto 


For 9x9 in. Experimental, not adopted (109/H1039 
and Ph. 134). 

Standard lens for F.52 (109/H.1039 and Ph. 135 and 
others). 

Experimental for 9x9 in. Not adopted (Ph.133). 
Redesigned (Calc. Ill) (Ph.308). 

For F.24 camera. Standard telephoto. 

For F.24 camera. Standard telephoto. 

For F.24 camera. Standard telephoto. 

Successive prototype for F.52 (161/H.1261, Ph.252, 
Ph.296, Ph.288). 

Prototype for F.52 (Ph.261). 

Prototype for F.24. Not adopted (35/H.885). 
Standard for F.52 camera (26/H.852, 44/H906 
112/H.1061, and Ph.305). 

Production model in early part of World War II 
(Ph.305). 

Redesigned production type (later in World War II) 
Ph.305. 

Prototype for F.52 (Ph. 258). Not adopted. 
Experimental for F.52. Not adopted (Ph.297). 
Experimental for F.52. Not adopted (Ph.297). 
Experimental for F.52. Not adopted (Ph.297). 

World War I lens, used in F.24 camera. 

1918 lens for 9x9 in. 

Twelve produced (Ph.335 and Ph.343). 

Experimental for F.52. Some produced (Ph.166, 
98/A1012). 

Standard for F.52 (Ph.289 and Ph.317). 

Standard for F.52 (Ph.289 and Ph.260). 

Standard for F.52 (Ph.289 and Ph.325). 

Standard for F.52 (Ph.318). 

Similar to Booth Telephoto. Prototype (Ph.318). 
Prototype. Development of Booth (Ph.328). 
Experimental for F.52 (Ph.342). 

Various lenses figured by Hilger or Burch (Ph.260, 
Ph.326). 

Used in F.52 before production of Booth Telephoto. 
Experimental for F.52 (Ph.316 and Ph.343). 
Prototype for F.52. Not accepted (Ph.263). 
Commercial type tested for F.52 but not adopted. 
German lens tested in U.K. (Ph.334). 

Tested for F.24 camera (Ph.306). 

German W.A. Survey lens (Ph.299). 

German telephoto (Ph.302). 

Tested against 36-in. Booth (Ph.325). 


* Lenses generally produced only as prototypes or in small quantities during World War II, which show the greatest promise for the 
future. 

t The lenses found most useful and produced in large quantities. 












APPENDIX 


585 


I. Chapter 12 


Some of the more important characteristics 
of the optical systems of the OSRD sights are 
collected in Table 1 on page 582. Although the 
significance of most of the column heads is 
quite obvious, several need the explanations 
given below: 

Field: Radius of field in mils. If suitable only 
for reticle pattern of concentric 
circles with no radial lines, the letter 
r is appended. 

P 0 Parallactic range at center of field. 

P e Parallactic range at edge of field. 

Notes: May include any necessary remarks. 
In particular the type of lens system 
for lens sights is indicated. The num¬ 


ber of lenses js given by the initial 
numeral in parenthesis. 

Model: For sights of which models have been 
made,' the aperture is given by the 
initial number. The letter A or B in¬ 
dicates the type of model. Type A is a 
model designed to serve as a produc¬ 
tion prototype, built to be mounted 
and used under service conditions. 
Type B is a model in which the reticle 
and collimator are mounted in their 
correct relative positions for optical 
testing, with or without illuminator 
and reflex mirror. A 0 indicates that 
no model was made. 


Optical characteristics of reflex sights. 


Designation 

/-ratio 

Field 

Po 

Pe 

Notes 

Model 

Lens sights 

Yerkes L9b 

2.0 

250 r 

1.1 

3.0 

(2) Cemented 

3.5-in. B 

Yerkes L9k (T-95) 

2.0 

280r 

1.1 

3.2 

(2) Cemented 

2.75-in. A 

Mount Wilson Ross-2 

2.7 

250r 

3.1 

2.7 

(2) Separated 

3.75-in. B 

Rochester S-l 

2.5 

70r 

1.3 

2.0 

(2) Separated 

3.5-in. B 

Rochester S-2 

3.0 

150 

0.6 


(2) Separated 

3.5-in. A 

Rochester Flightsight 

3.5 

12.5 

0.2 

0.2 

(2) Separated 

3.5-in. A 

Rochester T-67 

3.0 

50 

1.0 


(2) Cemented 

40-mm A 

Yerkes T-28 

2.0 

21 Or 

0.7 

0.8 

(3) Cem. doub. and sing. el. 

0 

Yerkes T-32 

2.0 

250r 

0.9 

1.2 

(3) Cem. doub. and sing. el. 

3.5-in. B 

Mount Wilson Ross-3 

1.4 

250 r 

2.4 

4.2 

(3) Cem. doub. and sing. el. 

4.0-in. B 

Polaroid 

Yerkes P-7 

1.9 

2.4 

250 

0.5 

1.7 

(4) 1 Cem. doub. 

3.0-in. B 

0 

Yerkes UP-10 

2.4 

250 

0.6 

1.0 

1 Sep. doub. 

(4)2 Sep. doub. 

3.5-in. B 

Rochester T-94 

1.6 

280r 

0.4 

4.5 

(4) 2 Cem. doub. 

60-mm A 


2.0 

210r 

0.2 

3.9 

(4) 2 Cem. doub. 

50-mm A 

Polaroid 

1.6 

105 

0.3 

0.6 

(4) 2 Cem. doub. 

3.5-in. A 

Lens Mangin sights 

Yerkes LSM-4 

1.3 

210 

0.3 

0.9 


0 

Yerkes LSM-7 

1.3 

210 

0.3 

0.9 

CHM plastic 

0 

Yerkes LSM-10 

1.3 

210 

0.3 

0.9 


4x6-in. B 

Solid sights 

Mount Wilson Hayward S-l 

1.3 

160 

0.0 

3.1 

Mangin 

lxl-in. A 

Yerkes M-16 

1.3 

210 

0.0 

0.5 

Lens-Mangin 

0 

Rochester S-2 

1.7 

125r 

1.0 


Glass lens plastic prism 

3.5-in. A 

Double Mangin sight 

Yerkes DM 

1.0 

75 




0 

Schmidt sights 

Straight Schmidt 

0.7 

100 

0.0 

1.3 


0 

Mount Wilson Bowen sight 

2.0 

200 

1.0 

1.0 

Exit pupil 4.5 in. 

9x4.5-in. A 







586 


APPENDIX 


II. Chapter 12 

Specifications—lens sights. 



Component 

Radius Thickness Material Aperture 

Component 

Radius Thickness 

Material Aperture 

A. 

Yerkes L9B 

F — 4 in. 




G. Rochester T-67 F = 119.8 mm 



+2.491 





+72.58 





Lens 

0.657 

BSC-2 

2.00 

Lens 


9.0 

BSC-1 

40 


Cemented 

Lens 

—1.611 

0.160 

LF-3 

2.00 

Cemented 

Lens 

—57.08 

4.0 

EDF-2 

40 


—6.78 

3.486 




—152.13 

113.5 





Reticle 

—1.263 



1.79 

Reticle 





B. 

Yerkes L9k 

F = 4 in. 




H. Yerkes T-28 

F = 4 in. 






+2.519 





+5.61 





Lens 

Cemented 

—1.564 

0.650 

BSC-2 

2.00 

Lens 

—7.78 

0.253 

BSC-2 

2.00 




0.157 

LF-2 

2.00 

Air 


0.006 





—6.27 

3.529 



Lens 

+ 1.835 

0.479 

BSC-2 

2.00 


Reticle 

—1.257 



2.00 

Cemented 

Lens 

—5.75 

0.143 

EDF-1 

2.00 

C. 

Mount Wilson Ross 2 lens 

F — 

5.3 in. 



+3.281 






+3.402 






3.175 




Lens 


0.304 

DBC-1 

2.00 

Reticle 

—0.994 



1.48 


Lens 

Reticle 


—3.402 

—3.135 

— 12.6 

— 1.68 


0.025 

0.063 

4.862 


EDF-3 


Rochester S-l F = 222 

mm 

+ 121.3 


Lens 

22.0 

—121.3 


Air 

3.92 

—108.2 


Lens 

7.02 

—422.0 

198.1 

Reticle —80.2 



C-l 


DF-2 



I. Yerkes T-32 

F = 4 in. 




2.00 


+5.39 




Lens 

—7.69 

0.242 

BSC-2 

2.00 


2.38 

Air 


0.006 



+1.772 





Lens 


0.467 

BSC-2 

2.00 

89 

Cemented 

Lens 

—5.72 

0.137 

EDF-1 

2.00 


+3.17 





1.071 


Reticle 


-0.99 


89 


J. Mount Wilson Ross 3 lens F — 
+3.813 


E. Rochester S-2 F = 266.7 mm 


Lens 

Cemented 

Lens 


—2.073 


0.376 

0.093 


2.8 in. 
DBC-1 
EDF-3 


1.70 

2.00 

2.00 



+ 145.5 




Air 

—32.6 

0.111 



Lens 

19.7 

C-l 

89 

+1.925 



Air 

—145.5 

4.7 



Lens 

0.278 

DBC-1 

2.00 

Lens 

—130.6 

6.0 

DF-3 

89 


+7.77 

2.369 


1.48 

—511.1 



Reticle 

—0.907 





242.9 








Reticle 

—93.6 




K. Polaroid f/1 

.9 F = 145 

mm 








+87.69 




Rochester 

Flightsight 

F = 305 

mm 


Lens 


21.0 

CHM 

76.20 


+166.5 




Cemented 

—83.90 




Lens 

—166.5 

30.2 

C-l 

89 

Lens 

+439.4 

5.07 

Styrene 

76.20 

Air 

—148.6 

5.38 



Air 

+83.20 

0.43 



Lens 

—579.4 

9.64 

DF-2 

89 

Lens 

+ 203.9 

10.28 

CHM 

76.20 



283.3 





120.0 



Reticle 

00 




Reticle 

.. . 















APPENDIX 


587 


Component Radius Thickness Material Aperture 
L. Yerkes P-7 F = 4.8 in. 



+2.661 




Lens 

Cemented 

—2.661 

0.444 

BSC-2 

2.00 

Lens 

00 

0.194 

DBF-1 

2.00 

Air 

+2.758 

1.332 



Lens 

+1.828 

0.194 

DBF-1 

2.00 

Air 

+2.852 

0.259 



Lens 

—41.1 

0.254 

BSC-2 

2.00 



2.828 



Reticle 

—7.03 



2.40 

Yerkes UP-10 

F rr: 4.8 
+2.908 

in. 



Lens 

—2.736 

0.421 

BSC-2 

2.00 

Air 

—2.581 

0.028 



Lens 

—19.85 

0.139 

DF-2 

2.00 

Aii- 

+4.000 

1.382 



Lens 

+2.286 

0.167 

DF-2 

2.00 

Air 

+5.08 

0.137 



Lens 

—5.08 

0.249 

BSC-2 

2.00 



2.886 



Reticle 

—5.76 



2.40 

Rochester T-94 F = 102.33 mm 




+82.8 




Lens 

Cemented 

—90.3 

14.0 

BSC-1 

60 or 50 

Lens 

00 

2.0 

DF-3 

60 or 50 

Aii- 

+ 82.8 

0.0 



Lens 

Cemented 

—90.3 

14.0 

BSC-1 

60 or 50 

Lens 

00 

2.0 

DF-3 

60 or 50 



84.3 



Reticle 

—32.0 




Polaroid //1.6 

F = 142 
+272.38 

mm 



Lens 

Cemented - 

—111.82 

23.89 

CHM 

88.9 

Lens 


7.28 

Styrene 

88.9 


—261.17 




Air 

+72.79 

0.00 



Lens 


30.07 

CHM 

88.9 

Cemented 

Lens 

—165.68 

10.26 

Styrene 

88.9 


+160.32 

100.06 



Reticle 





Yerkes LSM-4 No data available. 



Component Radius Thickness Material Aperture 


Yerkes LSM-7 

F — 3.9 in. Aperture 2.67x3.33 in. 

Radii 

Separations 

1. 4.60 

AB 0.15 

2. 11.2 

BC 1.10 CD 1.59 

3. 3.77 

DE 0.12 

4. 6.20 

CF 1.62 

Reticle 3.23 

Diameter 1.73 

Material CHM for all elements except inner mirror 
Mirror plate glass 0.04 thick. 

Yerkes LSM-10 

F — 3.9 in. Aperture 2x3 in. 

Radii 

Separations 

1. 4.44 

AB 0.19 

2. 9.98 

BC 1.35 CD 1.38 

3. 3.70 

DE 0.25 

4. 6.11 

CF 1.64 

Reticle 

3.33 Diameter 1.2 


Inner mirror glass 0.12 thick 
Material BSC-2 glass 

S. Mount Wilson Hayward sight S-l F = 1.89 in. in 
glass Aperture lxl 

Radii Separations 

Reticle 1.305 

AB 1.2565 

1. 5.7 

BC 0.6 

2. 3.2 

CD 0.022 

Reticle diameter 0.52 
Material BSC-1 


Yerkes M-16 F = 4 

in. in glass. Aperture 2x2 

Radii 

Separations and Material 

Reticle 3.26 


1. 17.4 

AB 2.40 BC 1.00 CHM 

2. 7.76 

CD 0.004 air 

3. 18.4 

DE 0.172 Styrene 

4. 5.69 

EF 0.291 air 

Reticle diameter 1.12 

Rochester S-3 F = 135.3 mm 

Component Radius 

Thickness Material Aperture 

+166.7 


Lens 

7.0 C-l 89 

00 

Air 

0.0 

+114.6 

Plastic Prism 

198 Plastic nn = 1.495 89 

Reticle 



V. Double Mangin sight no data. 

W. Straight Schmidt sight no data. 

X. Mount Wilson Bowen sight F = 4.474 in. 

Radius Separation 

Primary mirror 9.0 

4.526 

Reticle 4.474 

y. Eastman Kodak Fly’s-Eye sight See OSRD Re¬ 
port No. 6281. 









588 


APPENDIX 


III. Chapter 12 

The following diagrams are included to show great accuracy in drawing has been made, and 
the general forms of all the collimators listed several instances will be found where one draw- 
in Appendix II of Chapter 12. No attempt at ing represents several related collimators. 


TWO LENS TYPES 


[ 

A [7 

r\ 

li J i 

11 

J lA/ 


YERKES 
A L9b 
B L9k(T-95) 


MOUNT WILSON 
ROSS 

C 2 LENS 


ROCHESTER ROCHESTER 

D S-l G T —67 

E S-2 

F FLIGHT SIGHT 


THREE LENS TYPES 



YERKES 
H T- 28 
I T-32 


MOUNT WILSON 
ROSS 

J 3 LENS 
K POLAROID f/1.9 


FOUR 




7 \ 

1 

1 

1 / 


YERKES 
L P-7 


LENS TYPES 



M up-io 







































APPENDIX 


589 



LENS MANGIN 


A 



P LSM 4 
Q LSM 7 
R LSM 10 



YERKES 
























































590 


APPENDIX 



SOLID SIGHT 



ILLUMINATING 

SURFACE 


CORRECTOR 

PLATE 



I 


RETICLE 


RETICLE 



MOUNT WILSON 
Y BOWEN SIGHT 


X STRAIGHT SCHMIDT 




















GLOSSARY 


Aberration. In optics the failure of rays of light to 
converge to a point focus, measured by the departure 
in angular measure, or in linear measure in the focal 
plane, of the ray in question from the ideal point. 

Achromatic. Corrected, as well as possible, for color. 

AI Equipment. Airborne radar used in interceptor 
planes. 

Alicyclic Rings. A name applied to compounds con¬ 
taining a ring of carbon atoms but not belonging to 
the aromatic series. 

Altazimuth. A design for a mounting which employs 
two axes of rotation—one horizontal and the other 
vertical. 

Anastigmat. An optical system that is corrected over 
a wide angular field for astigmatism, and usually for 
curvature of field. The term is loosely used and has 
been applied to lenses of widely varying quality. 

Anti-Vibration. Capable of removing external vibra¬ 
tions from an isolated member. 

Anion. The product evolved at the anode in electrol¬ 
ysis. In general a negative ion. 

AOM. Anti-Oscillation mount. 

Aperture. The area of a lens through which the light 
may pass, or a diaphragm which limits the size of 
of a light beam in an optical instrument. 

Apochromatic. A type of color correction that com¬ 
bines three or more colors into a common focus, as 
contrasted with the usual achromatic correction for 
which two colors are combined. In its strictest sense, 
an apochromatic optical system is one corrected for 
secondary spectrum for three colors, and for spher¬ 
ical aberration and coma for two. The designation is 
often loosely used. 

Armorer. A technician trained in the adjustment of 
guns, turrets, and associated equipment. 

Aspheric Surface. An optical surface that departs 
from a spherical form. Optical instruments having 
rotational symmetry usually employ spherical sur¬ 
faces for production simplicity. 

Astigmatism. An off-axis aberration. Radial and tan¬ 
gential lines are in focus in different planes. 

Atomic Refractivity. The product of the specific re¬ 
fraction of an element by its atomic weight. 

Auxiliary Magnification. The magnification of an 
auxiliary telescope employed as an aid to the eye in 
the inspection of optical instruments. 

AVM. Anti-vibration mount. Often synonymous with 
anti-oscillation mount, although the latter usually 
pertains to angular vibration only. 

Baffle. A plate or wall to deflect, check or otherwise 
regulate flow. 

Ball-Cone Mount. A type of AOM developed at the 
Kodak Research Laboratories. 

Birefringence. The property of double refraction, 
which is said to be high or low according as the dif¬ 
ference between refractive indices is large or small. 

Bloom. A surface coating or appearance; a milky ap¬ 
pearance on the surface of glass. 


Blue Cane Glass. Calcium-sodium silicate glass in 
the form of rods, or “canes.” 

Bore-Sighting. Alignment of camera optical axis with 
that of the bore of the gun. 

Carboxyl Group. The group of most organic acids, 
such as formic, acetic, and benzoic, which have the 
univalent radical CCLH in common. 

Center-of-Gravity Mount. Arrangement of mount¬ 
ing elements with respect to the center of gravity of 
the isolated member such that linear motions cannot 
produce rotational motions and vice versa. 

Centipoise. A measure of viscosity. It is one hun¬ 
dredth of a poise. 

Chromatism. Chromatic aberration, the result of 
varying focus of a lens for different wavelengths of 
light. 

Chunk Glass. Large irregular fragments of glass 
produced by fracturing of the solidified glass mass in 
the melting pot. 

Circle of Confusion. The disk, of measurable di¬ 
ameter, by which a point in the object is represented 
in the image formed by a lens. 

Cobb Chart. A type of testing target involving pairs 
of bright lines on a dark background, grouped in 
descending size according to a fixed ratio. 

Collective. A lens near a focal plane whose purpose 
is to increase the light transmission of a telescope. 

Collimator. A lens or mirror system which renders 
the rays of light from a point parallel. 

Collinear. Lying on a straight line. 

Coma. An off-axis aberration in the focus due to the 
different zones of an optical system having different 
magnifications. 

Contour (cartography). A line on a map passing 
through points all at the same elevation, to show 
heights. 

Contour Follower. A cam which limits the vertical 
motion of a gun at certain bearings, to avoid hitting 
parts of the aircraft which would otherwise be ex¬ 
posed to fire. 

Contrast. The ratio of surface brightness of respec¬ 
tive objects. Measured by direct ratio, or by log of 
this ratio, or by density difference, or by slope of the 
straight line portion of the characteristic curve. Also 
occasionally measured in units of an adopted basic 
contrast. 

Control (cartography). Dimensional or angular in¬ 
formation used in mapping, to determine the scale of 
the map, the positions of points, etc. Usually consists 
of the coordinates or elevations of certain points, the 
distance between two points, or the direction of a 
line. 

Coplanar. Lying in a plane. 

Correcting Plate. A lens-like glass plate with 
slightly deformed surface that is used in an optical 
system to alter the course of the rays by differential 
amounts. 


591 


592 


GLOSSARY 


Crabbing. Side motion or drift of aircraft relative to 
the ground, usually caused by wind. 

Curvature of Field. A lens defect such that image 
points of a plane source do not lie in a plane. 

Damper. Element of anti-vibration mount whose func¬ 
tion is to reduce the natural frequency amplitude and 
to prevent oscillation. 

Definition. A property of an optical system which 
relates to the system’s ability to produce sharp, dis¬ 
tinct images. 

Deflection. The angle by which a gun must be aimed 
ahead of a moving target in order to hit it. 

Deflection Shooting. Shooting at a moving target 
which requires appreciable deflection in aiming. 

Dendritic. Pertaining to dendrite or to arborescent 
crystallization. 

Dial Gauge. A device which indicates on a dial very 
small variations in a fixed dimension. 

Diopter. A unit used to express the power of a lens. 
It is equal to the reciprocal of the focal length in 
meters. 

Dioptometer. A device which permits the measure¬ 
ment of the degree of convergence or divergence of 
light rays. 

Dispersive Power. The property of a medium to sepa¬ 
rate light of different wavelengths, as in a spectrum. 

Distortion. A lens defect such that the image pro¬ 
duced by the lens does not have the same shape as 
the object. 

Dodging. The controlled alteration of density between 
negative and print for the purpose of equalizing print 
quality, usually accomplished by shadowing the print¬ 
ing light. 

Doublet. An optical combination of two lenses, 
usually an achromatic pair. 

Dynamic Boresighting. Boresighting in the presence 
of vibration. 

EK. Eastman Kodak Company. 

Entrance Pupil. The effective area through which 
image forming rays may enter an optical system. 

Erecting Prism. A prism that rotates the field of 
view through 360 degrees to compensate for inversion 
of the objective. 

Erector. A lens combination which inverts the image 
in a telescope so that the observer sees the field right 
side up. 

Exit Pupil. The area through which rays may leave 
an optical instrument and enter the eye. 

Exposure. In photography, the total effect of the 
product of time and intensity. 

Eye Freedom. The transverse distance through which 
the observer’s eye can be moved and still see all of 
the reticle pattern at the normal eye position. 

Eyepiece. The lens or lenses by which the image 
formed by the objective may be examined by the ob¬ 
server. 

Eye Relief. The distance from eye lens to effective 
exit pupil of the system. 


Eye Space. The space from which all of the reticle 
pattern can be seen. 

/-Number. The ratio of aperture to focal length, much 
used in photography as a measure of lens speed. 

Field. The angular extent of the portion of the object 
space which may be seen through a telescope by mov¬ 
ing the eye only. 

Field-Flattener. A lens or deformed plate placed 
near the focal plane for the purpose of depressing an 
otherwise curved focal surface into a plane surface. 

Filtering Action. Mechanical filtering action anal¬ 
ogous to electrical filtering to attenuate all frequen¬ 
cies above a certain lower limit. 

Flat. A piece of optical glass one of whose surfaces 
has been ground and polished to a flat surface. 

Flexible Gun. A gun whose direction of aiming rela¬ 
tive to the aircraft can be changed. 

Fly’s-Eye Sight. A reflex sight of large exit pupil 
made up of a surface of many small lenses (and thus 
resembling a fly’s eye), each of which has its own 
reticle. To the observer the effect is the same as 
though there were only one large lens in use. 

Focal Length. The distance from the principal plane 
to the focus of a lens. 

Focal Plane. A plane perpendicular to the optical 
axis passing through the focal point on the axis. 

Focal Surface. The surface of best focus over a field, 
generally curved. 

Fork Lift. A small truck, ordinarily used for stowing 
boxes, provided with a horizontal fork which can be 
used to lift objects, usually about 9 to 12 ft above the 
ground. 

Foucault Test Chart. A group of equally spaced 
parallel black lines. The spacing between the lines is 
equal to the width of the lines. 

Frictional Damping. Damping in which the resisting 
force is essentially independent of velocity. Also 
called Coulomb and dry friction damping. 

Fringes. Dark bands in the field of an interferometer. 

Gimbal Mount. A type of AOM developed at the In¬ 
stitute of Optics, University of Rochester. 

Ground Speed. Velocity of aircraft relative to the 
ground. 

Gunner. The operator of a sight which controls a gun. 

Gunsight. As used in connection with flexible gun 
installation, refers to the gyro-controlled lead com¬ 
puting sight. A collimated reticle image is seen su¬ 
perimposed on the target by means of a reflecting 
glass plate. 

Halides. Binary compounds (chlorides, bromides, io¬ 
dides, or fluorides) of halogens with an element or 
radical. 

Hardstands. Hard surfaced areas for the parking of 
aircraft. 

Harmonization. The adjustment of the direction of 
aim of a gun and sight parallel to one another, some¬ 
times taking account of ballistic corrections. 

Harmonization (Mirror Boresight Method). A 
method based on the use of a mirror, mounted ac¬ 
curately at right angles to a mandrel which fits a 




GLOSSARY 


593 


collet in the gun barrel, to set parallel to one another 
the sight and the gun which it controls. 

Harmonization (Mirror Frame Method). A method 
based on the use of a group of flat mirrors mounted 
parallel to one another on a frame, for setting a 
group of guns and sights parallel. 

Harmonization (Prism Method). A method based 
on the use of two double-image prisms to establish 
two parallel lines of sight. 

Harmonization (Wire Method). A method based on 
the use of a wire under tension and level bubbles to 
establish two parallel lines of sight. 

Haze. In aerial photography the scattering back 
toward the camera lens of sunlight from air, moist¬ 
ure, and dust below the plane. 

Hydrolysis. A chemical decomposition involving ad¬ 
dition of the elements of water. 

Hydrophilic. Having, or denoting, strong affinity for 
water. 

Hyperchromat. An optical combination with en¬ 
hanced color characteristics, often used for eliminat¬ 
ing excess color in some other part of the system. 

Inclusions. Regions of non-homogeneous material in 
optical glass. 

Index-*/ Curve. A relation between the p-value and 
the index of refraction for inorganic glasses; in gen¬ 
eral the higher the rvalue, the lower the refractive 
index. 

Interferometer. A device which compares optically 
the lengths of two light paths. 

Intervalometer. Device for automatically making 
the power electrical connection to trip the camera 
shutter at a preset time interval. 

Invariant System. An optical system in which a 
motion or rotation of the system as a whole causes 
no apparent motion of the image. 

Inverted Telephoto. An optical system that is much 
longer than the equivalent focal length, usually ob¬ 
tained by the negative power preceding the positive. 

Jacks. Adjustable supports used to steady the wings 
and tail of aircraft on the ground. 

Kinetic Definition Chart Apparatus. A device, used 
as a target, designed to measure the definition or 
resolving power of an optical instrument. 

KRL. Kodak Research Laboratories. 

Lateral Color. An off-axis aberration due to dif¬ 
ferent magnification in the different colors. 

Lead. Deflection. 

Lead Computer. A device for computing the lead. 
Frequently connected mechanically to a sight to auto¬ 
matically deflect it by the proper angle. 

Lipophilic. Having, or denoting, a lack of affinity for 
water. 

Longitudinal Chromatism. The axial difference in 
focus of a lens for rays of different color. 

Louvre Shutter. A Venetian-blind type shutter. 

Macroscopic. Pertaining to objects of large size, rela¬ 
tive to the scale of values under consideration; the 
opposite of microscopic. 

Magnification Factor. Same as transmissibility. 


Mangin Mirror. A spherical mirror formed by alu¬ 
minizing or silvering the back surface of a lens so 
designed that the front surface corrects the spherical 
aberration of the mirror. 

Mar Resistance. The ability of a surface to with¬ 
stand scratching by particles of carborundum falling 
from a given height. 

Micrometer. A device for the accurate measurement 
of short distances. 

Microscopic. Pertaining to objects of small size, rela¬ 
tive to the scale of values under consideration; oppo¬ 
site of macroscopic. 

Microphotometry. The study of photometric problems 
on a microscopic scale, generally involving the photo¬ 
graphic emulsion. 

Mil. An angle equal to 0.001 radian. 

Minus-Blue Filter. In aerial photography, a filter 
that eliminates blue and violet light. Generally the 
equivalent of Wratten No. 12. 

Mirror Boresight Method. See Harmonization. 

Mirror Frame Method. See Harmonization. 

Monochromator. A device for isolating a single 
wavelength of light at a time; in practice, a narrow 
band of wavelengths. 

Mosaic. A composite picture made by piecing together 
prints of a number of negatives of contiguous areas, 
all of the same scale. 

Mosaic (Controlled). A mosaic in which the method 
of processing and assembling the photographs assures 
that the images are in correct horizontal relationship 
one to another, to within some tolerance. 

Mosaic (Photographic). Portions of many aerial 
photographs assembled in such a fashion that images 
match at the joints, thus giving a photographic rep¬ 
resentation of a large area. 

Mosaic (Uncontrolled). A mosaic in which the pho¬ 
tographs are assembled by matching images only. 

nn. The index of refraction at the sodium D line ?i5893 
A. 

Natural Frequency. The undamped resonant fre¬ 
quency. 

p-Value. The ratio (wd — 1 ) / (n F — n c ) where w D , n F , 
n c are the indices of refraction at wavelengths 5893, 
4861, 6563 A respectively. It is a measure of the 
dispersive power of a glass. 

Objective. The principal image forming lens of a 
telescope. 

Oblique Aerial Photograph. An aerial photograph 
taken with the camera axis significantly off the ver¬ 
tical. 

Oblique Aerial Photograph (High). One in which 
the horizon appears. 

Ocular. An eyepiece. 

Off-Axis Aberration. Aberration for points in the 
field away from the optical axis. 

Ophthalmic. Pertaining to optics of the eye; ophthal¬ 
mic glass is glass produced for spectacle lenses. 

Optic. The glass components of any optical device or 
the combination of the components. 

Optical Axis. An imaginary line passing through the 




594 


GLOSSARY 


centers of curvature of the various lens surfaces. 

Orange-Peel Surface. The descriptively named sur¬ 
face which results from the sudden, uneven chilling 
of the surface of a hot piece of glass in the plastic 
state as it comes in contact with the relatively cool 
surface of a metal mold. 

Panatomic X. A special emulsion of high speed and 
fine grain. Class A. 

Parallactic Range. The extreme parallactic shift as 
the eye traverses a diameter of the aperture of a 
sight, expressed in mils. 

Parallax. The apparent shift of the reticle image 
with respect to a distant object as the eye traverses 
the aperture of a sight. 

Partial Dispersion. The difference in the index of re¬ 
fraction at two designated wavelengths. 

Petzval Sum. A condition in optics involving the sum 
of simple functions of the radii and indices of an 
optical system, intimately related to curvature of 
field in the absence of astigmatism. 

Photometric Area. An area of uniform and known 
brightness, used for standardizing the general ex¬ 
posure level, and sensitivity of the emulsion. 

Photomicrography. Microscopy by photographic 
methods. 

Platen. A reference plate or surface, used for locat¬ 
ing the film. 

Plasticise. To render plastic; to break down. 

Polar Group. A union of atoms in which the chemical 
bond is electrostatic attraction between oppositely 
charged particles (ions). 

Polyhydroxy. Containing more than one hydroxyl 
group, the latter being a univalent group or radical 
consisting of one atom of hydrogen and one of oxy¬ 
gen. 

Porro Prism. An erecting prism frequently used in 
binocular systems made of two right-angle prisms. 
There are four internal reflections from the sides of 
the prisms. The hypotenuse faces in part are the 
entrance and exit faces of the prism. The other parts 
of the hypotenuse faces are usually in cemented 
contact. 

Prism. An optical device for changing the direction of 
a beam of light. 

Polar Axis. An axis, parallel to that of the earth, on 
which an instrument rotates. 

Prism Method. See Harmonization. 

Probable Error. An increment and decrement to be 
applied to the observed value of a quantity such that 
for a large number of observations half the values 
will lie within the interval so obtained and half 
without. 

Probe. A slender rod, used to measure the growth of a 
crystalline mass. 

Proboscope. A device by which the interior surfaces 
of the lenses in an optical instrument may be ex¬ 
amined. 

Rangefinder. A device for measuring the range of a 
target, whose dimensions may be unknown, by mea¬ 
suring the difference in its apparent direction as 


seen from the two ends of a fixed base line within the 
instrument. 

Rayleigh Limit. A principle enunciated by Lord Ray¬ 
leigh, that phase differences of the order of ^-wave¬ 
length of light will not detract noticeably from the 
limiting resolution of a converging pencil of light. 

Ray Tracing. A method of computing the perform¬ 
ance of a lens system by tracing a ray from lens sur¬ 
face to lens surface through the instrument. 

Refractive Index. The ratio of the velocity of light in 
a vacuum (or in air) to that in any other medium. 

Resolution. Generally, the fineness of detail resolv¬ 
able. Often used as the equivalence of resolving 
power, which is properly its reciprocal. 

Resolving Power. The ability of an instrument to 
show a group of parallel lines as separated, generally 
expressed in terms of the minimum angle of separa¬ 
tion of the lines still shown as separated. 

Resolving-Power Targets. A group of targets de¬ 
signed for quantitative study of the various factors 
that limit resolution. 

Reticle. An optical device which superimposes a 
group of lines and divisions upon the field of a tele¬ 
scope. 

Rubber Shell Mount. A type of AOM developed at 
the Technicolor Motion Picture Corporation. 

Scanner. A scanning device. 

Scanning Device. A device which, by optical and 
mechanical means, presents to an observer who looks 
into fixed eyepieces, a series of continuously or in¬ 
termittently changing fields which cover systematic¬ 
ally a sector of the horizon or of the sky. 

Scavenger. A material introduced into a chemical 
process to carry off impurities in a reaction. 

Scotoptic Vision. Vision at low levels of illumination. 

Secondary Spectrum. The residual color aberration 
remaining in an optical system after it has been 
achromatized for two wavelengths. 

Seeds. Very small inclusions. 

Shake Table. An instrument for testing the effects 
of linear and angular vibration upon AOM systems. 

Shock Mounting. A resilient mounting used to pro¬ 
tect such apparatus as flight instruments and radio 
equipment from damage by plane vibration and mech¬ 
anical shock. 

Slab Glass. Glass manufactured in long continuous 
sheets. 

Sleek. A glossy scuff on the surface of an optical 
element. 

Spherical Aberration. An optical error of a lens 
which results from a difference in focal length be¬ 
tween the central and marginal areas of the lens. 

Stacker. See Fork Lift. 

Stadiameter. A device for determining the range of a 
target whose linear dimensions are known by measur¬ 
ing the angle which it subtends at the observer. 

Static Boresighting. Boresighting in the absence of 
vibration. 

Stones. Inclusions somewhat larger than seeds. 



GLOSSARY 


595 


Strain. A defect in optical glass produced by non- 
uniform contraction during the cooling of the glass. 

Striae. A defect in optical glass produced by regions 
of slightly varying index of refraction. 

Super XX. A special emulsion of very high speed and 
medium graininess; for aerial purposes, with en¬ 
hanced red sensitivity. Class L. 

Sweep. Method of motion compensation by swinging 
the entire camera about a transverse axis. 

Telephoto. An optical system whose equivalent focal 
length exceeds its overall physical length to the focal 
plane. In ordinary photography, used loosely for any 
lens of long focal length, particularly in miniature 
photography. 

TMP. Technicolor Motion Picture Corporation. 

T. O. The Army Technical Order describing a pro¬ 
cedure. 

Transmissibility. The ratio of the amplitude of a 


mounted system to the amplitude of the disturbing 
vibration. 

Turbidity. The turbid character of the photographic 
emulsion that causes diffusion of light and spreading 
of the image on a microscopic scale. 

UR. University of Rochester. 

Vertical Aerial Photograph. An aerial photograph 
taken with the camera axis approximately vertical, 
usually within 5 degrees. 

vpm. Vibrations per minute. 

Wedge. A thin prism which produces small deviations 
in a light beam. 

Wire Method. See Harmonization. 

Zonal Aberration. The residual spherical aberration 
left over after optimum balancing of undercorrected 
third order spherical and overcorrected fifth and 
higher orders has been accomplished. 

Z-Number. Hardness number on the Rockwell scale. 





BIBLIOGRAPHY 


Numbers such as Div. 16-111.11-M5 indicate that the document listed has been microfilmed and that its 
title appears in the microfilm index printed in a separate volume. For access to the index volume and to 
the microfilm, consult the Army or Navy agency listed on the reverse of the half-tittle page. 


Chapter 1 

1. By'itish and Canadian Reports: 

Photographic Resolving Power of Lenses, PO-348, 6. 

P.R.C. 27, Photographic Research Committee of 
the National Research Council of Canada, pp. 1-11. 
Report on Consistency of Photographic Resolution 
Methods of Testing Photographic Lenses, L. C. 
Martin and C. A. Padgham, Ph. 148, Royal Air¬ 
craft Establishment under the Ministry of Air¬ 
craft Production, September 1943. 

A Review of the Problem of Resolution as it 
Affects the Design, Focusing, and Testing of 
Lenses for Use in Aerial Photography, APRC 134. 

The Precision of Photographic Lens Resolution 
Measurements, Ph. 320, Royal Aircraft Establish¬ 
ment under the Ministry of Aircraft Production, 
March 1945. 

Photographic Lens Testing, LOG A J-3530 (Liaison 
Office) Photographic Laboratory, Army Air 
Forces, Engineering Division, Apr. 29, 1944. 
Photographic Lens Testing at the Photographic 
Laboratory, Richard N. Nierenberg, LOG A J-5548 
(Liaison Office), Photographic Laboratory, Army 
Air Forces, Engineering Division, Apr. 29, 1944. 
Methods of Testing Photographic Lenses, W. M. 
Wreathall and D. J. Walters, Ph. 293, Royal Air¬ 
craft Establishment under the Ministry of Air¬ 
craft Production, July 1944. 

Experiments Showing the Dependence of Aerial 
Photographic Resolution on the Choice of Focus, 

Using Test Objects of High, Middle, and Low Con¬ 
trast, and a Series of Lenses at Different Aper¬ 
tures, APRC 142. 

2. “Development Problems in Aerial Cameras/’ H. C. 
Wohlrab, Translated by L. J. Goodlet, Luftwissen, 

Vol. 9, No. 2, February 1942, pp. 37-44. 

3. Spherically Symmetrical Lenses and Associated 
Equipment for Wide-Angle Aerial Photography, 
OSRD 6016, OEMsr-474, Report 16.1-118, Harvard 
University, Nov. 30, 1945. Div. 16-111.11-M5 

4. Optical Tests of Five Lenses for Aerial Photog¬ 
raphy, OSRD 5319, OEMsr-101, Report 16.1-100, 
Mount Wilson Observatory, Sept. 25, 1945, pp. 

12-15. Div. 16-111.11-M4 

4a. Ibid., pp. 10-11. 

4b. Ibid., pp. 1-15. 

4c. Ibid., pp. 6-8. 

5. Design and Development of an Automatically 
Focusing J+O-inch f 15.0 Distortionless Telephoto 
and Related Lenses for High Altitude Aerial 
Reconnaissance, OSRD 6017, OEMsr-474, Report 
16.1-119, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M6 


5a. Ibid., pp. 132-137. 

5b. Ibid., pp. 138-148. 

5c. Ibid., pp. 216-227. 

Design and Development of an 100-inch f/10 
Anastigmat for Aerial Reconnaissance at Extreme 
Altitudes, OSRD 6019, OEMsr-474, Report 16.1- 
121, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M7 

7. Miscellaneous Projects Partially Completed, Theo¬ 
dore Dunham, Jr., OSRD 6028, OEMsr-474, Re¬ 
port 16.1-130, Harvard University, Dec. 31, 1945. 

Div. 16-101-M7 

7a. Ibid., pp. 29-33. 

7b. Ibid., pp. 34-39. 

7c. Ibid., pp. 1-6. 

7d. Ibid., pp. 41-43. 

7e. Ibid., pp. 43-47. 

7f. Ibid., pp. 39-40. 

7g. Ibid., pp. 6-13. 

7h. Ibid., pp. 13-18. 

8. Apochromatic Photographic Aerial Lenses and 
Other Optical Instruments Making Use of Syn¬ 
thetic Fluorite, OSRD 6020, OEMsr-474, Report 
16.1-122, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M8 

8a. Ibid., pp. 45-48. 

8b. Ibid., pp. 54-56. 

9. Optical Tests of the Harvard College Observatory 
Distortionless Apochromat f/8, Focal Length 36 
Inches, OSRD 4519, OEMsr-101, Report 16.1-75, 
Mount Wilson Observatory, Oct. 10, 1944. 

Div. 16-111.11-M2 

9a. Ibid., pp 3 et seq. 

9b. Ibid., pp. 1-12. 

10. Quantitative Studies and Observations of Factors 

Limiting Resolution of Aerial Photographs, OSRD 
6029, OEMsr-474, Report 16.1-131, Harvard Uni¬ 
versity, Dec. 31, 1945. Div. 16-111.6-M4 

10a. Ibid., Parts I and II. 

10b. Ibid., pp. 151-154. 

10c. Ibid., p. 317. 
lOd. Ibid., pp. 32-106. 
lOe. Ibid., p. 401. 
lOf. Ibid., p. 352. 
lOg. Ibid., pp. 344, 348. 
lOh. Ibid., pp. 93-95. 
lOi. Ibid., pp. 99-105. 

11. Design and Development of a 36-inch f/8.0 Tele¬ 

photo for the K-18 Camera, OSRD 6025, OEMsr- 
474, Report 16.1-127, Harvard University, Dec. 31, 
1945. Div. 16-111.11-M10 

12. Wide Field f/1 Camera Lens, OSRD 6030, OEMsr- 

160, Report 16.1-108, University of Rochester, 
Sept. 15, 1945. Div. 16-111.411-MI 


597 


598 


BIBLIOGRAPHY 


13. Optical Plastic Material Synthesis, Fabrication 
and Instrument Design, OSRD 4417, OEMsr-70, 
Report 16.1-59, Polaroid Corporation, Feb. 1, 1945. 

Div. 16-161.1-M2 

14. Design and Development of Several Types of 7- 

inch f 12.5 Lenses for Night Photography, OSRD 
6018, OEMsr-474, Report 16.1-120, Harvard Uni¬ 
versity, Dec. 31, 1945. Div. 16-111.411-M2 

14a. Ibid., pp. 6-14, 23-53. 

15. Wide Field Fast Cameras, Louis G. Henyey and 

Jesse L. Greenstein, OSRD 4504, OEMsr-1078, 
Final Report 16.1-72, Yerkes Observatory, Apr. 
30, 1945. Div. 16-111.41-MI 

16. “New Catadioptric Meniscus Systems,” Maksutov, 
Journal of the Optical Society of America, May 
1944. 

17. Tests of Aerial Camera Lenses Made at Mount 
Wilson Observatory, OSRD 3629, OEMsr-101, Re¬ 
port 16.1-36, Mount Wilson Observatory, Mar. 15, 

1944. Div. 16-111.11-MI 

18. Optical Tests of the Bausch and Lomb Telestigmat 

f/8, Focal Length UO Inches, OSRD 4499, OEMsr- 
101, Report 16.1-67, Mount Wilson Observatory, 

Dec. 5, 1944. Div. 16-111.11-M3 

19. Resolution of Aerial Cameras in the Laboratory 
and in the Air, OSRD 4738, OEMsr-101, Report 

16.1-89, Mount Wilson Observatory, Mar. 31, 1944. 

Div. 16-111.6-M3 

20. Lens-film Resolving Power and Aerial Image 
Energy Distribution of Several Aerial Camera 
Lenses, OSRD 6127, OEMsr-392, Report 16.1-149, 
Eastman Kodak Company, Mar. 27, 1946. 

Div. 16-111.11-M11 

21. Two-mirror Schmidt Camera for Aerial Photog¬ 
raphy, OSRD 5644, OEMsr-101, Final Report 16.1- 
104, Mount Wilson Observatory, Sept. 25, 1945. 

Div. 16-111.15-M2 

22. Eighth Progress Report (Technical Data), Walter 
S. Adams and Charlton M. Lewis, Mount Wilson 
Observatory, Aug. 15, 1943. 

Seventh Progress Report (Technical Data), Walter 
S. Adams and Charlton M. Lewis, Mount Wilson 
Observatory, July 15, 1943. 

23. Report on the Mounting and Testing of f/l.A, 
Three-inch Focal Length Schmidt Optics, Herbert 
E. Grier, Interim Report, MIT, Schmidt Optics. 

24. A Practical Application of the Schmidt Camera 
to Night Photography, OSRD 6023, OEMsr-474, 
Report 16.1-125, Harvard University, Dec. 31, 

1945. Div. 16-111.41-M2 
24a. Ibid., pp. 9-12. 

24b. Ibid., pp. 14-15. 

24c. Ibid., pp. 10-11. 

25. Antivibration and Ground Speed Compensation 

Mounts for Aerial Cameras, OSRD 6123, OEMsr- 
392, Report 16.1-145, Eastman Kodak Company, 
October 1945. Div. 16-111.13-M3 

25a. Ibid., Appendix I, p. 76. 

25b. Ibid., Appendix I, p. 80. 


25c. Ibid., Figs. 10, 11, p. 20. 

25d. Ibid., Fig. 17, p. 34. 

25e. Ibid., Figs. 12, 13, pp. 22, 27. 

25f. Ibid., p. 30 et seq. 

25g. Ibid., p. 1 et seq. 

25h. Ibid., pp. 45-50. 

25i. Ibid., p. 63. 

25j. Ibid., p. 63 et seq. 

26. Stabilized Aerial Camera Mounts, OSRD 6124, 
OEMsr-392, Report 16.1-146, Eastman Kodak 
Company, December 1945. Div. 16-111.13-M5 
26a. Ibid., pp. 10-15. 

26b. Ibid., Part II. 

27. Gun Camera Antivibration Mount, OSRD 6125, 

OEMsr-392, Report 16.1-147, Eastman Kodak 
Company, October 1945. Div. 16-111.13-M4 

27a. Ibid., pp. 16-26. 

27b. Ibid., pp. 27-28 for mirror operation. 

28. Modification of the Metrogon Shutter to Increase 
Its Speed, OSRD 3025, OEMsr-101, Final Report 

16.1- 35, Mount Wilson Observatory, Nov. 1, 1943. 

Div. 16-111.12-MI 

29. Shutter Development for Aerial Photography, 
OSRD 4739, OEMsr-101, Final Report 16.1-90, 
Mount Wilson Observatory, Mar. 15, 1945. 

Div. 16-111.12-M3 

29a. Ibid., pp. 1-17. 

30. Aerial Camera Shutter, OSRD 4184, OEMsr-710, 

Final Report 16.1-57, Technicolor Motion Picture 
Corporation, Feb. 28, 1945. * Div. 16-111.12-M2 

30a. Ibid., p. 1 et seq. 

31. Investigation of High-Speed Aerial Camera 
Shutters (Part I), H. J. Hood and F. M. Bishop, 
OSRD 4552, OEMsr-622, Final Report 16.1-66, 
Eastman Kodak Company, Jan. 9, 1944. 

32. A Method for Checking the Focus of Aerial 
Cameras, OSRD 4185, OEMsr-710, Final Report 

16.1- 58, Technicolor Motion Picture Corporation, 

Feb. 28, 1945. Div. 16-111.15-MI 

33. Development and Construction of an Exposure 

Meter for Use ivith a Standard View Finder in 
Aerial Photography, OSRD 4392, OEMsr-1245, Re¬ 
port 16.1-61, University of Michigan, January 
1945. Div. 16-111.2-MI 

34. A Device for Testing the Flatness of Film, in the 
A-5 and A-7 Magazines under Service Conditions, 
OSRD 6024, OEMsr-474, Report 16.1-126, Harvard 
University, Dec. 31, 1945. Div. 16-111.14-MI 

35. Miscellaneous Projects for Instructional and Lab¬ 
oratory Purposes, OSRD 6027, OEMsr-474, Report 

16.1- 129, Harvard University, Dec. 31, 1945. 

Div. 16-180-M4 

36. Camera Stabilizer, John F. Taplin, OEMsr-1366, 

Division 7, Lawrence Aeronautical Corporation, 
Sept. 25, 1945. Div. 7-321.224-M5 

Chapter 2 

1. Two-mirror Schmidt Camera for Aerial Photog¬ 
raphy, OSRD 5644, OEMsr-101, Final Report 



BIBLIOGRAPHY 


599 


16.1- 104, Mount Wilson Observatory, Sept. 25, 

1945. Div. 16-111.15-M2 

2. Design and Development of an Automatically 
Focusing 40-inch, f/5.0 Distortionless Telephoto 
and Related Lenses for High Altitude Aerial Re¬ 
connaissance, OSRD 6017, OEMsr-474, Report 

16.1- 119, Harvard University, Dec. 31, 1945. 

Div. 16-111.1.1-M6 

2a. Ibid., pp. 158-188. 

3. Design and Development of an 100-inch ft 10 
Anastigmat for Aerial Reconnaissance at Extreme 
Altitudes, OSRD 6019, OEMsr-474, Report 16.1- 
121, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M7 

4. Apochromatic Photographic Aerial Lenses and 
Other Optical Instruments Making Use of Syn¬ 
thetic Fluorite, OSRD 6020, OEMsr-474, Report 

16.1- 122, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M8 

4a. Ibid., pp. 48-51. 

5. Design and Development of a 36-inch f/8.0 Tele¬ 

photo for the K-18 Camera, OSRD 6025, OEMsr- 
474, Report 16.1-127, Harvard University, Dec. 31, 
1945. Div. 16-111.11-M10 

6. Resolving Power Targets for Aerial Photography, 
Duncan E. MacDonald, OSRD 4445, OEMsr-203, 
Final Report 16.1-60, MIT, December 1944. 

Div. 16-111.6-M2 

6a. Ibid., p. 5 et seq. 

7. Resolution of Aerial Cameras in the Laboratory 
and in the Air, OSRD 4738, OEMsr-101, Report 

16.1- 89, Mount Wilson Observatory, Mar. 31, 1944. 

Div. 16-111.6-M3 

7a. Ibid., pp. 2-15. 

7b. Ibid., pp. 2-15. 

7c. Ibid., pp. 10-11. 

7d. Ibid., pp. 41-42. 

8. Quantitative Studies and Observations of Factors 

Limiting Resolution of Aerial Photographs, Part 
/, Flight Data and Test Equipment, OSRD 6029, 
OEMsr-474, Report 16.1-131, Harvard University, 
Dec. 31, 1945. Div. 16-111.6-M4 

8a. Ibid., pp. 99-105. 

9. Resolving Power in Aerial Photography, OSRD 
4047, OEMsr-101, Report 16.1-50, Mount Wilson 
Observatory, May 15, 1944. Div. 16-111.6-MI 
9a. Ibid., pp. 2-10. 

9b. Ibid., pp. 1, 10. 

9c. Ibid., pp. 6-9. 

10. Quantitative Studies and Observations of Factors 

Limiting Resolution of Aerial Photographs, Part 
II, Analysis of Data, Conclusions and Recom¬ 
mendations, OSRD 6029, OEMsr-474, Report 16.1- 
131, Harvard University, Dec. 31, 1945, pp. 255, 
256, 258, 259, 283, 371. Div. 16-111.6-M4 

10a. Ibid., pp. 390-393. 

10b. Ibid., p. 315. 

10c. Ibid., pp. 423-424. 

11. Antivibration and Ground Speed Compensation 


Mounts for Aerial Cameras, OSRD 6123, OEMsr- 
392, Report 16.1-145, Eastman Kodak Company, 
October 1945. Div. 16-111.13-M3 

11a. Ibid., p. 60 et seq. 
lib. Ibid., p. 65. 

12. Progress Report on Aerial Camera Motions, 
Duncan E. MacDonald, OSRD 5178, OEMsr-203, 
Report 16.1-98, MIT, June 1945. Div. 16-111.13-M2 

13. A Method for Checking the Focus of Aerial 

Cameras, OSRD 4185, OEMsr-710, Final Report 

16.1-58, Technicolor Motion Picture Corporation, 
Feb. 28, 1945. Div. 16-111.15-MI 

14. A Device for Testing the Flatness of Film in the 
A-5 and A-7 Magazines under Service Conditions, 
OSRD 6024, OEMsr-474, Report 16.1-126, Harvard 
University, Dec. 31, 1945. Div. 16-111.14-MI 

15. British Report APRC 102/H/1006, Photographic 
Research Committee of the National Research 
Council of Canada. 

16. British Report APRC Note 48, Photographic Re¬ 
search Committee of the National Research 
Council of Canada. 

Chapter 3 

1. Pacific Fleet Conditions and Requirements Regard¬ 
ing Photogrammetric Equipment and Processes, 
Philip Kissam and Merrill Flood, Office of Field 
Service, OSRD, June 5, 1944. 

2. A Report on Airplane Photographic Tests of 

Lenses, Film and Filters for Oblique Photography 
Suitable for Mapping Purposes, OSRD 6102, 
OEMsr-1039, Report 16.1-138, Aero Service Cor¬ 
poration, Oct. 31, 1945. Div. 16-111.3-M2 

3. Design and Development of Lenses for Rectifica¬ 
tion of Metrogon High Obliques, OSRD 6021, 
OEMsr-474, Report 16.1-123, Optical Research 
Laboratory, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M9 

10a. Ibid., p. 32. 

4. Fixed Projection Cameras for Rectifying High 
Oblique Aerial Photographs, Robert Singleton, 
OSRD 4709, OEMsr-1087, Report 16.1-80, Merrill 
Flood and Associates, Oct. 31, 1945. 

Div. 16-111.3-M8 

5. Test of Rectification and Plotting of a Pair of 
High Oblique Photographs, Robert Singleton, 
OSRD 6039, OEMsr-1087, Report 16.1-111, Merrill 
Flood and Associates, Oct. 31, 1945. 

Div. 16-111.3-M9 

Supplementary Reports 

6. Examination and Test of the Flashlight Perspec¬ 
tive Projector, Robert Singleton, OSRD 4027, 
OEMsr-1087, Progress Report 16.1-48, Merrill 
Flood and Associates, July 22, 1944. 

Div. 16-111.3-MI 

7. Soundings and Beach Contours with Airplane 
Cameras Mounted Athwartships, Philip Kissam, 



600 


BIBLIOGRAPHY 


OSRD 3990, OEMsr-1087, Progress Report 16.1-47, 
Merrill Flood and Associates, Aug. 3, 1944. 

Div. 16-111.3-M2 

8. Manual of Photogrammetry, The American Society 
of Photogrammetry, Pitman Publishing Corp., 

1944. 

9. A Procedure for Determining the Orientation of 
Aerial Photographs by Pairs , Robert Singleton, 
OSRD 4708, OEMsr-1087, Report 16.1-79, Merrill 
Flood and Associates, Feb. 23, 1945. 

Div. 16-111.3-M3 

10. A Stereoscopic Plotter for Contouring in Ortho¬ 
gonal Projection from Rectified Aerial Photo¬ 
graphs, Philip Kissam and Robert Singleton, 
OSRD 4711, OEMsr-1087, Report 16.1-82, Merrill 
Flood and Associates, Apr. 9, 1945. 

Div. 16-111.3-M4 

11. Summary Description of Existing Mapping Sys¬ 
tems in the United States, O. M. Miller and Robert 
Singleton, OSRD 4707, OEMsr-1087, Report 16.1- 
78, Merrill Flood and Associates, May 4, 1945. 

Div. 16-111.3-M5 

12. An Enlarging Camera for Use with a Fixed 
Rectifier, Robert Singleton and Marvin Thralls, 
OSRD 4710, OEMsr-1087, Report 16.1-81, Merrill 
Flood and Associates, May 24, 1945. 

Div. 16-111.3-M6 

13. A System for Mapping from High Oblique Aerial 

Photographs, Robert Singleton, Philip Kissam, 
and O. M. Miller, OSRD 4714, OEMsr-1087, Re¬ 
port 16.1-85, Merrill Flood and Associates, June 
18, 1945. Div. 16-111.3-M7 

14. Manual on the Pinhole Rectifying Camera, OSRD 

6100, OEMsr-1039, Report 16.1-136, Aero Service 
Corporation, Oct. 31, 1945. Div. 16-111.3-M10 

15. Manual on the Variable Ratio Printer, OSRD 

6101, OEMsr-1039, Report 16.1-137, Aero Service 
Corporation, Oct. 31, 1945. Div. 16-111.3-M11 

Chapter 4 

1. The NDRC Optical Inspection Project at the Penn¬ 
sylvania State College for the Period of October 
19US to November 19U5, Howard S. Coleman and 
Madeline F. Coleman, OSRD 6104, OEMsr-1197, 
Report 16.1-140, Pennsylvania State College, Oct. 

21, 1945. Div. 16-101-M5 

2. Optical Inspection, Howard S. Coleman, OSRD 

6103, OEMsr-1197, Report 16.1-139, Pennsylvania 
State College, Oct. 20, 1945. Div. 16-101-M4 

3. A Description of the Kinetic Definition Chart 

(K.D.C.) Apparatus and Its Uses, Howard S. 7. 
Coleman and Samuel W. Harding, OSRD 6005, 
OEMsr-1197, Report 16.1-132, Pennsylvania State 
College, Oct. 19, 1945. Div. 16-162.2-M2 8. 

4. “Vision in Optical Instruments,” Proceedings of 
the Physical Society, Vol. 48, 1936, p. 747. 

5. The Penn State 1-1 Michelson-Twyman Inter- 9. 
ferometer and Its Use in Determining Conform¬ 


ance with Design and in Quality Control of Lenses, 
Prisms, and Telescopic Systems, Howard S. Cole¬ 
man and David G. Clark, OSRD 6106, OEMsr- 
1197, Report 16.1-142, Pennsylvania State College, 
Oct. 21, 1945. Div. 16-162.4-MI 

6. Photoelectric and Photographic Procedures for the 

Evaluation of Optical Instrument Design, Howard 
S. Coleman and David G. Clark, OSRD 6107, 
OEMsr-1197, Report 16.1-143, Pennsylvania State 
College, Oct. 16, 1945. Div. 16-162.1-MI 

7. The Dioptometer and Its Use in the Inspection of 
Optical Instruments, Howard S. Coleman, OSRD 
6105, OEMsr-1197, Report 16.1-141, Pennsylvania 
State College, Oct. 19, 1945. Div. 16-162.3-MI 

8. Notes on the Scattering of Light in Optical Fire 

Control Instruments, Howard S. Coleman and 
Samuel W. Harding, OSRD 6108, OEMsr-1197, 
Report 16.1-144, Pennsylvania State College, Oct. 
19, 1945. Div. 16-112.3-MI 

9. Comparisons of M-70 Telescopes by Five Different 
Manufacturers, Laboratory Report 16, July 22, 

1944. 

Chapter 5 

1. Effects of Binocular Magnification on the Visibility 

of Targets at Low Levels of Illumination, S. 
Howard Bartley and Eloise Chute, OSRD 4433, 
OEMsr-1058, Final Report 16.1-62, Dartmouth 
College, Nov. 30, 1944. Div. 16-121-M2 

2. The Effects of Night Binocular Design Features 

on the Visibility of Targets at Low Levels of 
Illumination, Carl W. Miller and Lloyd H. Beck, 
OSRD 6128, OEMsr-1229, Report 16.1-150, Brown 
University, Oct. 25, 1945. Div. 16-121-M4 

3. Summary of Experimental Data (Supplement to 

OSRD 6128), Carl W. Miller and Lloyd H. Beck, 
OSRD 6129, OEMsr-1229, Report 16.1-151, Brown 
University, Oct. 25, 1945. Div. 16-121-M5 

4. Physiological Factors Determining the Perform¬ 
ance of Night Binoculars, H. K. Hartline, I. H. 
Wagman, L. J. Milne, A. J. Rawson, V. Legallais, 
and D. Scott, OSRD 6099, OEMsr-1228, Report 
16.1-135, University of Pennsylvania, Oct. 31, 

1945. Div. 16-121.1-M2 

5. Visibility in Meteorology, W. E. K. Middleton, Uni¬ 
versity of Toronto Press, 1941. 

6. The Influence of Binoculars and Telescopes on the 
Visibility of Targets at Twilight, S. Hecht, C. D. 
Hendey, and S. Shlaer, CAM(NRC) Report 312, 
Columbia University, June 1944. 

Frequency of Seeing at Low Illumination, H. K. 
Hartline and R. McDonald, CAM(NRC) Report 
110, University of Pennsylvania, January 1943. 
“Energy, Quanta, and Vision,” S. Hecht, S. 
Shlaer, and M. Pirenne, Journal of General Phys¬ 
iology, Vol. 25, No. 6, July 1942, pp. 819-840. 

A Study of Pupil Size at Low Levels of Illumina¬ 
tion, Irving H. Wagman, OSRD 6098, OEMsr- 




BIBLIOGRAPHY 


601 


1228, Report 16.1-134, University of Pennsylvania, 
Oct. 15, 1945. Div. 16-121.1-MI 

10. A Short Statistical Survey of the Variation of 
Pupil Diameter with Field Brightness, A.R.L./N.- 
2/0.502, Admiralty Research Laboratory, Ted- 
dington, Eng., March 1942. 

11. Effect of Size and Variability of the Pupil of the 

Eye on the Choice of Exit Pupil Size for Night 
Binoculars, H. K. Hartline, Minutes and Pro¬ 
ceedings of the Twelfth Meeting of the Army, 
Navy, and OSRD Vision Committee, June 12, 1945, 
p. 30. Div. 6-201-M5 

12. Effect of Errors in Setting the Interpupillary Dis¬ 
tance of Binocular Telescopes on Target Detection 
at Night, H. K. Hartline, Minutes and Proceedings 
of the Twelfth Meeting of the Army, Navy, and 
OSRD Vision Committee, June 12, 1945, p. 35. 

Div. 6-201-M5 

13. Factors Determining the Performance of Night 
Binoculars, H. K. Hartline, OEMsr-1228, Interim 
Report, University of Pennsylvania, December 

1944. 

14. Visibility of Targets at Low Levels of Illumina¬ 
tion, OEMsr-160 Section D-3, NDRC, University 
of Rochester. 

15. “A Theoretical Basis for Intensity Discrimination 
in Vision,” S. Hecht, Proceedings of the National 
Academy of Sciences, Vol. 20, 1934b, p. 644. 

16. An Experimental Study of the Utility at Night of 
Binoculars of Different Powers and the Effect of 
Providing these Instruments with Non-Reflecting 
Film, Report NPL/SCAN/4, National Physical 
Laboratory, Department of Scientific and Indus¬ 
trial Research, Great Britain, June 1942. 

Chapter 6 

1. The Prism Method for Harmonization of B-29 Re¬ 
mote Control Turrets, OSRD 5402, OEMsr-160, 
Report 16.1-101, University of Rochester, Aug. 1, 

1945. Div. 16-112.13-MI 
la. Ibid., p. 7 et seq. 

2. The Wire Method for Harmonization of B-29 Re¬ 
mote Control Turrets, OSRD 5403, OEMsr-203, 
Report 16.1-102, MIT, Aug. 1, 1945. 

* Div. 16-112.12-MI 

2a. Ibid., p. 25 et seq. 

3. The Mirror Boresight Method for Harmonization 

of B-29 Remote Control Turrets, OSRD 5404, 
OEMsr-474, Report 16.1-103, Merrill Flood and 
Associates, Aug. 1, 1945. Div. 16-112.11-MI 

4. The Harmonization of Aircraft Remote Fire Con¬ 

trol System, Philip Kissam, OSRD 4787, OEMsr- 
1087, Report 16.1-93, Merrill Flood and Associates, 
Oct. 10, 1945. Div. 16-112.1-M2 

4a. Ibid., Manual, p. 16. 

4b. Ibid., Fig. 116. 

4c. Ibid., Appendix E. 

5. A Mirror Frame Method for Harmonizing B-29 


Guns and Sights, OSRD 4277, OEMsr-474, Report 
16.1-53, Harvard University, Dec. 31, 1945. 

Div. 16-112.11-M2 

6. Technical Order No. 11-7(}A-1, Army Air Forces, 
1945. 

Chapter 7 

1. “The Production of Large Single Crystals of 
Lithium Fluoride,” Donald C. Stockbarger, Review 
of Scientific Instruments, Vol. 7, 1936, p. 133. 

2. Artificial Optical Fluorite, OSRD 4690, OEMsr-45, 

MIT, May 1, 1946. Div. 16-161.11-M2 

3. Report on Possible Natural Sources of High Grade 
Fluorite, OEMsr-563, Section D-3, NDRC Report 
308, Princeton University, Sept. 30, 1942. 

4. “Preparation of Crystals of Sparingly Soluble 
Salts,” W. C. Ferneliuw and K. D. Detling, 
Journal Chemical Education, March 1934, p. 176. 

5. National Bureau of Standards, Report No. IV— 
4/445-18/45, Feb. 23, 1946. 

6. “A Note on the Photographic Measurement of the 
Transmission of Fluorite in the Extreme Ultra¬ 
violet,” E. G. Schneider, The Physical Review, Vol. 
45, 1934, p. 152. 

7. National Bureau of Standards, Report TP 104934, 
Feb. 6, 1945. 

8. Optical Working of Synthetic Crystals, OSRD 

4506, OEMsr-1177, Report 16.1-74, Perkin-Elmer 
Corporation, April 1945. Div. 16-161.11-M3 

9. National Bureau of Standards, Test 445-48/44, 
June 30, 1944. 

Chapter 8 

1. Journal of the Optical Society of America, Vol. 29, 
1939, p. 291. 

2. French patent, 803, 169, Rohm and Haas, Sept. 24, 
1936. 

3. U. S. Patent 2,071,907, Sept. 23, 1937. 

4. Chemistry of Synthetic Resins, 2 Vols., Carleton 
Ellis, Reinhold Publishing Co., New York, 1935. 

5. Industrial Engineering Chemistry, Vol. 28, 1936, 

p. 1160. 

6. Optical Plastic Material, Synthesis, Fabrication 

and Instrument Design, OSRD 4417, OEMsr-70, 
Report 16.1-59, Polaroid Corporation, Feb. 1, 1945, 
Table 1, p. 68 et seq. Div. 16-161.1-M2 

6a. Ibid., p. 51. 

6b. Ibid., pp. 62-80. 

6c. Ibid., pp. 93-120. 

6d. Ibid., pp. 215-233. 

6e. Ibid., pp. 73-76. 

6f. Ibid., pp. 234-238. 

7. Hard Protective Coatings for Optical Plastics, 
Howard J. Lucas, L. Reed Brantley, and others, 
OSRD 4119, CIT, Nov. 30, 1944. Div. 16-161.12-M2 
7a. Ibid., pp. 14-15. 

7b. Ibid., pp. 20-30. 



602 


BIBLIOGRAPHY 


7c. Ibid., pp. 50-55. 

7d. Ibid., pp. 33-34. 

8. Tests of Optical Plastic Elements and of Reflex 

Sights, Max Petersen, OSRD 4788, OEMsr-203, 
Report 16.1-94, Massachusetts Institute of Tech¬ 
nology, Oct. 15, 1945. Div. 16-161.1-M3 

8a. Ibid., p. 6 et seq. 

9. Proceedings of the Physical Society of London, 
Selwyn, Vol. 55, 1943, p. 286. 

10. Bureau of Standards Circular C-428. 

11. Report of Naval Gun Factory [Washington, D. C.] 
to BuOrd., Letter JJ/Plastics (227) (T) dated 
Aug. 1, 1944. 

12. Journal of the Optical Society of America, L. A. 
Jones and R. N. Wolfe, Vol. 35, 1945, p. 559. 

Chapter 9 

1. Spherically Symmetrical Lenses and Associated 
Equipment for Wide Angle Aerial Photography, 
OSRD 6016, OEMsr-474, Report 16.1-118, Har¬ 
vard University, Nov. 30, 1945, pp. 53-103. 

Div. 16-111.11-M5 

2. Design and Development of 100-inch f/10 Anas- 
tigmat for Aerial Reconnaissance at Extreme 
Altitudes, OSRD 6019, OEMsr-474, Report 16.1- 
121, Harvard University. Dec. 31, 1945. 

Div. 16-111.11-M7 

3. Apochromatic Photographic Aerial Lenses and 
Other Optical Instruments Making Use of Syn¬ 
thetic Fluorite, OSRD 6020, OEMsr-474, Report 

16.1- 122, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M8 

4. Miscellaneous Development Work for Other OSRD 
Projects, OSRD 6026, OEMsr-474, Report 16.1- 
128, Harvard University, Dec. 31, 1945. 

Div. 16-180-M3 

5. Miscellaneous Projects for Instructional and Lab¬ 
oratory Purposes, OSRD 6027, OEMsr-474, Re¬ 
port 16.1-129, Harvard University, Dec. 31, 1945. 

Div. 16-180-M4 

6. Miscellaneous Projects Partially Completed, Theo¬ 
dore Dunham, Jr., OSRD 6028, OEMsr-474, Re¬ 
port 16.1-130, Harvard University, Dec. 31, 1945. 

Div. 16-101-M7 

7. The Optical Research Laboratory at Harvard, 
James G. Baker, OSRD 4740, OEMsr-474, Report 

16.1- 91, Harvard University, Dec. 31, 1945. 

Div. 16-101-M6 

8. Methods of Making Roof Prisms, OSRD 1073, 
Mount Wilson Observatory, Aug. 1, 1942. 

Div. 16-161.3-MI 

8a. Ibid., p. 6 et seq. 

9. Diamond Milling of Roof Prism Blanks, OSRD 
4735, OEMsr-101, Report 16.1-86, Mount Wilson 
Observatory, Mar. 31, 1945. Div. 16-161.3-M2 
9a. Ibid., pp. 21-26. 

10. ^Methods of Producing by Molding of High-Pre¬ 
cision Optical Parts of Glass, J. H. Webb and 


Loyd A. Jones, OEMsr-421, Progress Report 

16.1- 8, Eastman Kodak Company, Feb. 11, 1943. 

Div. 16-161.2-MI 

11. The Molding of Glass for Optical Purposes, OSRD 

4500, OEMsr-421, Report 16.1-68, Eastman Kodak 
Company, Sept. 13, 1945. Div. 16-161.2-M2 

12. The Mark 11* Illuminated Sight, Raymond W. 

Wengel, OSRD 6281, OEMsr-56, Problems 2492- 
GG1, 2492-GG2, and others, Division 7 Report to 
the Services 104, Development Dept., Eastman 
Kodak Company. Div. 7-111-M2 

13. Photoengraved Optical Reticle Research Project, 
OEMsr-293, NDRC Section D-3 Report, Edward 
Stern and Company, Inc., Philadelphia, Pa., Feb. 
28, 1942. 

14. An Investigation of Photographic Methods of Mak¬ 
ing Reticles, Richard M. Badger, William Shand, 
Jr., and others, OSRD 3219, OEMsr-389, Service 
Project NO-98, Report 16.1-34, California Insti¬ 
tute of Technology, Dec. 31, 1943. Div. 16-161.4-M3 

15. [Evaporated Films], NDCrc-118, Service Projects 
AC-11 and CE-27, Section D-3 Report, California 
Institute of Technology. 

16. [Evaporated Films], OEMsr-529, Service Projects 
AC-11 and CE-27, Section D-3 Report, Vard, Inc. 

17. The Evaporation of Thin Films , OSRD 4789, 

OEMsr-160, Report 16.1-95, University of Roch¬ 
ester, Mar. 1, 1945. Div. 16-161.5-M2 

Chapter 10 

1. Aids to Night Vision, OSRD 1482, OEMsr-160, 

Progress Report 16.1-23, University of Rochester, 
Mar. 1, 1943. Div. 16-121-MI 

la. Ibid., p. 9. 

2. Aids to Night Vision, III, OSRD 1709, OEMsr-160, 

Progress Report 16.1-28, University of Rochester, 
June 15, 1943. Div. 16-123-M2 

2a. Ibid., p. 5. 

3. Wide-Field Telescopes, OSRD 6033, OEMsr-160, 

Progress Report 16.1-112, University of Rochester, 
Oct. 8, 1945. Div. 16-121-M3 

4. Binocular Developments, OSRD 4114, OEMsr-579, 

Final Report 16.1-33, Bausch and Lomb Optical 
Company, Dec. 28, 1944. Div. 16-120-MI 

4a. Ibid., pp. 12-15. 

4b. Ibid., pp. 10-11. 

5. Miscellaneous Projects for Instructional and Lab¬ 
oratory Purposes, OSRD 6027, OEMsr-474, Report 

16.1- 129, Harvard University, Dec. 31, 1945. 

Div. 16-180-M4 

6. Design of Wide-Angle Telescopes for Tanks, 
OSRD 3888, OEMsr-160, Progress Report 16.1-45, 
University of Rochester, Dec. 1, 1943. 

Div. 16-122-MI 

6a. Ibid., pp. 8-10. 

7. Tank and Antitank Telescopes, Louis G. Henyey, 
Jesse L. Greenstein, and W. A. Hiltner, OSRD 




BIBLIOGRAPHY 


603 


4503, OEMsr-1078, Report 16.1-71, Yerkes Observ¬ 
atory, October 1945. Div. 16-122-M2 

7a. Ibid., pp. 22-24. 

7b. Ibid., pp. 27-42. 

7c. Ibid., pp. 43-51. 

8. Tank Periscope Binocular, T-9, OSRD 6031, 

OEMsr-160, Report 16.1-109, University of Roch¬ 
ester, Sept. 1, 1945. Div. 16-133-MI 

9. Optical Plastic Maternal Synthesis, Fabrication 
and Instrument Design, OSRD 4417, OEMsr-70, 
Report 16.1-59, Polaroid Corporation, Feb. 1, 1945. 

Div. 16-161.1-M2 

10. Development of a Precision Theodolite Telescope, 
OSRD 1848, OEMsr-385, Progress Report 16.1-31, 
University of Rochester, Aug. 15, 1943. 

Div. 16-141-MI 

11. Study of Submarine Periscope Design, Louis G. 

Henyey and Jesse L. Greenstein, OSRD 6130, 
OEMsr-1078, Final Report 16.1-152, Yerkes Ob¬ 
servatory, October 1945. Div. 16-131-M2 

12. Apochromatic Photographic Aerial Lenses and 
Other Optical Instruments Making Use of Syn¬ 
thetic Fluorite, OSRD 6020, OEMsr-474, Report 

16.1-122, Harvard University, Dec. 31, 1945. 

Div. 16-111.11-M8 

13. Unit Power Periscopes, Louis G. Henyey, Jesse L. 

Greenstein, and W. A. Hiltner, OSRD 4502, 
OEMsr-1078, Final Report 16.1-70, Yerkes Ob¬ 
servatory, September 1945. Div. 16-130-MI 

13a. Ibid., pp. 5-12. 

13b. Ibid., pp. 20-30. 

14. Development of an Aircraft Periscope, Mark 35, 

Model 0 and of an Experimental Range Finder for 
Antisubmarine Aerial Patrol Planes, OSRD 6022, 
OEMsr-474, Report 16.1-124, Harvard University, 
Dec. 31, 1945. Div. 16-132-M4 

15. P-80 Periscope Design, OSRD 6037, OEMsr-160, 

Report 16.1-116, University of Rochester, Oct. 1, 

1945. Div. 16-132-M2 

Chapter 11 

1. Miscellaneous Optical Designs, Louis G. Henyey 
and Jesse L. Greenstein, OSRD 4505, OEMsr-474, 
Report 16.1-73, Yerkes Observatory, October 1945. 

Div. 16-180-M2 

la. Ibid., p. 35 et seq. 

2. Smithsonian Physical Tables, Frederick E. Fowle, 
Vol. 88 Smithsonian Miscellaneous Collection, 
Smithsonian Institution of Washington, D. C., 8th 
Edition, 1933, p. 330. 

3. Miscellaneous Development Work for Other OSRD 
Projects, OSRD 6026, OEMsr-474, Report 16.1-128, 
Harvard University, Dec. 31, 1945. Div. 16-180-M3 

Chapter 12 

1. Reflex Sights, Louis G. Henyey, Jesse L. Green¬ 
stein, and W. A. Hiltner, OSRD 4501, OEMsr- 


1078, Final Report 16.1-69, Yerkes Observatory, 
Apr. 30, 1945, p. 2. Div. 16-112.2-M2 

la. Ibid., p. 23. 

lb. Ibid., p. 9. 

lc. Ibid., p. 34. 

2. Reflex Sights, OSRD 4736, OEMsr-101, Report 

16.1- 87, Mouiit Wilson Observatory, Mar. 20, 1945, 

p. 15. Div. 16-112.2-MI 

2a. Ibid., p. 1. 

2b. Ibid., p. 11. 

3. The Mark Illuminated Sight, Raymond W. 
Wengel, OSRD 6281, Problem Nos. 2492-GG1, 
2492-GG2, and others, Division 7 Report to the 
Services 104, Eastman Kodak Co., 1945. 

Div. 7-111-M2 

4. Development of Special Reflex Gun Sights, OSRD 
6032, OEMsr-60, Report 16.1-110, University of 
Rochester, Sept. 27, 1945, p. 14. Div. 16-112.2-M3 
4a. Ibid., p. 3. 

4b. Ibid., p. 8. 

4c. Ibid., p. 11. 

5. Optical Plastic Material, Syyithesis, Fabrication 

and Instrument Design, OSRD 4417, OEMsr-70, 
Report 16.1-59, Polaroid Corporation, Feb. 1, 1945, 
pp. 190, 194. Div. 16-161.1-M2 

Chapter 13 

1. Stadiameters, OSRD 6035, OEMsr-160, Report 

16.1- 114, University of Rochester, Oct. 2, 1945. 

Div. 16-112.4-MI 

2. Wide Field Telescopes, OSRD 6033, OEMsr-160, 

Report 16.1-112, University of Rochester, Oct. 8, 
1945. Div. 16-121-M3 

Chapter 14 

1. Aids to Night Vision, NDRC Interim Report, In¬ 

stitute of Optics, University of Rochester, Mar. 1, 
1943. Div. 16-170-MI, M2 

2. Anti-Oscillation Mounted Night Sights, NDRC 

Interim Report, Institute of Optics, University of 
Rochester, Mar. 1, 1943. Div. 16-123-MI 

3. Anti-Oscillation Mounted High Power Telescope, 

OSRD 6036, OEMsr-160, Report 16.1-115, Insti¬ 
tute of Optics, University of Rochester, Oct. 1, 
1945. Div. 16-123-M5 

4. Antivibration-Mounted Binocular and Monocular, 
Joseph Mihalyi, H. J. Hood, and F. M. Bishop, 
OSRD 4444, OEMsr-1090, Final Report 63, East¬ 
man Kodak Company, Feb. 10, 1945. 

Div. 16-123-M3 

5. Improvement of the Definition in Aerial Photog¬ 
raphy, OEMsr-392, NDRC Interim Report, Kodak 
Research Laboratories, Nov. 6, 1942. 

5a. Ibid., p. 12. 

5b. Ibid., p. 13. 

5c. Ibid., p. 7. 




604 


BIBLIOGRAPHY 


6. The Eastman Anti-Oscillation Binocular Mount, 
Section 16.1 NDRC Report, July 19, 1943. 

7. Binocular Anti-Vibration Mount, Kodak Research 
Laboratories, Aug. 27, 1945. 

7a. Ibid., Fig. 10. 

7b. Ibid., p. 5. 

8. Anti-Vibration Mounts for Binoculars, OSRD 6126, 

OEMsr-392, Report 16.1-148, Eastman Kodak Com¬ 
pany, Nov. 20, 1945. Div. 16-123-M7 

8a. Ibid., p. 10. 

9. Periscopic Scanning Device, OSRD 4182, OEMsr- 
617, Final Report 16.1-55, Technicolor Motion 
Picture Corporation, Feb. 28, 1945. Div. 16-132-MI 
9a. Ibid., p. 4. 

9b. Ibid., p. 8. 

10. Anti-Oscillation Mount for Binoculars, OSRD 
4183, OEMsr-617, Final Report 16.1-56, Techni¬ 
color Motion Picture Corporation, Feb. 28, 1945. 

Div. 16-123-M4 

10a. Ibid., p. 2. 

10b. Ibid., p. 4. 

10c. Ibid., p. 28. 

11. Anti-Vibration Mounting of Airplane Instruments, 

S. J. Zand and L. N. Swisher, Bulletin 100 C, 
Lord Manufacturing Company, Erie, Pennsylvania. 

12. Airplane Photography, H. E. Ives, J. B. Lippincott 
Co., 1920. 

13. Report on Development of Aids to Night Vision, 

OSRD 1479, OEMsr-160, Progress Report 16.1-14, 
Institute of Optics, University of Rochester, Feb. 
1, 1942. Div. 16-123-MI 

13a. Ibid., App. IX. 

13b. Ibid., App. XV. 

14. Anti-0scillatioyi Mount Tests, OSRD 6034, OEMsr- 
160, Report 16.1-113, Institute of Optics, Uni¬ 
versity of Rochester, Oct. 2, 1945. Div. 16-123-M6 
14a. Ibid., App. II, Fig. 1. 

14b. Ibid., App. III. 

14c. Ibid., App. V. 

14d. Ibid., App. VI. 

15. Mechanical Vibrations, J. P. Den Hartog, 2nd 
Edition, McGraw-Hill Book Company, 1940. 

15a. Ibid., pp. 85-88. 

15b. Ibid., pp. 407-411. 

16. Vibration Problems in Engineering, S. Timoshenko, 
2nd Edition, D. Van Nostrand Company, 1937. 

Chapter 15 

1. The 0.1-mil Recording Phototheodolite, J. Leslie 

Quigley, OSRD 5921, OEMsr-503, Problem DD- 
2517, Final Report 16.1-107, Eastman Kodak Com¬ 
pany, Dec. 31, 1945. Div. 16-141-M2 

la. Ibid., pp. 28-32. 

2. Refractive Errors in Observing an Object in the 
Earth’s Atmosphere, L. Charles Hutchinson, AMP 
Working Paper 15 [AMG-C], Oct. 31, 1945. 

AMP-502.1-M35 


Chapter 16 

1. Optical Scannmg Devices, Walter S. Adams, Theo¬ 

dore Dunham, Jr., and others, OSRD 1420. 
OEMsr-115, Final Report 16.1-13, Mount Wilson 
Observatory, July 31, 1942. Div. 16-143-MI 

la. Ibid., p. 8. 

lb. Ibid., p. 13. 

lc. Ibid., p. 20. 

l d. Ibid., p. 6. 

le. Ibid., p. 18. 

2. Miscellaneous Optical Designs, Louis G. Henyey 
and Jesse L. Greenstein, OSRD 4505, OEMsr-1078. 
Report 16.1-73, Yerkes Observatory, October 1945. 

Div. 16-180-M2 

3. Interim Report, Mount Wilson Observatory. 
OEMsr-115, 1942. 

4. Wide Field Telescopes, OSRD 6033, OEMsr-160. 

Report 16.1-112, University of Rochester, Oct. 8. 
1945. Div. 16-121-M3 

Chapter 17 

1. High-Speed Antiglare Shutter Production Design . 
Joseph Mihalyi and D. C. Harvey, OSRD 4446, 
OEMsr-707, Problem DD-2510D, Final Report 

16.1- 64, Eastman Kodak Company, Sept. 28, 1945. 

Div. 16-144-M2 

2. Aids to Night Vision, Anti-Oscillation Mounted 
Night Sights, OSRD 1479, OEMsr-160, Progress 
Report 16.1-14, University of Rochester, Mar. 1. 

1943. Div. 16-123-MI 

3. Antivibration Mounts for Binoculars, OSRD 6126, 

OEMsr-392, Report 16.1-148, Eastman Kodak 
Company, Nov. 20, 1945. Div. 16-123-M7 

4. Antivibration Mounted Binocular and Monocular, 

Joseph Mihalyi, H. J. Hood, and F. M. Bishop, 
OSRD 4444, OEMsr-1090, Problem DD-1623, 

Final Report 16.1-63, Eastman Kodak Company, 
Feb. 10, 1945. Div. 16-123-M3 

5. Development of Photoelectric Control Apparatus 
for High-Speed Antiglare Shutter to Protect 
Night Vision of Pilots from Enemy Flares, Seville 
Chapman, OSRD 6006, OEMsr-100, Final Report 

16.1- 133, Stanford University, Sept. 30, 1945. 

Div. 16-144-M3 

5a. Ibid., p. 17 et seq. 

Chapter 18 

1. Rapid Processing Equipment for Periscope Pho¬ 
tography, D. C. Harvey and Joseph L. Boon. 
OSRD 4551, OEMsr-622, Problem DD-2518A, Re¬ 
port 16.1-65, Eastman Kodak Company, Dec. 21. 

1944. Div. 16-131-MI 

Chapter 19 

1. Two Star Navigating Instrument, OSRD 5645, 
OEMsr-101, Report 16.1-105, Mount Wilson Ob¬ 
servatory, Sept. 25, 1945. Div. 16-142-MI 





OSRD APPOINTEES 
DIVISION 16 


Chief 

George R. Harrison 


Paul E. Klopsteg 

Deputy Chiefs 

Richard C. Lord 

Herbert E. Ives 

Consultants 

F. E. Tuttle 

H. R. Clark 

Technical Aides 

Richard C. Lord 

J. S. Coleman 


H. K. Stephenson 

0. S. Duffendack 

Members 

Arthur C. Hardy 

Theodore Dunham, Jr. 


Herbert E. Ives 

E. A. Eckhardt 


Paul E. Klopsteg 

Harvey Fletcher 


Brian O’Brien 

W. E. Forsythe 


F. E. Tuttle 


SECTION 16.1 

Chief 

Theodore Dunham, Jr. 

G. W. Morey 

Consultants 

H. F. Mark 

F. L. Jones 


H. F. Weaver 

Lillian Elveback 

Technical Aides 

S. W. McCuskey 

Ira S. Bowen 

H. F. Weaver 

Members 

R. R. McMath 

W. V. Houston 


G. W. Morey 

W. R. Brode 

F. E. Wright 

section 16.2 

Chief 

Brian O’Brien 

Consultants 

V. K. Zworykin 


Technical Aide 

Charles E. Waring 


Members 

W. E. Forsythe Julian H. Webb 

Harvey E. White 


OSRD APPOINTEES 


Lewis Knudson 


S. Q. Duntley 


Edwin G. Boring 


W. L. Enfield 


H. S. Bull 


Alan C. Bemis 
Saul Dushman 


section 16.3 

Chief 

Arthur C. Hardy 

Consultants 

Edward R. Schwarz 

Technical Aides 

Ernest T. Larson 

Members 

F. C. Whitmore 

section 16.4 

Chief 

0. S. Duffendack 

Considtants 

W. H. Radford 

Technical Aides 
James S. Owens 
Members 


W. E. Forsythe 


W. E. Forsythe 


E. Q. Adams 


William Herriott 


D. W. Bronk 
A. C. Hardy 
Theodore Matson 
A. H. Pfund 


section 16.5 

Chiefs 

Deputy Chiefs 

Consultants 

Technical Aides 
Val E. Sauerwein 
Members 


V. K. Zworykin 


Parry H. Moon 


Arthur W. Kenney 


L. A. Jones 


H. G. Houghton, Jr. 


Winston L. Hole 

H. G. Houghton, Jr. 
George A. Morton 

Herbert E. Ives 

Brian O’Brien 

A. C. Downes 

John T. Remey 

W. B. Rayton 
A. B. Simmons 
G. F. A. Stutz 
Harvey E. White 


606 


CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACTS 


Contract No. 

Contractor 

Subject 

OEMsr-1039 

Aero Service Corporation 

/ 

Aerial mapping methods. 

OEMsr-579 

Bausch & Lomb 

Binoculars. 

OEMsr-1229 

Brown University 

Binocular testing. 

OEMsr-389 

California Institute of Technology 

Reticles. 

OEMsr-657 

California Institute of Technology 

Hardening plastics. 

OEMsr-1058 

Dartmouth College 

Binocular testing. 

OEMsr-421 

Eastman Kodak Company 

Molded glass. 

OEMsr-622 

Eastman Kodak Company 

Shutter development and periscope camera. 

OEMsr-1090 

Eastman Kodak Company 

Night glasses. 

OEMsr-392 

Eastman Kodak Company 

Aerial cameras. 

OEMsr-503 

Eastman Kodak Company 

Theodolites. 

OEMsr-707 

Eastman Kodak Company 

Anti-glare shutter. 

OEMsr-1087 

M. Flood and Associates 

Mapping methods. 

OEMsr-474 

Harvard College 

Optical design. 

OEMsr-203 

Massachusetts Institute of Technology 

Testing. 

OEMsr-45 

Massachusetts Institute of Technology 

Optical crystals. 

OEMsr-101 

Mount Wilson Observatory 

General optical problems. 

OEMsr-1197 

Pennsylvania State College 

Inspection methods. 

OEMsr-1177 

Perkin-Elmer Corporation 

Crystal lenses. 

OEMsr-70 

Polaroid Corporation 

Optical plastics. 

OEMsr-lOO 

Stanford University 

Glare protector. 

OEMsr-710 

Technicolor Motion Picture Corporation 

Langer shutter. 

OEMsr-617 

Technicolor Motion Picture Corporation 

Scanning devices. 

OEMsr-1078 

University of Chicago (Yerkes) 

Design of optical instruments. 

OEMsr-1115 

University of Michigan 

Stabilizing devices. 

OEMsr-1245 

University of Michigan 

Exposure meter. 

OEMsr-205 

University of Pennsylvania 

Tropical deterioration. 

OEMsr-1228 

University of Pennsylvania 

Binocular performance. 

OEMsr-871 

University of Pittsburgh 

Clouding of optical glass. 

OEMsr-160 

University of Rochester 

Night vision. 






SERVICE PROJECT NUMBERS 


The projects listed below were transmitted to the Executive 
Secretary, NDRC, from the War or Navy Department through 
either the War Department Liaison Officer for NDRC or the 
Office of Research and Inventions (formerly the Coordinator of 
Research and Development), Navy Department. 


Service 

Project 

Number 

Subject 

AC-11 

AC-26 

AC-29 

AC-88 

AC-99 

AC-113 

AC-114 

AC-127 

AC-131 

Optical glass substitutes. 

Night glasses and night sights. 

Photographic equipment research. 

Testing of aerial photographic equipment. 

Shock-proof gun camera mounts. 

Quick developing camera (assigned to Section 16.5). 

Location of tactical targets by air search. 

Aircraft armament harmonization studies. 

Development of an optical viewfinder for use in aerial photog¬ 
raphy. 

CE-8 

Enlarging field of view of night glasses without sacrifice of 
power and without increase in weight. 

CE-9 

CE-21 

CE-27 

NA-124 

NA-140 

NA-176 

NA-200 

NO-97 

Quantity producing of optical glass. 

Telescope for one-second theodolite. 

Elimination of reflection from glass surfaces. 

Development of photogrammetric instruments and techniques. 
ASW project combining searchlight and night glasses. 
Development of photographic lens shutter. 

Wide angle projection system (dome trainer). 

Application of nonreflecting films to the surfaces of optical 
instruments. 

NO-98 

NO-103 

NO-127 

NO-210 

New methods of reticle manufacture. 

Night interceptor sight. 

Pressure proof binoculars. 

Night binocular design study. 

Extension: Visibility data of infrared sources. 

NR-111 

NS-105 

Design and construction of periscopic aircraft sight. 
Anti-vibration mounted double system binoculars. 

Amendment: Development of anti-vibration mounting for 
shipborne binoculars for lookout purposes. 

NS-236 

NS-242 

OD-43 

OD-48 

Development of new eyepiece design for 7x50 binoculars. 
Periscope camera with quick-developing film. 

Investigation of film formation on optical surfaces. 
Phototheodolites for aerial position finding. 

Extension: Development of precision timing equipment for 
fire control data recorders. 

OD-116 

OD-119 

OD-128 

Anti-oscillation mount for binoculars in tanks. 

Optical fire control equipment (telescopes T-44 and T-76). 
Development and manufacture of pilot models of telescope 
T-108. 





SERVICE PROJECT NUMBERS (Continued) 


Service 

Project 

Number Subject 


OD-129 

Development and construction of pilot models of binoculars of 
the periscopic type. 

OD-138 

Development of improved methods of testing and inspecting 
optical instruments. 

Extension: Study of the effects of striae on the perform¬ 
ance of optical instruments. 

OD-146 

Study of sighting devices for computing sight M7 and M45. 

OD-149 

Design of telescopes T-118 and T-119. 

OD-180 

Telescope T-136. 

16.1-1 

Studies of stabilizing devices. 

16.1-2 

Testing of optical systems. 

16.1-3 

Stadiameters. 

16.1-4 

Scanning devices. 

16.1-5 

Reflex sights. 

16.1-6 

Vision studies. 

16.1-7 

Submarine periscope design and photographic performance. 

16.1-8 

Anti-glare shutter. 

16.1-9 

Panofsky stabilizer. 

16.1-10 

Foxhole periscope. 

16.1-11 

Panofsky stabilizer. 


609 








INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. 

For access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


A-8 camera mount 

effect on resolution, 160 
flight tests, 113-114 
performance, 159 
test curves, 112-113 
A-ll camera mount 

angular velocity during motion, 
159 

effect on resolution, 160 
performance, 159 
test curves, 112-113 
A A (antiaircraft) guns, reflex 
sight, 499 

AAF N-9 gunsight, 490-491 
Aberration modifiers 
function, 256 

recommendations, 261-262 
Aberrations in optical instruments 
see Astigmatism; Chromatic ab¬ 
erration; Coma; Field cur¬ 
vature in lenses; Spheri¬ 
cal aberration; Vignetting 
in aerial lenses; Zone errors 
Abrader for testing plastics, 371- 
372 

Abrasion number, 373 
Achromatic lenses 
dispersive power, 349 
meniscus lenses, 64, 93 
use of styrene, 347 
Aerial photography 

see Cameras, aerial; Mapping 
methods, aerial 
Aero Service Corporation 

aerial mapping methods, 175-203 
pinhole rectifying camera, 182- 
183,186-191 

variable ratio printer, 196 
Aero-Ektar lenses, 59-61 
7-in., 60 
24-in., 155, 167 
astigmatism, 60-61 
comparison with polaroid lenses, 
59-60 

f/2.5, 5x5; 339 

for periscope photography, 575 
resolution, 78-79, 153-155 
spherical aberration, 339 
tests, 65 

wedge patterns, 70 
Aerostigmat lens, 65 
Air-bellows damper for camera 
mounts, 109-110 


Aircraft guns and sights, harmo¬ 
nization 

see B-29 guns and sights, har¬ 
monization 

Aircraft periscopes, 464-470 
aberrations, 465 
mechanical design, 466-468 
optical design, 464-466 
P-51; 469 
P-80; 469-470 
scanning prism, 464 
unit power periscope, 464-469 
use of Mark 23 bombsight, 468- 
469 

Aircraft scanning chair, 560, 563 
Akeley phototheodolite, 529 
Alkaline halides, 337-338 
disadvantages, 338 
for color correction, 340 
optical constants, 337 
recommendations, 145 
Allyl methacrylate (cross-linking 
plastic), 350-352 
evaluation, 351-352 
hardness measurements, 363 
polymerization, 352 
synthesis, 350-351 

Altazimuth four-mirror scanner, 
559-560 

Altazimuth two-mirror scanner 
applications, 551, 563 
method of stabilizing, 555 
reduction of eye fatigue, 555 
rotation of field of view, 554-555 
rubber eyecup, 554-555 
Altimeter for photogrammetric 
sounding, 178 
Aluminum films, 432-433 
Aluminum oxide for coating plas¬ 
tics, 374 

^-amino-ethyl methacrylate, 348 
AN (abrasion number), 373 
Anastigmat lenses 
12-in., f/4.5, 9x18; 64 
12-in., f/5, 9x9; 57 
36-in., f/8, 9x18; 56-57 
100-in., f/10, 9x18; 50-52 
tests, 65, 70 
zone errors, 51-52 

Annealing furnace for manufactur¬ 
ing fluorite crystals 


disadvantages, 324 
heat source, 324 
operation, 324 
structural strain, 323-324 
Antiaircraft guns, reflex sight, 499 
Antiglare shutters for night binoc¬ 
ulars 

see Shutters for night binoculars, 
antiglare 

Antioscillation mounts, 510-527 
application to boresighting, 521 
ball-cone and rubber shell- 
mounts, 513-515, 526-527 
comparison of designs, 515-516 
field performance, 521-522 
for two-mirror Schmidt camera, 
95 

gimbal mounts, 510, 512-513, 526- 
527 

Lord shock absorbers, 95 
method of suspension, 512 
recommendations, 526-527 
requirements, 511-512 
vibrations, 511-512 
Antioscillation mounts, damping 
methods 
evaluation, 526 
frictional damping, 522-526 
isopentane, 95 
purpose, 520-523 
sylphon bellows, 95 
transmissibility, 522-523 
viscous damping, 522-526 
Antioscillation mounts, laboratory 
tests, 516-522 
II-c binocular mount, 519 
angular amplitude of optical sys¬ 
tem, 517-518 
objectives, 516 
performance curves, 519 
range of frequencies, 518 
shake tables, 516-517 
simultaneous impression of linear 
and angular vibrations, 518- 

519 

source of vibrations in aircraft, 

520 

testing method, 517-519 
transmissibility, 520 
Antitank telescopes 

eccentric collective, 451-452 
mirror erectors, 451-452 
recommendations, 471 


611 


612 


INDEX 


T-108; 365-368, 376-381 
T-118; 365, 451-452 
Antivibration mounts, 105-130 
center-of-gravity mount, 105-106, 
117-119,145,160-162 
Eastman-NDRC, 113-114, 145, 

156,160 

effect on resolution, 156-157, 160 
effectiveness, 511 
gimbal, 109-111,128 
ground-speed compensation 
mount, 111, 124-126 
gun camera mounts, 126-130 
laboratory tests, 112-113 
multiple mount for F-5E air¬ 
craft, 111-112 

pantagraph mount for T-94 gun- 
sight, 489-490 

recommendations, 145, 171-173 
stabilized mounts, 119-124 
theory, 105-109 

transmissibility of magnification 
factor, 511 

use of intervalometer, 124 
Antivibration mounts, flight tests, 
113-117 
A-8; 113-114 

Eastman-NDRC mount, 113-114 
effect on resolution, 115 
effectiveness, 116-117 
Apochromatic folded collimator, 
335-336 

Apochromatic lenses, 52-57, 333-337 
1.8-in., f/20; 335 
36-in., f/8; 52-55 
36-in., f /11 telephoto, 63-64, 69 
48-in., f/8, 9x9; 54-55 
100-in., f/8, 9x18; 56-57 
aerial tests 54-55 
barium fluoride, 63-64 
dialytique objective, 335 
fluorite, 334-335 

folded telescope and collimator, 
312, 335-336, 576 
resolution, 153-155 
telephoto, 70 
tests, 65, 70 
triplet objective, 336 
use of calcium fluoride, 63-64 
use of fluorite, 52, 333-335 
Apo-periscope lens, 336-337 
Argus Corporation, reflex sights, 
494 

Armco ingot iron molds for glass 
molding, 410 

Armor glass, use in reflex gun- 
sights, 483 


Artificial fluorite crystals 

see Fluorite crystals, synthetic 
ASA (artificial sky apparatus), 
256-258 

effect of coated optics, 257-258 
effect of scattered light on reso¬ 
lution, 205 

effect of striae, 257-258 
Askania phototheodolite, 529 
Astigmatism, 53-54 

advantageous factors, 38 
effect on KDC efficiency, 251-253 
formula, 251-253 
measurement, 244, 247, 366 
modifier, 249, 251-253 
optical design requirements, 36 
prediction of interferometer pat¬ 
tern, 251-253 
Rayleigh limit, 250-253 
reduction by use of anastigmat 
lens, 50-52, 56-57, 64-65, 70 
tests, 244, 247 

Automatic focusing of aerial cam¬ 
eras, 45-46,164 
Axial color, 366 

B-29 fire-control system, stadiam- 
eter 

double-image, 508 
mirror coatings, 508-509 
recommendations, 509 
specifications, 508 

B-29 guns and sights, harmoniza¬ 
tion, 289-311 

causes of errors, 308-309 
effect of plexiglas, 308 
effect of temperature and wind, 
309 

gun cameras, 308-309 
Mark II wire method, 295-299 
Mark III wire method, 299-302 
middle distance yard method, 
289, 290 

mirror boresight method, 302- 
303, 308 

mirror frame method, 290, 304- 
307 

plane in flight, 308-309 
prism method, 290-295 
recommendations for future re¬ 
search, 306-307, 309-311 
requirements for field harmoniz¬ 
ation, 289-290 

suggested improvements, 309 
use of binoculars, 299-300 
use of periscopes, 305 
use of plane mirrors, 289 
Bakelite, surface hardness, 373 


Ball-cone antioscillation mounts 
damping system, 513-514 
recommendations, 526-527 
Barium crown, mold pressing, 414 
Barium fluoride 

apochromatic lens, 63-64 
recommendations for future re¬ 
search, 145, 340-341 
synthetic crystals, 332 
transparency, 338 
Barnesite for glass polishing, 391 
Bartol camera shutter, 139 
Bausch and Lomb 40-in., f/8 tele¬ 
photo lens, 67-69 
astigmatism and field curvature, 
67-68 

chromatic aberration, 67-68 
internal collimation, 67 
resolving power, 69 
spherical aberration, 68-69 
use of visual resolving power for 
tests, 67-68 

Bausch and Lomb Optical Company 
7x50 binocular, 442-444, 452 
10x50 binocular with 7-degree 
field, 442 

Erfle telescopic eyepiece, 437 
fixed-focus binoculars, 452, 455 
1-224 fluorite crystal, 325-326 
reticle production, 418 
Beam splitters, polarizing, 429-432 
angle of polarization, 429 
conditions for polarization, 430- 
431 

performance, 432 
zinc sulfide and cryolite layers, 
430 

Bell and Howell Corporation, reflex 
sights, 491 
Benzoyl peroxide 

use in polymerizing cyclohexyl- 
methacrylate, 355 
use in polymerizing methyl meth¬ 
acrylate, 343 

Beryllium-aluminum alloy for shut¬ 
ter blades, 132 

Between-the-lens camera shutter, 
163 

Bichromated glue-relief process of 
making reticles, 418, 419 
Bifocal bipower tank telescope, 449- 
450 

Binoculars 
6x,510 

7x50; 437, 442-444, 452 
8x56; 552 
10x50; 442 

antioscillation mounts, 510-527 
disadvantages for lookouts, 551 



INDEX 


613 


eye guards, 282-283, 286, 287 
for double dove prism scanner, 
552 

KDC efficiency, 233-234 
Mark 7 offset wedge attachment, 
365, 366 
resolution, 216 
retinal illumination, 278 
use in harmonizing guns and 
sights, 299-300 

Binoculars, general types, 452-455 
fixed-focus, 452, 455 
Galilean, 367 
periscopic, 452-453 
pressure-proof binoculars, 455 
submarine, 452 
tank, 452-453 
wide-angle, 442-444 
Zeiss, 437, 442 

Binoculars, specifications, 218-223 
cojlimation, 220-221 
exit pupil, 218-219 
image definition, 216 
inspection procedure, 217 
light transmission, 219 
magnification, 218 
reticles, 221 
shock test, 220-221 
size of true field, 218-219 
weatherproofing, 223 

Binoculars for night vision, 263- 
288 

7x50; 437, 442-444, 452 
antiglare shutters, 565-571 
Brown University tests, 271-276 
Dartmouth College tests, 268-271 
dummy binocular, 443-444 
Eastman binoculars, 565-566 
factors affecting visual perform¬ 
ance, 283-286 

recommendations for future re¬ 
search, 286-288, 571 
University of Pennsylvania tests, 
276-282 

Binoculars for night vision, errors 
alignment, 279-280 
interpupillary settings, 277 
observation errors, 269 

Binoculars for night vision, tests, 
268-282 

binocular gain, 273-274 
brightness levels, 268, 270-271 
brightness loss, 273-274 
comparison with naked-eye ob¬ 
servations, 270-271, 279 
correction for guessing, 272 
effect of angular motion, 275 
exit pupil variation, 273, 277, 
283-284 

eye coordination, 275 


eye guards, 282-283, 286, 287 
folded binoculars, 283, 286 
hand-held binoculars, 275, 279- 
282, 284 
head-rests, 275 

high - magnification binoculars, 
281 

limiting pupillary aperture, 278- 
279 

low brightness photometer, 272 
magnification tests, 271, 273-275 
observations on a moving vessel, 
279-280 

observing conditions, 264-268, 
271-272 
range, 283-284 
retinal illumination, 278 
targets, 266, 268, 271-272, 274- 
275 

threshold of visibility, 266-267, 
274, 285 

time of exposure, 275 
types tested, 268 
variations in design, 275, 286 
visual physiology, 265, 276-277 
Biotar lens, 60 
Birefringence 

fluorite crystals, 327 
optical glass, 362-363 
Blue cane glass, molding process, 
413-416 

Bombsight (Mark 23), 468-469 
Boresighting telescopes, 309 
Bornyl methacrylate, 349 
Borosilicate glass, molding process, 
414 

Boston University Optical Research 
Laboratory 
camera mounts, 160 
resolution of aerial photographs, 
95 

Bowen reflex gunsight, 485-487, 
494-498 

advantages, 495-496, 498 
collimator, 486 
disadvantages, 496-498 
for AT-6 aircraft, 494-496 
for P-51 B aircraft, 496-498 
parallactic range, 487 
recommendations, 504 
reticle speed ring, 495-496 
use in lead-computing sights, 498 
Brewster angle (reflection of light), 
485 

Brightness level in binoculars, 268, 
270-271, 273-274 
British research 
aerial lenses, 24 
collimation of binoculars, 220 


colloidal-silver process of making 
reticles, 418, 419 

lead-sulfide process of making 
reticles, 418 

silver-line process of making ret¬ 
icles, 418, 419 

Brock optical rectifiers, 196, 199 
Brown University 

binocular tests, 271-276 
eyepieces for 7x50 binoculars, 
442 

Bubbles, detection in optical glass. 
209 

Buckbee-Mears process of reticle 
production, 418, 420 

Calcium fluoride 

for apochromatic lenses, 63-64 
for low-reflection films, 425 
Calcium fluoride crystals 

see Fluorite crystals, synthetic 
Calcium-sodium silicate, molding 
process, 413-416 

California Institute of Technology 
louvre shutters, 99-100 
photographic methods of reticle 
production, 417-421 
Camera mounts 

see Antioscillation mounts; Anti¬ 
vibration mounts 
Camera shutters 

see Shutters for aerial cameras 
Cameras, aerial, 1-277 

see also Mapping methods, aerial 
antivibration mounts, 104-130 
aperture, 32 

cold chamber for tests, 143-144 
correcting plates, 147 
exposure time, 91, 141, 163-164 
film, 91-92 

film-flatness tester, 141-142, 165 
focus checker, 140 
ground-sweep mechanism, 97 
image movement, 156, 171 
lens, 23-92 

recommendations, 144-146, 171. 
173-174 

resolution in aerial photography, 
147-174 

shutters, 99-100,130-140, 145,163 
stabilizer, 144 
trimetrogon, 177,192 
use of gyroscope, 144 
vibration of aircraft, 156-164 
Cameras, aerial, types 

24-in. standard, 132-133, 153, 155, 
159,173 

for water depth determination, 
200-202 

K-18; 55-56, 62-64 



614 


INDEX 


lens rectifying, 188 
pinhole rectifying, 182-188, 186- 
187,191 

point-light-source, 186-187 
ratio, 196 

rectifying, 186-190,195-197 
Schmidt, 92-104,147, 360 
Sonne stereostrip, 200-201 
Cameras for periscope photog¬ 
raphy, 572-573 

Cameras for phototheodolites 
camera drive, 530 
Eastman camera mechanism, 
533-536, 540 

exposure time, 530, 533-536 
film size and shrinkage, 530 
frequency of picture taking, 530 
lens, 529 

mounting, 529-530 
self-energizing clutch, 533-536 
Carnegie Institution of Washington 
see Mount Wilson Observatory 
Cassegrain lens system, 539 
Center-of-gravity camera mount, 
117-119 

cancellation of shutter recoil ef¬ 
fects, 160 

effect on resolution, 160-162 
flight tests, 118 

recommendations, 118-119, 145 
reduction of translational vibra¬ 
tion, 118 
theory, 105-106 

Chicago Aerial Surveys, 198-199 
Chiolite for low-reflection films, 425 
CHM 

see Cyclohexylmethacrylate 
Chromatic aberration 

chromatic difference of magnifi¬ 
cation and distortion, 35 
definition, 35 
Gaussian point, 35 
longitudinal color, 53 
measurement, 244, 247-248 
primary spectrum, 35 
reduction by use of alkaline 
halides, 340 

reduction by use of apochromatic 
lens, 52-57, 333-337 
Chromium films, 433 
Coated optics 

effect on scattering light, 257- 
258, 262 

lithium fluoride, 268 
nitrogen dioxide, 348, 375 
optically neutral paints, 149 
plastics, 370-375 

use of silicon tetrachloride, 371- 
375 

Cobb low-contrast chart, 79 


Cold chamber for camera tests, 143- 
144 

Collimators 
diagrams, 588 

folded apochromatic, 335-336 
for KDC apparatus, 226, 229 
for testing lenses, 140, 164-165 
glass-fluorite, 576 
high-resolution projection lens, 
474 

inspection of reticles, 222 
long-focus lenses, 444 
measurement of focal length in 
lenses, 211 
portable, 164-165 
use of fluorite crystals, 312 
Colloidal-silver process of making 
reticles 

evaluation, 419, 420 
method, 418 
Color aberrations 

see Chromatic aberration 
Color photography 

36-in., f/11, 9x18 apochromatic 
lens, 63-64 

disadvantage of telephoto lens, 
36-37 

fluorite lens, 54-55 
recommendations, 145 
Coma (lens aberration) 
effect of vignetting, 37 
effect on KDC efficiency, 254 
measurement, 366 
modifier, 249, 253-256 
Rayleigh limit, 250-251, 254 
varying as fourth power of aper¬ 
ture, 33-34, 38 

varying as square of aperture, 
33-34, 37-38 

Condenser lens, plastic, 474-475 
Cone vision, 276 
Contact film printing, 63 
Contrast in aerial photographs 
see also Resolution in aerial pho¬ 
tography 

effect of haze, 29-31 
effect of perfect and imperfect 
lenses, 31 

microscopic contrast, 28, 91, 171, 
173 

recommendations for future re¬ 
search, 91,171 
requirements, 24-25 
Cooke lenses 
Aviar, 65 

triplet objective, 62, 437, 444-447 
Copper films, 433 
Coronagraphic triplet objective 
design, 475-476 
spectrographic use, 475 


use of quartz monochromator, 
475 

Correcting plates 

for aerial cameras, 147 
for periscopes, 459-462 
Coulomb damping 

see Frictional damping of optical 
systems 

Crabbing angle of aircraft, 115 
Cross hairs 
see Reticles 
Cross-linking plastics 

allyl methacrylate, 350-352, 363 
p-divinyl benzene, 350 
ethylene dimethacrylate, 349-351, 
363, 373 
hardness, 370 
properties, 349-350 
storage, 350 
synthesis, 349-351 

Crown glass lens, 100-in., f/8, .9x18 
apochromatic, 56-57 
Cryolite 

for low-reflection films, 425, 426 
use in polarizing beam splitters, 
430 

Crystalline calcium fluoride 
see Fluorite crystals, synthetic 
Curvature of field in lenses 
see Field curvature in lenses 
Cyclohexyl cyclohexylmethacrylate, 
349 

Cyclohexylmethacrylate 
advantages, 346-347 
baking time for lenses and 
prisms, 359 

polymerization, 346-347, 355 
refraction index, 342, 361 
use in M-16 solid reflex gunsight, 
501 

use in photographic lens, 338-330 
Cyclohexylmethacrylate, properties, 
361-365 
density, 363 

dispersive power, 342, 361-362 
homogeneity, 346, 362 
molecular weight, 347 
scratch resistance, 364 
secondary spectrum, 455-456 
softening temperatures, 363-364 
strain, 362-363 
surface flatness, 362 
surface hardness, 363, 373 
tensile, impact, and flexural 
strength, 365 
thermal conductivity, 364 
transmission, 363-364 
water absorption, 346, 363 
wear ratio, 373 



INDEX 


615 


Cyclohexylmethacrylate monomer, 
chemical production 
materials for synthesis, 352-354 
preparation of the charge, 353- 
354 

removal of inhibitor prior to use, 
354 

storage, 353 

Damping of optical instruments, 
106-109 

air-bellows damper, 109-110 
effect on filtering action, 106-107 
frictional dampers, 108-109, 127, 
522-526 

magnification factor, 107 
sylphon bellows damper, 95-97 
use of silicone fluid, 123 
viscous damping, 107-108, 522- 
526 

Dartmouth College, binocular tests, 
268-271 

Data recorders for phototheodo¬ 
lites, 545-546 

Day photography, aerial lenses, 
40-57 

anastigmat, 50-52, 56-57 
apochromatic, 52-57 
telephoto, 44-50, 55-56 
wide-angle, 40-44 

Definition in optical instruments, 

213- 217 
binoculars, 216 
lenses, 212-213 
periscopes, 216 

prisms, wedges, and windows, 

214- 215 
telescopes, 216 

Design of optical instruments 
see Optical design evaluation, 
procedure and equipment 

Dextrex degreaser, use in making 
roof prisms, 396 

Dial gauge for inspection of 
prisms, 213-214 

Diamond milling of lenses, 390 

Diamond tools for optical manufac¬ 
ture, 393-394 

Diffuse film printers, 168-169 

Dioptometer, 205, 244-248 
function, 245 

inspection of reticles, 222, 244 
measurement of astigmatism, 
244, 247 

measurement of chromatic aber¬ 
ration,244, 247-248 
measurement of parallax, 244- 
247 

measurement of spherical aber¬ 
ration, 244, 247-248 


principal parts, 244 
supplementary apparatus, 247- 
248 

unit of the diopter, definition, 245 
Direct view striaescope, 208 
Dispersion specifications for optical 
glass, 205-207 

p-divinyl benzene, synthesis, 350 
Dove prism 

double dove prism scanner, 552- 
553 

use in two-star navigating de¬ 
vice, 577-578 

Dri-film for periscope photography, 
575-576 

Dry-friction dampers for camera 
mounts, 108-109 

Eastman Kodak Company 
8-J-35 lens, 70, 78-79 
A-8 camera mount, 113-114 
aerial lens tests, 69-89 
ball-cone antioscillation mounts, 
513-514, 526-527 

fly’s-eye gunsight, 487-488, 501- 
504 

gimbal antioscillation mounts, 
510, 512-513, 526-527 
glass molding, 406-417 
glue-silver process of making ret¬ 
icles, 418-420 
night binoculars, 565-566 
periscope photography, 572-576 
phototheodolites, 528-550 
shake table for testing optical 
mounts, 516-517 

stabilized camera mounts, 119- 
124 

tank telescope, 445 
Eastman recording phototheodo¬ 
lite, 531-545 

aided tracking and telescopes, 
541-543 

alignment of instrument, 544-545 
alignment of the axes, 543-544 
angle measurements, 531-533, 
540-541 

camera mechanism, 533-536, 540 
Edgerton lamps, 530, 541, 544 
electric controls, 544 
exposure control, 539-540 
lens, 536-537 
lens mounts, 538-539 
lens resolution, 537-538 
levels, 543 
seals, 544 

use of telescope, 542-543 
worm and worm wheel, 531-533 
Eastman-NDRC antivibration cam¬ 
era mount 


flight tests, 113-114 
performance, 160 
recommendations, 145 
spring mount, 112-113 
use of sweep mechanism, 156 
Edgerton lamps 

for phototheodolites, 530, 541, 544 
recommendations, 145 
use in camera resolution studies, 
166-167 

8x56 binocular, 552 
8-J-35 Eastman lens, 70, 78-79 
8-T-87 lens, 70 
Ektar lenses 

see Aero-Ektar lenses 
Elevator furnace for manufactur¬ 
ing fluorite crystals, 315-322 
crucibles, 320 
diffusion pumps, 316 
gradient baffle, 318-320 
heaters, 316-320 
helices, 318 
installation, 320 
operation, 320-322 
power supply, 320 
vacuum tanks, 315-316 
Erfle telescopic eyepiece, 437, 448 
Etching process of making reticles, 
418-419 

Ethylene dimethacrylate, 349-351 
hardness measurements, 363, 373 
synthesis, 350-351 

Evaporation process of coating 
plastics, 370-371 

Evaporation process of depositing 
films 

low-reflection films, 425-427 
metallic films, 432-434 
Exposure meter for aerial cameras, 
141 

Eye guards for binoculars 
effectiveness, 282-283, 286 
recommendations, 287 
Eye measurements 
pupil studies, 276-277 
resolving power, 224-225 
sensitivity to light, 473-474 
use of dioptometer, 245 
Eyepieces for 7x50 binocular, 442- 
444 

Eyepieces for telescopes 
aberrations, 435-436 
apparent field, 436 
aspheric surface, 435-436 
characteristics, 435-436 
Erfle, 437, 448 
eye-point, 437 
orthoscopic, 450-451 
parabolic, 441-442 




616 


INDEX 


Petzval sum, 435 
wide-angle, 435-437 

f/0.7 Schmidt optical system, 365, 
366 

f/1 curved field lens, 57-58 
f/1 Schmidt camera, 103-104 
f/1.5 plastic condenser lens, 474-475 
f/1.6 reflex gunsight, 365, 366, 494 
f/2.5 lenses 

5x5 (Aero-Ektar), 339 
7-in. (Polaroid), 70 
7-in., 5x5 (Harvard), 60-62 
7-in., 5x5 (Harvard-Polaroid), 60 
f/2.8 lenses 

7.38-in., 365, 366 
7.5-in., 5x5; 59-60 

f/2.85, 6-in. lens, recommendations, 
145 

f/3, 7-in., 5x5 plastic lens, 59 
f/3.5 lenses 

5.950-in. (wide angle), 41, 78 
24-in., 9x18 for night photog¬ 
raphy, 64 

f/4.5, 12-in., 9x18 anastigmat lens, 
64 

f/5 lenses 

12-in., 9x9 (anastigmat), 57 
40-in. (NDRC telephoto), 76-78 
40-in., 9x9 (telephoto), 44-49, 
155-156,167 
f/6 lenses 

60-in., 9x9 (telephoto), 44-45, 48- 
49 

60-in., 9x18 (telephoto), 48-50 
triplet lens for K-18 camera, 62- 
63 

f/8 lenses 

36-in. (apochromatic), 52-55 
36-in., 9x18 (anastigmat), 56-57 
36-in., 9x18 (telephoto), 55-56, 
70-76, 340 

40-in. (Bausch and Lomb tele¬ 
photo) , 67-69 

48-in., 3 1 /4x4 1 / 4 : (telephoto), 62 
48-in., 9x9 (apochromatic), 54-55 
100-in., 9x18 (apochromatic), 56- 
57 

f/10, 100-in., 9x18 anastigmat lens, 
50-52, 76-77 
f/II lenses 

15-in. telephoto, 537 
36-in., 9x18 apochromatic, 63-64, 
69 

f/20 apochromatic lenses 
1.8-in. fluorite objective, 335 
triplet objective, 336 
Fabry 

kinetic definition chart appa¬ 
ratus, 204 


resolving power of the eye, 224 
Fairchild stereocomparators, 177 
Field curvature in lenses 

correction in periscopes, 462-463 
curved platen, 36 
curved-field lens, 57-58 
field flatteners, 36, 62 
measurement, 366 
15-in. f/II telephoto lens, 537 
56-M-56 lens, 70 
Figure-4 reflex gunsights, 485 
N-9; 490-491 

principle of operation, 490 
S-l; 490-491 
S-2; 491 
S-3; 491 
Film 

effect of exposure time, 25-26 
effect of haze, 26 

effect on camera resolution, 168- 
169 

for phototheodolites, 538 
infrared, 42, 183 
Kodalith orthochromatic, 70 
magazines, 97-99,172 
microfile, 183, 538 
micrometer film holder, 164-165 
panchromatic, 68-69 
Pan-X, 28, 91-92,168 
recommendations for future re¬ 
search, 171, 434 

requirements for aerial cameras, 
25-29, 91-92 

Super-XX, 28, 69, 91-93, 538 
Tri-X night film, 58 
turbidity, 32 

wide-angle photography, 40, 42 
Film, high-reflection, 427-430 
multilayer films, 428-430 
recommendations, 434 
titanium dioxide, 428 
zinc sulfide, 427-428 
Film, low-reflection, 423-427 
calcium fluoride, 425 
chiolite, 425 
cryolite, 425, 426 
evaporation process, 425-427 
index of refraction, 377-378 
magnesium fluoride, 425 
methods of eliminating reflection, 
425 

methods of hardening, 425-426 
recommendations for future re¬ 
search, 434 
theory, 423-426 
Film, metallic 
aluminum, 432-433 
black surfaces, 433 
chromium, 433 


evaporation process of deposit¬ 
ing, 432-434 

natural density filters, 433 
recommendations, 434 
silver and copper, 433 
Film printing methods 

see Photographic processing 
techniques 

Film-flatness tester for aerial pho¬ 
tography, 141-142, 165 
Filters for lenses 
infrared, 44-45, 48-49 
minus-blue, 44-45 
natural density filters, 433 
yellow and red for haze, 163 
Fire-control system, stadiameter, 
508-509 

5x tank telescope, 445-446 
5.950-in., f/3.5 wide-angle lens, 41, 
78 

Fixed-focus binoculars, 452, 455 
Flare in aerial lenses, 34 
Flash night photography, 57, 101- 
103 

Flats (optical glass) 

fringe patterns for plastics, 383- 
387 

interferometer tests, 242-243 
specifications, 215 
Flightsight (reflex gunsight) 

3x telescope, 437 
nightshooting, 491 
parallactic range, 491-493 
use with radar, 491 
Flint glass lens 

7-in., f/2.5, 5x5; 60-61 
100-in., f/8, 9x18; 56-57 
Fluorite, natural 
fringe patterns, 327 
use in microscope objectives, 333 
Fluorite correctors for periscopes. 
459-463 
G-8; 461-463 
H-12; 461 
P-55, 459-461 

Fluorite crystals, synthetic, 312-341 
advantages, 334 

barium and strontium fluoride. 

332 

defects, 312-314 
disadvantages, 333-334 
effective temperature gradient, 
312-313 

fluorspar, 313-315 

for infrared photography, 312, 

333 

freezing of crystals, 312-313 
1-224; 325-326 
IX-7; 325-326 




INDEX 


617 


laboratory method of growing, 
312-313 

recommendations, 145, 340-341 
working and spontaneous speed, 

313 

Fluorite crystals, synthetic, manu¬ 
facturing process, 314-325 
annealing furnace, 323-324 
elevator furnace design, 315-322 
evaluation of crystallizing fur¬ 
naces, 325 

multiple crystals, 325 
natural fluorite, 314-315 
pot furnace design, 322-323 
preparation of the stock, 314-315 
synthetic calcium fluoride, 315 
Fluorite crystals, synthetic, optical 
applications, 333-337 
aerial camera lens, 36-37, 148, 
332, 333 

apochromat objectives, 334-335 
folded apochromatic telescope 
and collimator, 312, 335-336, 
576 

wide-angle lenses, 148, 444 
Yerkes apo-periscope objective, 
336-337 

Fluorite crystals, synthetic, optical 
properties, 325-327 
birefringence, 327 
dispersion, 312 
fringe patterns, 326, 327 
homogeneity, 326-327 
refractive index, 312, 325-326 
secondary spectrum, 312 
transmission, 326 
ultraviolet cutoff point, 326 
variation in optical path length, 
327 

Fluorite crystals, synthetic, surface 
working techniques, 327-332 
blocking, 331 
edging, 332 
effect of scratches, 330 
effect of thermal shock, 327 
grinding, 330, 331 
polishing, 330, 331 
subsurface fracturing, 330 
Fluorspar for artificial fluorite 
crystals 

effect on color of crystal, 313 
multiple crystal growth, 313-314 
preparation, 314-315 
Fly’s-eye reflex gunsight, 501-504 
armor glass for reflex mirror, 
487, 501 

boresighting mount, 501-502 
cooling device, 503-504 
eye relief, 487 
illumination, 503-504 


lens plate, 501-503 
multiple lens collimating system, 
487-488 

parallax errors, 501-502 
pilot visibility, 487 
principle of operation, 487-488 
recommendations, 504 
reticles, 501-503 

Focal errors in aerial lenses, 35-36 
Focal length of lenses, specifica¬ 
tions, 211-212 

Focal plane camera shutters, 133- 
140 

continuously operating, 140 
effect of vibration on resolution, 
163 

Langer shutter, 133-135, 145 
recommendations, 145 
Technicolor shutter, 135-139 
Focal settings for aerial cameras 
attachment for checking focus, 
140 

automatic focusing, 45-46, 164 
effect on resolution, 164-165 
errors, 33-36 
recommendations, 172 
Formulas 

astigmatism, 251-253 
azimuth and elevation of stars 
and moon, 549 

eye freedom in reflex gunsights, 
481 

KDC efficiency, 229 
natural frequency of a mechan¬ 
ical filtering system, 106 
Fort Belvoir, Va., use of oblique 
photographs, 179 
Foucault test object, 224, 226 
IV submarine periscope 
aberrations, 459 
G-8 corrector, 461-463 
properties, 459 
40-in. lenses 

Bausch and Lomb f/8 telephoto, 
67-69 

Harvard f/5 telephoto, 44-49, 
155-156,167 

NDRC f/5 telephoto, 76-78 
48-in. lenses 

f/8, 3 1 /4x4 1 /4 (telephoto), 62 
f/8, 9x9 (apochromatic), 54-55 
Foveal (central vision), 265 
Foxhole periscopes, 468-469 
Frankford Arsenal 

Cooke triplet objective for tele¬ 
scope, 444-445 
tank telescope, 445 
Fraunhofer telescope objective, 444 
Frictional damping of optical sys¬ 
tems, 523-526 



comparison with viscous damp¬ 
ing, 108-109, 522 
definition, 522 

effect on boresighting in gun 
camera, 127 

energy dissipated per cycle, 524 
evaluation, 525, 526 
limitations, 524-525 
Fringe patterns 
flats, 383-387 
fluorite crystals, 326, 327 
natural fluorite, 327 
prisms, 387 

specifications for optical parts, 
243 

G-8 fluorite corrector for IV peri¬ 
scope, 461-463 
Galilean binoculars, 367 
Galilean telescope, 459 
Garlock seal for phototheodolites, 
544 

Gauges for kinetic definition chart 
apparatus, 230 
General Electric Company 
Dri-film, 575-576 

H6 high-pressure mercury lamp, 
66 

stadiameter for B-29 fire-control 
system, 508-509 

Geological Survey, Clarendon, Vir¬ 
ginia, use of oblique photo¬ 
graphs, 179 

German stereoplanigraph, 175 
Gimbal antioscillation mounts, 512- 

513 

recommendations, 526-527 
use with 6x binoculars, 510 
Gimbal antivibration camera 
mounts 

double-ring, 109-111 
for gun cameras, 128 
performance, 128 
Glass, optical 
see Optical glass 
Glass substitutes 
see Plastics 

Glass-fluorite folded collimator, 576 
Glue-silver process of making reti¬ 
cles, 418-420 

Gould and Eberhardt Company, 
worm and worm wheel, 531 
Ground-speed compensation camera 
mount, 124-126 

control of sweep motor speed, 125 
effectiveness, 125 
rotating prism unit, 125-126 
theory, 124-125 

variable-speed electric motor, 111 



618 


INDEX 


Ground-sweep mechanism for aerial 
cameras, 97 

Gun camera antivibration mounts, 
126-130 

boresighting inaccuracies, 127- 
128 

dampers, 127 

for flexible gun installations, 128- 
130 

gimbal mount, 128 
mirror mounts, 129-130 
spring mount, 112-113, 128 
theoretical considerations, 126- 
128 

vibration filtering action, 127 

Guns and sights, harmonization 
see B-29 guns and sights, har¬ 
monization 

Gunsights, reflex 
see Reflex gunsights 

Gurley Company, theodolite tele¬ 
scope, 458 

Gyroscope, use in aerial camera, 
120-121,144 

H6 high-pressure mercury lamp, 66 

H-12 fluorite corrector for 1.9 
periscope, 461, 463 

Harmonization of guns and sights 
see B-29 guns and sights, har¬ 
monization 

Harshaw Chemical Company, op¬ 
tical crystals, 312 

Hartmann film-flatness test, 142, 
165, 258 

Harvard 36-in., f/8, 9x18 wide- 
angle telephoto lens, 55-56 
aerial tests, 55 

image-energy distribution, 70-76 
recommendations for future re¬ 
search, 55 

spherical achromatism, 340 

Harvard 36-in., f/8 apochromatic 
lens, 52-55 

astigmatism and field curvature, 
52-54 

cold chamber measurements, 52- 
53 

color aberrations, 53 
distortion, 53-54 
long barrel, 52-55 
resolution, 54 
short barrel, 52 
tertiary spectrum, 70 
thermostating for focus stability, 
52-53 

vignetting, 53-54 
wedge pattern, 78 

Harvard 40-in., f/5 telephoto lens, 
44-49 


aberrations, 76-77 
automatic focusing with altitude, 
45-46 

control of image quality, 47-48 
effect of haze, 155-156, 167 
image-energy distribution, 76 
infrared filter, 44-45, 48-49 
Micarta tubing for insulation, 46 
minus-blue filter, 44-45 
mounting, 44-46 
resolution, 155-156,173 
sylphon bellows arrangement, 46 
wedge pattern, 78 
Harvard University 
aerial lenses, 60-64 
aircraft periscope, 464-469 
anastigmat lens, 50-52 
apochromatic lens, 56-57 
center-of-gravity camera mount, 
117-119 

cold chamber for camera tests, 
143-144 

collimator lenses, 444 
fabrication of large lens ele¬ 
ments, 390-394 

ground-speed compensation 
camera mount, 125-126 
high-resolution projection lens, 
474 

lens rectifying camera, 188 
lenses for wide-angle photog¬ 
raphy, 40-44, 55-56 
mirror frame method of harmo¬ 
nizing guns and sights, 290, 
304-307 

night flash Schmidt camera, 101- 
103 

P-55 fluorite corrector for 1.4 
periscope, 459-461 
Polaroid lens, 60 

resolution of aerial photographs, 
148, 165-166 

techniques for working fluorite 
surfaces, 331 
telephoto lens, 44-50 
Hasselkus lenses, 64 
Hayward solid reflex gunsights, 
498-501 

antiaircraft guns, 499 
M-l rifle, 498-499 
Mark 17 gunsight, 499-501 
rockets, 499 
Haze 

definition of aerial haze, 166-167 
effect of altitude, 167 
effect on aerial camera resolu¬ 
tion, 29-31, 165-168, 173 
effect on contrast in aerial photo¬ 
graphs, 29-31 



effect on Harvard 40-in, f/5 tele¬ 
photo lens, 155-156, 167 
effect on photographic emulsions, 
26 

use of yellow or red filters, 163 

High-reflection film, 427-430 
multilayer films, 428-430 
recommendations, 434 
titanium dioxide, 428 
zinc sulfide, 427-428 

High-speed lenses 

concentric surfaces, 64-65 
lens-mirror systems, 64 

Houston Company, gimbal mounts, 
519 

Hydrographic Office 
function, 175-176 
pinhole rectifying camera, 188 

Hypergon lens 
distortion, 190-191 
in rectifying camera, 189, 195 
manufacturing problems, 191 
performance, 189-190 

1.1 interferometer, 241-244 

adjustments for bringing fringes 
into view, 242-243 
light source, 244 

plane-parallel dividing plate, 244 
testing of prisms, flats, and 
lenses, 240, 242-243 
use as a production test instru¬ 
ment, 242-244 

1-224 fluorite crystal, refractive in¬ 
dex, 325-326 

Image definition in optical instru¬ 
ments, 212-217 
binoculars, 216 
lenses, 212-213 
M-71 telescope, 216 
periscopes, 216 

prisms, wedges, and windows, 
214-215 
telescopes, 216 

Image formation by plastic lenses 
see Plastic lenses, image forma¬ 
tion tests 

Image movement in aerial cameras 
compensation by sweep mounts, 
156 

recommendations for future re¬ 
search, 171 

Inclusions in optical glass, manu¬ 
facturing tolerances, 209 

Infrared photography 
film, 42, 183 
filters, 44-45, 48-49 
study of human eye pupils, 276- 
277 

use of fluorite crystals, 312, 333 



INDEX 


619 


Inorganic optical materials, prop¬ 
erties, 343, 362-363 
Inspection procedure for optical 
elements, 210-216 
see also Optical testing methods 
acceptable striae grade, 207 
flats, 215 
lenses, 210-213 

prisms, wedges, and windows, 
213-215 
reticles, 215 
Interference patterns 
flats, 383-387 
fluorite crystals, 326-327 
natural fluorite, 327 
plastic lens, 381 
prisms, 387 

specifications for optical parts, 
243-244 
Interferometer 

checking of optical flats, 215 
computation of interference pat¬ 
terns, 243-244 
function, 239, 258-260 
1.1; 241-244 

inspection of parts and sub- 
assemblies, 244 

Michelson-Twyman, 204, 239-244 
pattern for astigmatic images, 
251-253 

recommendations for future re¬ 
search, 261-262 
testing of lenses, 242-243 
testing of prisms, 213-214, 240, 
242-243 

tests on homogeneity of plastics, 
362 

Twyman-Green, 383-387 
Interpupillary settings in binoc¬ 
ulars, 277 

Intervalometer, use in camera 
mounts, 113, 115, 124 
Isopentane, use in camera mount- 
tings, 95 

IX-7 fluorite crystal, refractive in¬ 
dex, 325-326 

Japanese binocular, KDC efficiency, 
233-234 

Johannson gauge blocks for testing 
lenses, 391 

K-17 camera shutter, 132-134 
driving system, 132 
durability, 132-133 
performance, 132 
recommendations, 132 
shutter speeds, 134 
K-18 camera lenses 

12-in., f/4.5, 9x18 anastigmat, 64 


36-in., f/8, 9x18 wide-angle tele¬ 
photo, 55-56, 70-76 
f/6 triplet lens, 62-63 
K-24 camera lenses, 60-62 
astigmatism, 60-61 
Biotar design, 60 
high-index flint glass, 60-61 
high-index white glass, 61 
hyperchromatic doublet, 61 
ophthalmic glass, 60 
resolution, 61 

K and E (Keuffel and Esser) Com¬ 
pany 

photosensitive resist, 420 
reticle production process, 418 
KDC (kinetic definition chart) ap¬ 
paratus, 224-239 

auxiliary telescope, 227-229, 235 

collimator, 226, 229 

function, 204 

gauges, 230 

Model 2-B, 225-226 

Model 4, 226-229 

recommendations for future re¬ 
search, 261-262 
telescope, 494 B, 235 
test object, 204 
use for inspection, 238-239 
KDC (kinetic definition chart) ap¬ 
paratus, precision, 234-238 
effect of magnification, 235 
effect of target illumination, 234 
expected accuracy, 237 
probable error, 235-238 
systematic errors, 236-237 
KDC (kinetic definition chart) ap¬ 
paratus, test measurements, 
226-234 

binoculars, 233-234 
lenses, 212-213 
M-71 telescope, 216, 224, 236 
M-72 and M-76 telescopes, 236 
measurements compared to 
visual grading, 231 
off-axis measurements, 230-231 
optical target, 226-227 
resolving power, 224 
striae studies, 207 
test procedure, 229-230 
variation among observers, 237- 
238 

KDC (kinetic definition chart) 
efficiency 

concept of KDC efficiency, 204, 
217, 224-225 

effect of astigmatism, 251-253 
effect of coma, 254 
effect of spherical aberration, 
250-251 
formula, 229 



telescopes, 216-217, 258-260 
Kellner eyepieces for 7x50 binoc- 
cular, 443 

Keuffel and Esser Company 
photosensitive resist, 420 
reticle production process, 418 
Kinetic definition chart 

see KDC apparatus; KDC 
efficiency 

Kodak Research Laboratories 
see Eastman Kodak Company 
Kodalith orthochromatic film, 70 
Kollmorgen periscopes 
see Submarine periscopes 

L9k gunsight, 488 
Langer camera shutter, 133-135 
recommendations, 145 
screen mechanism, 133-134 
Lateral color 

see Chromatic aberration 
Lawrence Aeronautical Corpora¬ 
tion, stabilized camera 
mount, 119 

Lead-sulfide process of producing 
reticles, 418-420 
evaluation, 419-420 
opaque subcoat, 418 
Lens rectifier for oblique photo¬ 
graphs 

distortion, 188-191 
hypergon lens, 189 
manufacturing problems, 191 
rectifying camera, 188 
resolution tests, 189-190 
specifications, 188-189 
spherical lens, 189 
Lens reflex gunsights, 483, 495-496 
Lenses 

see also Lenses for aerial 
cameras; Lenses for tele¬ 
scopes 

achromatic, 93, 347, 349 
focal length, 211-212 
for periscope photography, 462- 
463, 575 

for phototheodolites, 529-530, 
536-537, 545 

for reflex gunsights, 483, 502- 
503, 586-587 

high-resolution projection lens, 
473-476 

hypergon, 189-191, 195 
meniscus lenses, 93, 339 
plastic, 59, 338-339, 376-383, 474- 
475 

projection printing lens, 63 
Rayleigh limit of aberrations, 
249-250 



620 


INDEX 


Rayleigh limit of resolution, 31- 
32 

wedge patterns, 70, 78, 171 
Lenses, manufacturing techniques, 
390-394 
beveling, 392 
casting molds, 356-357 
centering, 390-391, 393-394 
diamond milling, 390 
edging, 390-392 
grinding, 390, 392 
mountings, 393 

photographic lens design, 339- 
340 

physical characteristics of 10-in. 

diameter lens, 391 
polishing, 391 
radii checking, 391-392 
recommendations, 393-394 
temperature effects, 392 
use of vacuum chucks, 393-394 
Lenses, specifications, 210-213 
beauty defects, 213 
bubbles, 209 
centering, 210-211 
definition, 212-213 
focal length, 211-212 
performance, 173 
physical dimensions, 210-211 
Lenses, testing instruments 
collimator, 140, 164-165 
interferometer, 242-243 
Johannson gauge blocks, 391 
KDC apparatus, 212-213 
microphotometer, 29 
monochromator, 66, 394-395 
Lenses for aerial cameras, 23-92 
Aero-Ektar, 59-61, 153-155 
aerostigmat, 65 
anastigmat, 50-52, 56-57, 63-64 
apochromatic, 52-57, 333-337 
apochromatic telephoto, 63-64 
Biotar, 60 
Cooke Aviar, 65 

Cooke triplet objective, 62, 437, 
444-447 

curved field, 57-58 
filters, 44-45, 48-49, 433 
fluorite lenses, 36-37, 332-334 
long-focus, 148, 444 
metrogon, 64 
ophthalmic glass, 60 
pentac, 65 
Polaroid, 59 

recommendations, 50, 56-58, 61 
resolving power, 153-155 
summary of types, 582-584 
telephoto, 44-50, 55-56, 62, 67-69 
telestigmat, 65 
Telikon, 39-40, 62, 340 


thermostatic control, 164 
Topogon, 39-40 
wide-angle, 40-44 
wide-field designs of high speed, 
64-65 

Lenses for aerial cameras, aberra¬ 
tions, 33-40 
astigmatism, 36, 53-54 
centering and flare, 34 
chromatic aberration, 33-35, 51 
coma, 33-34, 37-38 
curvature of field, 36-37, 57-58, 
62, 366 

distortion, 34, 39, 51, 53-54 
double or multiple images, 34 
focal errors, 33-36 
image errors, 104 
longitudinal color, 36-37, 53 
residual errors, 34, 38, 52 
scattered light, 34 
secondary spectrum, 36-37 
silhouetting effect, 34 
spherical aberration, 31-32, 37- 
38, 51-52, 68-69 

suggestions for improvement, 39 
vignetting, 34, 37-39, 53-54 
zonal aberration, 33-34, 37-38, 
51-52, 57-58 

Lenses for aerial cameras, day 
photography, 40-57 
anastigmat, 50-52, 56-57 
apochromatic, 52-57 
telephoto, 44-50, 55-56 
wide-angle, 40-44 

Lenses for aerial cameras, design, 
23-40 

comparison of perfect and imper¬ 
fect lens, 29-31 
contrast ratio, 24 
effect of aperture size, 32 
effect of haze, 29-31 
film considerations, 25-29, 91-92 
military needs, 23-24 
optical requirements, 32 
recommendations, 33, 62, 76-77, 
144-145 

Lenses for aerial cameras, labora¬ 
tory tests, 65-92 

see also Lenses, testing instru¬ 
ments 

average lens-film performance, 
77-78, 84-87 
contrast factors, 90-91 
exposure level, 91 
film properties, 91-92 
image characteristics, 53-54, 67 
image-energy distribution meas¬ 
urements, 70-77 
lens resolution, 78-80, 88 



lens types, 65, 70 
lens-film resolving-power meas¬ 
urements, 78-79, 81-84 
light source, 66 
recommendations, 171 
requirements, 26 
target types, 89-90 
telephoto lens, 67-69 
wedge patterns, 171 
Lenses for aerial cameras, night 
photography, 57-62 

6- in., f/1 curved-field, 57-58 

7- in., f /2.5, 5x5; 60-62 
24-in., f/3.5, 9x18 lens, 64 
Polaroid, 59 

recommendations for future re¬ 
search, 61 

Lenses for aerial cameras, specific 
types 

6- in., f/1 curved-field, 57-58 
7rin., f/2.5, 5x5 (Harvard), 60- 

62 

7- in., f/2.5, 5x5 (Polaroid), 60 
7-in., f/2.8 (Polaroid), 70 
7-in., f/3, 5x5; 58-59 
7.38-in., f/2.8; 365-366 
7.5-in., f/2.8, 5x5; 59-60 

12-in., f/4.5, 9x18 anastigmat, 64 
12-in., f/5, 9x9 anastigmat, 57 
24-in., f/3.5, 9x18; 64 
36-in., f/8, 9x18 anastigmat, 56- 
57 

36-in., f/8, 9x18 wide-angle tele¬ 
photo, 55-56, 70-76, 340 
36-in., f/8 apochromatic, 52-55 
36-in., f/11, 9x18 apochromatic, 
63-64, 69 

40-in., f/5, 9x9 telephoto, 44-49, 
155-156, 167 

40-in., f/8 telephoto, 67-69 
48-in., f/8, 3 1 /4x4 1 / 4 telephoto, 62 
60-in., f/5, 9x18 telephoto, 48-50 
60-in., f/6, 9x9 telephoto, 44-45, 
48-49 

100-in., f/8, 9x18 apochromatic, 

56-57 

100-in., f/10, 9x18 anastigmat, 
50-52, 76-77 
Lenses for telescopes 

apochromatic, 312, 335-336, 576 
cemented triplet cornographic, 
475-476 

Cooke triplet, 444-447 
Fraunhofer, 444 
optical constants, 444 
telephoto, 450 

Lens-Mangin gunsights, 483-484. 
504 

Light scattering 

camera lens, 166-168, 172 




INDEX 


621 


coated optics, 257-258, 262 
effect on camera resolving* power, 
166-168, 205 
fluorite crystals, 314 
plastic lens, 378 

recommendations for aerial 
cameras, 172 

Light sensitivity of human eye, 
473-474 

Light transmission 
binoculars, 219 
M-71 telescope, 219 
photoelectric photometer, 219 
Lithium fluoride for optical coat¬ 
ings, 268 

Long-focus camera lenses 
collimator lens, 444 
fluorite, 148 
wide-angle, 148 

Longitudinal color in lenses, 36-37, 
53 

Lord shock absorbers for antioscil¬ 
lation camera mountings, 95 
Louvre camera shutter, 93, 99-100 
speed, 100 
spring drive, 99 
variation of exposure time, 99 
Low-reflection film 

see Film, low-reflection 
Lucite 

see Methyl methacrylate 

M-l rifle, reflex sight for, 498-499 
M-10 periscope, specifications 
image definition, 216 
reticles, 221-223 
weatherproofing, 223 
M-16 reflex gunsight, 501 
M-17 telescope, use in phototheodo¬ 
lites, 542-543 

M-21 to M-24 reflex gunsights 
see T-67 reflex gunsight 
M-70 telescope, KDC efficiency, 
231-233 

M-71 telescope, specifications, 216- 
223 

collimation, 219-221 
exit pupil, 218 
image definition, 216 
KDC efficiency, 216, 224, 236 
light transmission, 219 
magnification, 218 
optical and geometrical axis, 219- 
220 

reticles, 221-222 
weatherproofing, 223 
M-72 telescope, KDC efficiency, 236 
Magazines for film, 97-99, 172 
Magnesium fluoride 

for coating plastics, 374 


for low-reflection films, 425 

Magnesium oxide for optical ele¬ 
ments, 338 

Magnification factor 

antivibration camera mounts, 
511 

binoculars, 218, 271, 273-275, 281 
M-71 telescope, 218 

Maksutov, theory of lens-mirror 
systems, 64 

Mangin gunsights, 483-485 
double mirrors, 485 
Lens-Mangin, 483-484 
loss of light, 485 
recommendations, 504 
solid sights, 485 

Manufacturing techniques for 
optical instruments 
see Optical manufacturing tech¬ 
niques 

Mapping methods, aerial, 175-203 
“controlling the map,” 175, 183- 
185, 192-193 

definitions and fundamental ge¬ 
ometry, 179-181 

lenses for rectification, 188-191 
MFA method, 191-200 
navy requirements, 175-179 
oblique photographs, 185-200 
personnel, 178, 202 
plotting instruments, 175, 181- 
183, 186 

recommendations, 202-203 
triangulation, 178 
water depth determination, 178, 
200-202 

wide-field photogrammetry, 178- 
179 

Mark I, 35-mm periscope camera, 
572 

Mark II wire method of harmoniz¬ 
ing guns and sights, 295-299 
accuracy, 297 
equipment, 295-296 
evaluation, 299 
length of time required, 297 
mounting, 296, 299 
principle, 295 
procedure, 296-297 
recommendations for future re¬ 
search, 310 
tests, 297 

triple mirror, 296-297 

Mark III wire method of harmoniz¬ 
ing guns and sights, 299-302 
accuracy, 301-302 
equipment, 299-300 
evaluation, 301-302 
personnel, 300 
procedure, 300-301 


purpose, 299 

recommendations for future re¬ 
search, 310 
tests* 301 

Mark IV periscope, 336-337 
Mark 7 offset wedge 

attachment for binoculars, 366 
performance, 365 
Mark 14 gunsight 

see Fly’s-eye reflex gunsight 
Mark 17 gunsight 

half-silvered mirror sight, 501 
illumination, 499-501 
solid reflector sight, 499-501 
Mark 23 bombsight, 4*68-469 
Massachusetts Institute of Tech¬ 
nology 

collimator, 474 
optically neutral paints, 149 
resolving-power camera targets, 
148 

synthetic fluorite crystals, 312- 
341 

wire method of harmonizing guns 
and sights, 290 

Materials for optical elements 
see Optical materials 
Meniscus lenses 
achromatic, 93 
optical requirements, 339 
Mercury lamp, high-pressure, 66 
Merrill Flood and Associates 
aerial mapping methods, 191-200 
Mark III wire method of harmo¬ 
nizing guns and sights, 299- 
302 

mirror boresight method of har¬ 
monizing guns and sights, 
302-303 
Metallic film 

see Film, metallic 
Methacryl acetoacetate for syn¬ 
thesis of optical plastics, 348 
Methacrylates 

see also Cyclohexylmethacrylate; 

Methyl methacrylate 
allyl methacrylate, 350-352, 363 
/3-amino-ethyl methacrylate, 348 
bornyl methacrylate, 349 
ethylene dimethacrylate, 349-351, 
363, 373 

nitromethyl-propyl methacrylate, 
348 

Methyl methacrylate, 348 
condenser lens, 474-475 
dispersive power, 342-343 
hardness measurements, 363, 373 
homogeneity, 345 
polymerization, 342-343, 345 
refractive index, 342-343 





622 


INDEX 


water absorption, 346 
Metrogon camera shutter, 130-132 
durability, 131-132 
exposure time, 132 
moment of inertia, 131 
performance, 131 
Metrogon lenses, 64 
MFA aerial mapping method, 191- 
200 

location of control points, 192- 
193 

photography, 192 
plotting, 193, 198-199 
rectification of oblique photo¬ 
graphs, 192-197 

MFA method of harmonizing guns 
and sights, 299-303 
Mark III wire method, 299-302 
mirror boresight method, 302-303 
suggested improvements, 309 
Micarta tubing for camera insula¬ 
tion, 46 

Michelson-Twyman interferometer, 
239-244 

interferograms, 240, 326 
measurement of definition in 
lenses, 212-213 
observed field, 204 
optical parts, 239 
optical path length, 240 
Pennsylvania State College 1.1 
interferometer, 241-244 
principles, 239-240 
recommendations for future re¬ 
search, 261 
Microfile film 
evaluation, 183 

performance in phototheodolites, 
538 

Micrometer film holder, 164-165 
Microphotometer 

determination of microscopic 
contrast in image, 91, 173 
lens tests, 29 

measurements of radial and 
tangential resolution, 89 
Microscope, petrographic, 314 
Microscope objectives, use of nat¬ 
ural fluorite, 333 

Microscopic contrast in aerial 
photographs 

determination by microphotom¬ 
etry, 91, 173 

equivalent target contrast, 28 
recommendations for future re¬ 
search, 171 

Miller single eyepiece plotter, 175, 
182-183 

Miller stereoscopic plotting instru¬ 
ment, 175, 181-182 


Minus-blue lens filter, 44-45 
Mirror boresight method of har¬ 
monizing guns and sights, 
302-303, 308, 310 

Mirror frame method of harmo¬ 
nizing guns and sights, 290 
304-307 

Mirror lens system 
for phototheodolites, 536-537 
limitations, 65 

use of achromatic menisci, 64 
Mirror mounts 

aerial cameras, 123-124 
gun cameras, 129-130 
Molding presses for optical glass, 
407-417 

Armco ingot iron molds, 410 
design details, 408, 411-412 
induction heating, 415 
limitations, 416-417 
molds for Schmidt reflector, 410- 
411, 413 

Nichrome heaters, 411-413 
pressure, 408-409 
requirements, 416-417 
stainless steel molds, 410 
use of pure hydrogen, 408-409 
use of stellite, 409-410, 413 
working temperature, 409 
Monochloronapthaline for inspec¬ 
tion of optical glass, 206 
Monochromator for lens testing, 
66, 394-395 

Monocular telescopes, 437-442 
3x; 441-442 
3x21; 440-441 
6x42; 440-441 
7x35; 441 
7x50; 441-442 

Schmidt prism erector, 437, 440 
Mount Wilson Observatory 
aerial lens tests, 65-69, 88-89 
anastigmat lens tests, 51-52 
Bowen reflex gunsight, 486-487, 
494-498, 504 

collimators, 140, 164-165 
Hayward solid sights, 498-501 
methods for making roof prisms, 
395-406 

micrometer film holder, 164-165 
modification of metrogon camera 
shutter, 130-132 

multiple slit focal plane camera 
shutter, 133-135 

resolving-power camera targets, 
149 

scanning devices, 551-564 
tests on camera resolution, 165- 
167 


two-mirror Schmidt camera, 92- 
101, 147 

two-star navigating device, 577- 
581 

wide-angle photography tests, 
43-44 

Mounts, antioscillation 
see Antioscillation mounts 
Mounts, antivibration 
see Antivibration mounts 

N-9 gunsight, 490-491 
National Bureau of Standards, 
homogeneity of fluorite crys¬ 
tals, 326 

Navigating device, two-star, 577- 
581 

astigmatized image lines, 577- 
578, 580 

design and construction, 577-580 
measurement of target course 
and distance, 579-580 
method of displaying direction 
and distance, 577 
pentareflector, 577 
principle of operation, 577 
recommendations, 580-581 
stabilization of instrument, 580 
tests, 580 

use of cylindrical lenses, 577-578 
New Departure Corporation, bear¬ 
ings for phototheodolites, 

533 

Nichrome heaters for mold presses, 
411-413 

Night binoculars 

see Binoculars for night vision 
Night photography 
aerial lenses, 57-62, 64 
flash photography, 57, 101-103 
recommendations, 61, 145 
Schmidt aerial cameras, 92-103 
Nitrogen dioxide for coating plas¬ 
tics, 348, 375 

N itromethyl-propyl methacrylate, 
348 

Objectives for optical instruments 
see Lenses 

Oblique photographs for aerial 
mapping, 185-200 
see also Mapping methods, aerial 
characteristics, 183 
film, 183 
plotters, 186 
rectification, 186-197 
Oblique sketchmaster for aerial 
mapping, 175, 186 
1.4 submarine periscope, 458-463 
aberrations, 459 



INDEX 


623 


eyepiece, 461-462 

P-55 fluorite corrector, 459-461 

properties, 459 

1.8-in., f/20 apochromatic fluorite 
objective, 335 
1.9 submarine periscope 
aberration, 459 
H-12 corrector, 461, 463 
properties, 458 
100-in. lenses 

f/8, 9x18 apochromatic, 56-57 
f/10, 9x18 anastigmat, 50-52, 76- 

77 

110-in. periscope for P-51 airplane, 
469 

Ophthalmic glass for aerial camera 
lenses, 60 

Optical bench for lens testing, 66, 

78 

Optical design evaluation, pro¬ 
cedure and equipment, 249- 
258 

see also Optical testing methods 
aberration modifiers, 256 
artificial sky apparatus, 256-258 
astigmatism modifier, 249, 251- 
253 

coma modifier, 249, 253-256 
energy distribution in image, 258 
experimental ray tracing, 258 
photoelectric and photographic 
procedures, 258 

Yerkes spherical aberration mod¬ 
ifier, 250-251 

Optical elements, inspection pro¬ 
cedure, 210-216 

see also Optical testing methods 
acceptable striae grade, 207 
flats, 215-216 
lenses, 210-213 

prisms, wedges, and windows, 
213-215 
reticles, 215 
Optical fluorite 

see Fluorite crystals, synthetic 
Optical glass 

birefringence, 362-363 
flats, 215-216, 383-387 
inorganic, 343 
organic, 343-344 

Optical glass, molding process, 406- 
417 

Armco ingot iron molds, 410 
aspherical elements, 407, 414-415 
barium crown, 414 
borosilicate, 414 

calcium-sodium silicate, 413-416 
molding presses, 407-413, 415-416 
molds of unusual shape, 413-414 
orange-peel surface, 407 


plane plates, 413-414 
production tests, 414-415 
Schmidt reflectors, 410-411, 413 
stainless steel molds, 407 
stellite molds, 409-410, 413 
temperature, 407 
thickness control, 414 
Optical glass, specifications, 205- 
210 

color and dimensions, 209 
inclusions, 209 

index and dispersion, 205-207 
inspection methods, 206 
molded blanks, 206 
refractometer, 206 
slab glass, 205 
strain, 209-210 
striae, 207-209 

tolerances for various types, 206 
Optical harmonization of guns and 
sights 

see B-29 guns and sights, harmo¬ 
nization 

Optical instruments, resolving 
power 

see Resolving power of optical 
instruments 

Optical instruments, specifications, 
216-223 

cleanliness, 219-220 
definition, 216-217 
interference fringes, 243-244 
light transmission, 219 
magnification, entrance and exit 
pupil, 218-219 

mechanical features, 219-221 
reticle, 209, 221-224, 244-246 
weatherproofing, 223 
Optical instruments, summary of 
volume, 1-22 

Optical manufacturing techniques, 
389-434 

glass molding, 406-417 
high-efficiency films, 427-429 
lenses, 390-394 
low-reflection films, 423-427 
metallic films, 432-434 
paraboloidal molds, 394-395 
polarizing beam splitters, 429- 
431 

quartz monochromator, 394-395, 
475 

reticles, 417-421 
roof prisms, 395-406 
Optical materials 
alkaline halides, 145, 337-338, 340 
barium fluoride, 145, 332, 338, 
340-341 

cyclohexylmethacrylate, 338-339, 
342, 346-347, 361-365 


fluorite, 145, 312-341 
high-index material, 339-340 
inorganic, 343 
magnesium oxide, 338 
organic, 343-344, 362-363 
plastics, 338-339, 342-388 
rare earth glass, 337, 339 
recommendations, 145 
spinel, 145, 337-338 
strontium fluoride, 332, 338 
styrene, 338-339, 342, 347, 361- 
365 

summary of types, 337 
Optical plastics 
see Plastics 

Optical testing methods, 204-262 
evaluation of instrument design, 
249-258 
glass, 205-210 

inspection of complete instru¬ 
ment, 216-223 

inspection of parts, 210-216 
light distribution in image for¬ 
mation, 204-205 
recommendations, 258-262 
specification and inspection re¬ 
quirements, 223-224 
telescopes, 238-239 
use of magnification, 225 
Optical testing methods, equipment, 
224-249 

dioptometer, 222, 244-248 
kinetic definition chart appara¬ 
tus, 224-239 

Michelson-Twyman interferom¬ 
eter, 212-213, 239-244, 261 
proboscope, 205, 219-220, 248-249 
Orange-peel surface of glass, 407 
Organic optical materials, 343-344 
birefringence, 362-363 
dissociation energy, 343 
van der Waals forces, 344 
Orthochromatic film, 70 
Orthographic plotting from recti¬ 
fied oblique photographs, 
198-199 

Orthomethyl styrene, 349 
Orthoscopic telescope eyepieces, 
450-451 

Ozone process of coating plastics 
see Silicon tetrachloride for coat¬ 
ing plastics 

P-51 periscope, 469 
P-55 fluorite corrector for 1.4 peri¬ 
scope, 459-461 
aberrations, 460-461 
specifications, 460 
P-80 periscope, 469-470 
Panatomic-X film 








624 


INDEX 


properties, 91-92 
resolution, 168 
target contrast, 28 
Panchromatic film, 68-69 
Panoramic scanner, 553-554 
Pantagraph mount for T-94 gun- 
sight, 489-490 
Parallax 

in reflex gunsights, 482, 487, 491- 
493, 501-502 
in reticles, 221-222 
measurement, 244-247 
Parallel-light film printers, 168 
Parallel-line patterns for camera 
resolution studies, 149-150, 
171 

PD-5 lens for periscope photog¬ 
raphy, 462-463 
Pennsylvania State College 
dioptometer, 222, 244-248 
1.1 interferometer, 241-244 
kinetic definition chart appara¬ 
tus, 224-239 

optical testing methods, 204-262 
Pentac lens, 65 

Pentareflector for two-star navi¬ 
gating device, 577 
Periscope photography, 572-576 
camera, 572-573 
color characteristics, 575-576 
dri-film, 575-576 

entrance window conditions, 575- 
576 

factors determining quality, 574- 
575 

focus of periscope, 576 
lenses, 462-463, 575 
Mark I camera, 572 
rapid processing equipment, 572- 
573 

recommendations, 576 
resolution, 574-576 
Wratten filter, no. 12; 575 
Periscopes, 458-470 
aircraft, 464-470 
fluorite correctors, 459-463 
foxhole, 468-469 
image definition, 216 
inspection procedure, 217 
M-10; 216, 221-222 
Mark IV, 336-337 
recommendations, 470-471 
submarine, 458-464, 471 
use in harmonizing guns and 
sights, 305 

Periscopic binoculars, 452-453 
Perkin-Elmer Corporation 

lenses for phototheodolites, 537 
objective for coma modifier, 253 


refractive index of fluorite crys¬ 
tals, 325-326 

techniques for working fluorite 
surfaces, 330-332 
telephoto lens, 45 
Petrographic microscope, 314 
Petzval lens, 472 
Petzval sum 

advantage of low-index ma¬ 
terials, 339-340 

eyepiece for wide-field systems, 
435 

flatness of lens field, 56-57 
3x21 monocular telescope, 440- 
441 

Yerkes tank telescope, 447-448 
Phosphor, use in reflex gunsights, 
494 

Photo-alidade (aerial mapping in¬ 
strument), 175 

Photoelectric photometer, 219 
Photoelectric scanning apparatus, 
258 

Photoetching method of making 
reticles, 418-420 
Photogrammetry 

see Mapping methods, aerial 
Photographic emulsions 
see Film 

Photographic Interpretation 
Center, 178, 200-202 
Photographic processing techniques 
contact printing, 63 
diffuse printing box, 168-169 
effect on camera resolution, 168- 
169 

parallel-light printer, 168 
periscope photography, 572-573 
point-source printer, 168 
recommendations, 168-169, 171- 
172 

Photographic rectification 

see Rectification of oblique photo¬ 
graphs 

Photographs for aerial mapping, 
185-200 

see also Mapping methods, aerial 
characteristics, 183 
film, 183 
plotters, 186 
rectification, 186-197 
Photography, aerial 

see Cameras, aerial; Mapping 
methods, aerial 
Photography, color 
fluorite lens, 54-55 
recommendations, 145 
36-in., f/11, 9x18 apochromatic 
lens, 63-64 


Photography, day, aerial lenses, 
40-57 

anastigmat, 50-52, 56-57 
apochromatic, 52-57 
telephoto, 44-50, 55-56 
wide-angle, 40-44 
Photography, infrared 
film, 42,183 
filters, 44-45, 48-49 
study of human eye pupils, 276- 
277 

use of fluorite crystals, 312, 333 
Photography, night 
aerial lenses, 57-62, 64 
flash photography, 57, 101-103 
recommendations, 145 
Schmidt aerial cameras, 92-103 
Photography, periscope 
see Periscope photography 
Photography, strip, 103-104, 178 
Photointerpreters, 173, 177 
Photometers 

low brightness, 272 
photoelectric, 219 
sky photometer, 268 
Photomicrographs for lens studies, 
53, 66-67 

Photopic vision, 265 
Photosensitive resists, 420 
Phototheodolites, 528-550 
Akeley instrument, 529 
applications, 528 
Askania instrument, 529 
central control station, 545-548 
Eastman recording phototheodo¬ 
lite, 531-545 

exposure time, 530, 533-536, 539- 
540 

film, 183, 538 
installation, 546-548 
recommendations, 549-550 
specifications, 528 
Phototheodolites, design 
alignment of reticles, 544 
angle-recording dials, 530 
atmospheric refraction, 531 
ball bearings, 533 
bearing accuracy, 530 
camera, 529-530, 533-536, 540 
Edgerton lamps, 530, 541, 544 
focal length, 530 
lens, 529-530, 536-537, 545 
leveling and misleveling, 530 
tracking telescope, 528 
Phototheodolites, tests, 548-550 
azimuth worm wheel, 548 
dynamic tests, 549-550 
periodic error in worms, 550 
photographic resolving power of 
lens, 537-538, 549 



INDEX 


625 


static tests, 549-550 
Physiology of human vision 
see Visual physiology 
Pinhole rectifying camera, 186-191 
distortion, 188 
effectiveness, 182-183 
lens rectifier, 188-191 
performance, 188 
Plastic lenses, 376-383 
7-in., f/2.5, 5x5; 60 
7-in., f/3, 5x5; 59 
7.5-in., f/ 2.8, 5x5; 59-60 
condenser lens, 474-475 
design considerations, 338-339 
resolving power, 377-378 
surface curvatures, 381-383 
Plastic lenses, image formation 
tests, 376-381 

image of a line source, 379-381 
light scattering, 378 
monochromatic image of a pin¬ 
hole, 378-379 
resolution, 377-378 
slit image method, 376-378 
use of gelatin wedge, 379 
Plastics, 342-388 

allyl methacrylate, 350-352, 363 
cyclohexylmethacrylate, 346-347, 
352-354, 361-365 
disadvantages, 342 
evaluation, 351-352, 387-388 
flats, 383-387 
lenses, 59, 338-339 
methyl methacrylate, 342-343, 
345, 372-373 

optical requirements, 344-345 
prisms, 387 

recommendations for future re¬ 
search, 388 

styrene, 338-339, 342, 347, 361- 
365 

Plastics, abrasion tests, 371-373 
abrader, 371 
craze lines, 373 
reflectometer, 372-373 
scatter number, 373 
wear ratio, 373 

Plastics, casting technique, 355-361 
baking cycle, 359-360 
centering and trimming, 360 
injection of the polymer, 358-359 
machining of spherical lenses, 
360 

partial polymerization, 355-356 
polymerization rates, 359 
polyvinyl alcohol sheeting, 357 
preparation of molds, 356-358 
removal from mold, 360 
yield, 360-361 
Plastics, homogeneity 


absorption of water, 345-346 
cyclohexylmethacrylate, 346, 362 
methyl methacrylate, 345 
monomer loss, 345 
styrene, 346, 362, 386-387 
variations in cross-linking, 346 
Plastics, manufacturing process, 
352-361 

allyl methacrylate, 352 
cyclohexylmethacrylate monomer, 
352-354 
polishing, 352 
polymerization, 352 
purification of styrene, 347, 354 
Plastics, optical systems, 365-370, 
455-457 

athermalization, 367-369, 456-457 
characteristics, 365-367 
cyclohexylmethacrylate, 455-456 
design limitations, 369-370 
reflector aerial gunsights, 456 
styrene, 455-456 
types, 365-367 
wide-angle eyepieces, 456 
Plastics, surface coatings, 370-375 
aluminum oxide, 374 
evaporation process, 370-371 
magnesium fluoride, 374 
silicon tetrachloride — ozone 
process, 371-375 
Plastics, synthesis, 347-351 
cross-linked polymers, 349-352, 
363, 370 

ether linkages, 348-349 
heavy metals, 348 
high-index, high dispersive power 
materials, 348-349 
increasing alicyclic rings, 349 
low dispersive power materials, 
349 

nitrogen, 348 
silicon, 348 

styrene substitutes, 349 
sulfur, 348 

Plexiglas, effect on harmonization 
of guns and sights, 308 
Plotting instruments for aerial 
mapping, 181-183 
Miller single eyepiece plotter, 
175, 182-183 

Miller stereoscopic plotter, 175, 
181-182 

oblique sketchmaster, 175, 186 
photo-alidade, 175 
stereocomparators, 177 
stereoplanigraph, 175 
Point-light-source camera, 186-187 
Point-source film printers, 168 
Polariscope for homogeneity tests 
of optical plastics, 362 




Polarizing beam splitters, 429-432 
angle of polarization, 429 
conditions for polarization, 430- 
431 

performance, 432 
zinc sulfide and cryolite layers, 
430 

Polaroid aerial lenses 
7-in., f/2.5, 5x5; 60 
7-in., f/2.8; 70 
7-in., f/3, 5x5; 59 
7.5-in., f/2.8, 5x5; 59-60 
comparison with Ektar lenses, 
59-60 

Polaroid Corporation 
f/1.6 reflex gunsight, 365-366, 494 
optical plastics, 342-388, 455-458 
T-118 antitank telescope, 365- 
366, 451-452 

Polycyclohexylmethacrylate 
see Cyclohexylmethacrylate 

Polystyrene 
see Styrene 

Polyvinyl alcohol sheeting for lens 
molds, 357 

Porro prisms, applications 
periscopic binoculars, 452-453 
6x42 telescope, 440 
use of styrene, 347, 363 
wide-angle telescopes, 436-437 

Porro system scanner, 555-559 
angular sweep, 558 
arrangement of planes of reflec¬ 
tion, 555-558 

direction of line of sight, 555-556 
monocular scanner, 558-559 

Pot furnace for manufacturing 
fluorite crystals, 322-323 

Potassium perchlorate for explo¬ 
sively - propelled binocular 
shutters, 568 

Pressure chamber for camera tests, 
143-144 

Pressure-proof binoculars, 455 

Preston Laboratories, samples of 
S-2 reflex sight, 491 

Princeton University, survey of 
sources of raw fluorspar, 
314-315 

Prism method of harmonizing guns 
and sights, 290-295 
errors, 295 
evaluation, 307-308 
field prism, 290, 293 
field target, 290-291 
gun target and boresight tele¬ 
scope, 291 
principle, 290 
procedure, 292-293 





626 


INDEX 


recommendations for future re¬ 
search, 309 
sight prism, 291 
tests, 293-295 
Prisms 

advantages for scanning, 556-558 
casting molds, 356-357 
dove prisms, 552-553, 577-578 
fringe patterns, 387 
image definition, 214-215 
inspection procedure, 213-215 
interferometer tests, 214, 240, 
242-243 
plastics, 387 

Porro prisms, 347, 436-437, 440, 
452-453 

resolving power, 214-215, 387 
roof prisms, 395-406 
rotating prism unit for camera 
mounts, 125-126 

Schmidt prisms, 437-438, 440, 
553 

Proboscope, 205, 219-220, 248 
Processing techniques, photogra¬ 
phic 

see Photographic processing 
techniques 

Projection lens, high-resolution, 
473-474 

low level of illumination, 473 
optical design, 474 
use as a collimator, 474 
Projection lens, wide-angle, 40 
Projection printing lenses, 63 
Projection striaescope, 208 
Projector for dome trainer, 472-473 
optical design, 473 
Petzval lens, 472 
Pyrex glass for lens molds, 356 
Pyrogallol, polymerization inhibi¬ 
tor, 353 

Quartz, protective coat for films, 
434 

Quartz monochromator, 394-395, 
475 

Quartz wedge, measurement of 
strain in optical glass, 209- 
210 

Radar, use of flightsight, 491 
Radax Ultraperfex preloaded ball 
bearings for phototheodo¬ 
lites, 533 

Radio altimeter for photogram- 
metric sounding, 178 
Radium, use in reflex gunsights, 
494 

Rangefinders, comparison with 
stadiameters, 505 


Rare earth glass, 337, 339 
Ratio cameras, 196 
Ray Control Company, high-resolu¬ 
tion lenses, 474 
Rayleigh limit 

astigmatism, 250-253 
coma, 250-251, 254 
depth of focus, 51 
lens aberrations, 249-250 
lens resolution, 31-32 
light resolution, 35 
recommendations, 260-261 
spherical aberration, 250-251 
Recommendations for future re¬ 
search 

aerial mapping methods, 202-203 
aerial photography equipment, 
144-146, 171, 173-174 
binoculars, 286-288, 571 
camera mounts, 145,171-173, 526- 
527 

camera resolution, 91, 165, 170- 
174 

center-of-gravity camera mounts, 
118-119, 145 
films, 171, 434 

harmonization of B-29 guns and 
sights, 306-307, 309-311 
K-17 camera shutter, 132 
lens design, 33, 62, 76-77, 144-145 
lenses for aerial cameras, 50, 56- 
58, 61 

manufacturing techniques for 
lenses, 393-394 
optical plastics, 388 
optical testing methods, 258-262 
periscope photography, 576 
photographic processing tech¬ 
niques, 168-169, 171-172 
phototheodolites, 549-550 
reflex gunsights, 504 
scanning devices, 563-564 
stadiameters, 508-509 
synthetic fluorite crystals, 145, 
340-341 

telescopes and periscopes, 470-471 
two-mirror Schmidt camera, 93 
two-star navigating device, 580- 
581 

visual physiology, 282 
wide-angle photography, 41-42, 
55, 145 

Rectification of oblique photo¬ 
graphs, 186-197 
distortion, 186 

geometric definition, 179-181 
lens rectifier, 188-191 
multiple-stage rectification, 194- 
195 

one tilt projection, 195 


orthographic plotting, 198-199 
principles of rectification, 193- 
194 

projection through a lens, 186 
three tilt projections, 196 
two tilt projections, 196 
Rectifying cameras 
Brock, 196, 199 
hypergon lens, 189, 195 
one-stage fixed camera, 196-197 
pinhole, 182-183, 186-188 
ratio camera, 196 
resolution tests, 189-190 
two-stage fixed camera, 196, 197 
Reflectometer for abrasion tests, 
372-373 

Reflex gunsights, 477-504 
Bowen sight, 494-498, 504 
figure-4; 490-491 
flightsight, 437, 491-493 
fly’s-eye sight, 501-504 
Hayward solid sights, S-l; 498- 
501 

L9k (T-95), 488 

M-16 solid sight, 501 

Polaroid f/1.6 sight, 365, 366, 494 

recommendations, 504 

T-67; 493-494 

T-94; 488-489 

Reflex gunsights, characteristics, 
477-483, 585 

aberrations, 482, 487, 491-493, 
501-502 

aircraft sights, 477 
brightness and uniformity of 
projected image, 479-480 
deflection shooting, 477, 478 
eye freedom, 477, 480-481 
lead-computing mechanism, 478- 
479 

obstruction of field of view, 483 
operation, 478 
power consumption, 483 
reliability, 483 

reticle pattern, 478, 479, 482 
size, 483 
speed ring, 478 
spherical aberration, 482 
transmission of reflex mirror, 482 
use of armor glass, 483 
Reflex gunsights, optical systems, 
483-488 

Bowen sight, 485-487 
fly’s-eye sight, 487-488 
lens sights, 483, 586-587 
Mangin sights, 483-485, 504 
Schmidt sights, 485-486 
solid sights, 484-485 





INDEX 


627 


Refraction index 

cyclohexylmethacrylate, 342, 361 
effect of time, 362 
fluorite crystals, 312, 325-326 
low-reflection films, 377-378 
methyl methacrylate, 342-343 
styrene, 342, 347, 361 
zinc sulfide, 430-431 
Refractometer for inspection of 
optical glass, 206 

Residual errors (lens aberration), 

34, 38, 52 

Resist, photosensitive, 419-420 
Resolution in aerial photography, 
147-174 

24-in. standard camera, 153, 155, 
173 

comparison of laboratory and 
flight tests, 155-156 
comparison of lenses, 153-155 
dependence on f-number of lens, 
78 

measurements, 78-79, 81-84, 89, 
169 

radial and tangential resolution, 

35, 61 

Rayleigh limit, 31-32 
requirements, 24-25 
telephoto lens, 69, 153-155 
tests, 78-80, 88, 152-153, 166-167 
wide-angle lenses, 41-42 
Resolution in aerial photography, 
limitations, 115, 156-169 
see also Lenses for aerial 
cameras, aberrations 
aerial haze, 29-31, 165-168, 173 
air turbulence, 165, 172 
camera mounts, 160 
evaluation of limiting factors, 
169 

exposure time, 31, 163-164 
focal settings, 164-165 
image flare, 155 

photographic emulsions and proc¬ 
essing techniques, 168-169 
scattered light in lens and 
camera, 166-168 

target contrast, 69, 90-91, 155- 
156 

translational motion of aircraft, 
156 

vibration of aircraft, 156-164 
Resolution in aerial photography, 
recommendations, 170-174 
camera mounts, 171-173 
flight tests, 172-173 
laboratory tests, 171-172 
specifications, 173 
strip photography, 173 
target contrast, 91, 171 


Resolution striaescope, 207 
Resolving power of optical instru¬ 
ments 

binoculars, 216 

effect of scattered light, 166-168, 
205 

measurement, 224, 366 
periscopes, 574-576 
phototheodolites, 537-538, 549 
plastic lenses, 377 
prisms, 214-215 
tests, 189-190, 244 
Resolving power of the eye, 224-225 
Resolving-power targets for 
camera tests, 148-152 
asphalt airport runways, 150-151 
circular painted canvas patterns, 
149 

high-contrast targets, 151 
masonite sheets of resolution pat¬ 
terns, 149, 152 
optically neutral paints, 149 
parallel-line patterns on con¬ 
crete slabs, 149-150, 171 
pattern reflectivity, 150, 151 
photometric areas, 150-151 
radial patterns, 149-150 
recommendations, 171 
rectifying camera tests, 190 
resolution patterns, 151-152 
Reticles 

alignment in phototheodolites, 
544 

for fly’s-eye gunsight, 501-503 
pattern for gunsights, 478, 479, 
482 

photographic reticle plate, 503 
speed ring for gunsights, 495-496 
Reticles, photographic manufactur¬ 
ing techniques, 417-421 
advantages, 417-418 
bichromated glue-relief process, 
418, 419 

Buckbee-Mears method, 418, 420 
colloidal-silver process, 418-420 
Eastman glue-silver process, 418- 
420 

etching of an opaque subcoat 
under a glue resist, 418 
evaluation, 419-421 
Keuffel and Esser process, 418 
lead-sulfide process, 418-420 
line widths, 420-421 
photoetching method, 418-420 
photosensitive resists, 420 
recommendations, 421 
silver-line process, 418-420 
Reticles, specifications, 221-223 
inclusions, 209 
inspection methods, 215 


parallax, 221-222, 245-246 
use of collimator, 222 
use of dioptometer, 222, 244 
Robinson-Houchin Company, 
gimbal mount, 519 
Rocket guns, reflex sights, 499 
Rockwell hardness tester, 363 
Rod vision, 276 

Roll film Schmidt camera, 101-103 
Rolling cylinder scanner, 561-563 
adjustment of inter ocular dis¬ 
tance, 563 
optical layout, 561 
scanning without rotation of 
field, 561 

Roof prisms, manufacturing tech¬ 
niques, 395-406 
air jet spherometer, 405 
blocking process, 404-405 
Dextrex degreaser, 396 
fine grinding, 405 
general principles, 355-357 
jigs, 396 

milling procedure, 396-404 
polishing, 405 

roof angle corrections, 405-406 
shaping, 396 

wheels for grinding, 395-396 
Ross spherical correction, 34 
Ross survey lens, 65 
Rotating ball camera shutter, 40 
Royal Aircraft Establishment at 
Farnborough, lens tests, 89 
Rubber-shell antioscillation mounts, 
513-515 

damping method, 513-514 
recommendations, 526-527 

S-l reflex gunsight, 490-491 
S-2 reflex gunsight, 491 
S-3 reflex gunsight, 491 
Scanning devices, 551-564 

aircraft scanning chair, 560, 563 
altazimuth four-mirror scanner, 
559-560 

altazimuth two-mirror scanner, 
554-555, 563 
applications, 563 
automatic scanning, 552 
double dove prism scanner, 552- 
553 

panoramic scanner, 553-554 
photoelectric apparatus, 258 
Porro system scanner, 555-559 
principles of optical and mechan¬ 
ical design, 552 
recommendations, 563-564 
requirements, 551-552 
rolling cylinder scanner, 561-563 
vibration, 551 



628 


INDEX 


Schmidt aerial cameras, 92-104 
correcting plate, 147, 360 
f/1 camera with 8-in. focal 
length, 103-104 

for electric flash night photog¬ 
raphy, 101-103 

Schmidt aerial cameras, two-mir- 
ror, 92-101, 147 
antioscillation mount, 95 
camera frame, 95 
correcting plate, 92-93 
film magazine, 97-99 
ground sweep mechanism, 97 
length, 92-93 

Louvre shutter, 93, 99-100 
method of focusing, 95 
optical constants, 93 
recommendations for future re¬ 
search, 93 

resolution measures on Super-XX 
film, 93 

solid glass type, 100-101 
Schmidt f/0.7 optical system, 365, 
366 

Schmidt gunsights, 485-486 
Schmidt prism erector, 437, 440 
Schmidt reflectors, molding process, 
410-411, 413 

Schmidt rotating prism, 553 
Scotopic vision, 265, 473 
Scout camera lenses, 62 
Seeds, detection in optical glass, 
209 

Servo-controlled camera mount, 
119-123 

control mechanism, 119-120 
damping system, 122-123 
design of servo mechanism, 121- 
123 

follow-up mechanism, 120 
gimbalized gyroscope, 120-121 
integrating mechanism, 121 
silicone fluid for damping, 123 
7-in. lenses 

f/2.5, 5x5 (Harvard), 60-62 
f/2.5, 5x5 (Harvard-Polaroid), 
60 

f/2.8 (Polaroid), 70 
f/3, 5x5 (Polaroid), 59 
7.38 in., f/2.8 aerial camera lens, 
365, 366 

7.5-in., f/2.8, 5x5 plastic lens, 59-60 
7x telescope, 440 
7x35 monocular telescope, 441 
7x50 binoculars, 441-444 
10-mm exit pupil, 443 
dummy binocular, 443-444 
eyepieces, 442-444 
for submarines, 452 
increased eye distance, 443 


optical characteristics, 437 
parabolic eyepiece, 441-442 
reduced diameter at eye end, 442 
visual tests, 437 

Shake tables for testing optical 
mounts, 516-517 
Shellburst film, 183, 538 
Shock mounts 

see Antioscillation mounts; Anti¬ 
vibration mounts 

Shutters for aerial cameras, 130- 
140 

Bartol model, 139 
continuously-operating blades, 
139-140 

continuously-operating focal 
plane shutter, 140 
for wide-angle lenses, 40-41 
K-17 shutter, 132-133 
Louvre, 99-100 
Metrogon shutter, 130-132 
multiple slit focal plane shutter, 
133-139 

recoil velocities, 163 
recommendations, 145, 171-172 
vane-driven shutter, 139 
vibration, 163 

Shutters for night binoculars, an¬ 
tiglare, 565-571 
explosive propellant, 568 
high-speed shutter, 566-568 
mechanical shutter, 571 
photoelectric control circuit, 568- 
571 

purpose, 565-566 
recommendations, 571 
Sights for guns 

see Reflex gunsights 
Silhouetting effect in aerial lenses, 
34 

Silicon for optical plastics, 348 
Silicon tetrachloride for coating 
plastics, 371-375 
desiccator, 374 
humidifier conditioning, 374 
method of obtaining hard films, 
373-375 

nitrogen dioxide treatment, 375 
ozonizer, 374 

resistance of plastics to abrasion, 
371-373 

Silicone fluid for damping camera 
mounts, 123 
Silver films, 433 

Silver-line process of producing 
reticles, 418-420 
6-in. lenses 

dialytique apochromatic, 335 
f/1 curved field lens, 57-58 
f/2.85 wide-angle, 145 



6-H-62 lens, 70 
6x binoculars, 510 
6x40 telescope, 365, 366 
6x42 telescopes, 437 
monocular, 440-441 
Porro type, 440 
60-in. lenses 

f/5, 9x18 telephoto, 48-50 
f/6, 9x9 telephoto, 44-45, 48-49 
Sketchmaster, oblique, 175, 186 
Sky apparatus, artificial, 256-258 
effect of coated optics, 257-258 
effect of scattered light on reso¬ 
lution, 205 

effect of striae, 257-258 
Sky photometer, 268 
Sonne stereostrip camera, 200-201 
Specifications 

f/20 apochromatic triplet objec¬ 
tive, 336 

H-12 fluorite corrector for 1.9 
periscope, 461 
lens gunsights, 586-587 
lens rectifier for oblique photo¬ 
graphs, 188-189 
lenses, 173, 210-213 
M-71 telescope, 216-223, 236 
optical glass, 205-210 
optical instruments, 216-223 
P-55 fluorite corrector for 1.4 
periscope, 460 
phototheodolites, 528 
stadiameter for fire-control sys¬ 
tem, 508 

tank telescope, 445 
wedges, 213-215 
windows, 214-215 
Spectrograph, vacuum, 326 
Sperry Gyroscope Company, gyro 
recorder used for aerial vi¬ 
bration tests, 158 
Spherical aberration 

effect on KDC efficiency, 250-251 
measurement, 205, 247-248, 366 
modifier, 249-251 
oblique, 38 

Rayleigh limit, 250-251 
reduction by apochromatic lens, 
52-57, 333-337 

symmetrical errors of zone, 37- 
38 

tests, 244 

Spherical achromatism, 340 
Spinel, 145, 337-338 
Split-field tank telescope 
aberrations, 450-451 
characteristics, 450 
orthoscopic eyepiece, 450-451 



INDEX 


629 


recommendations for future re¬ 
search, 471 

telephoto objectives, 450 
Spring camera mounts, 112-113, 
128 

Stabilized mounts for aerial cam¬ 
eras, 119-124 

Eastman servo-controlled mount, 
119-123 

mirror mount, 123-124 
Stadiameters, 505-509 

advantage over rangefinders, 505 
for B-29 fire-control system, 508- 
509 

recommendations, 509 
three-power, 506-508 
unit-power, 505-506 
Stanford University, photoelectric 
trigger circuit for binocular 
shutters, 568-571 

Stellite for molding presses, 409- 
410, 413 

Stereocomparators (aerial map¬ 
ping instrument), 177 
Stereoplanigraph (aerial mapping 
instrument), 175 

Stereoscopic plotting instrument, 
175,181-182 

Stereostrip camera, 200-201 
Stern and Company, photographic 
methods of reticle produc¬ 
tion, 417-421 

Stiles-Crawford visual effect, 276 
Stones, detection in optical glass, 
209 

Strain specifications for optical 
glass, 209-210 

Striae specifications for optical 
glass, 207-209 
Striaescopes, 207-208 
direct view, 208 
elements, 205 
projection, 208 
resolution, 207 

Strip photography, 103-104, 178- 
179 

Strobolux lamps for determining 
camera vibration, 157-158 
Strontium fluoride, synthetic, 332 
Styrene 

derivatives, 349 
lenses, 338-339, 347, 359 
polymerization, 347, 356 
Porro prisms, 347, 363 
purified form, 347, 354 
resolution of prisms, 387 
storage, 354 

use in M-16 solid reflex gunsight, 
501 

Styrene, properties, 361-365 


dispersive power, 342, 361-362, 
455-456 

homogeneity, 346, 362, 386-388 
refraction index, 342, 347, 361 
scratch resistance, 364-365 
softening temperatures, 363-364 
strain, 362-363 
surface accuracy, 362 
surface hardness, 363, 373 
tensile, impact, and flexural 
strength, 365 
thermal conductivity, 364 
transmission, 363-364 
transparency, 347 
water absorption, 346-347, 363 
wear ratio, 373 
Submarine binoculars, 452 
Submarine periscope photography 
see Periscope photography 
Submarine periscopes, 458-464 
1.4; 458-463 
1.9; 459, 461,463 
IV; 459, 461-463 
aberrations, 459 

correction for field curvature, 
462-463 

inverted Galilean telescope, 459 
properties, 458-459 
recommendations, 471 
suggestions for improvement, 463 
Super-XX film 

combined with red filter, 69, 93 
performance, 538 
properties, 91-92 
resolution measures, 155 
Sweep mechanism for camera 
mounts, 124-125 

compensation for image move¬ 
ment, 156 

intervalometer, 113, 115, 124 
Sylphon bellows, use in camera 
mountings, 46, 95 

Synthane sockets for camera 
mounts, 110-111 
Synthetic fluorite crystals 

see Fluorite crystals, synthetic 

T-14.64 tank telescope, 449 
T-67 reflex gunsight 
application, 493 
illuminant, 493-494 
moment of inertia, 493-494 
parallactic range, 493-494 
reticle pattern, 493-494 
T-76 tank telescope, 445 
T-93 tank telescope, 440 
T-94 gunsight, 488-489 
T-95 gunsight, 488 
T-108 antitank telescope 
athermalization, 367 


eye relief feature, 366 
image formation, 376-381 
performance, 365 

T-116 telescope, image formation 
t^sts 

see Plastic lenses, image forma¬ 
tion tests 

T-118 antitank telescope 
eccentric collective, 451-452 
mirror erectors, 451-452 
performance, 365 
Tank binoculars, 452-453 
Tank telescopes, 444-451 
5x, 445-446 

bifocal bipower telescope, 449- 
450 

Cooke triplet objective, 444-447 
Eastman telescope, 445 
recommendations, 471 
split-field telescope, 450-451, 471 
T-76; 445 

Yerkes telescopes, 446-449 
Target contrast 

effect on camera resolution, 69, 
90-91,155-156 

equivalent target contrast, 28 
high-contrast targets, 151 
low-contrast targets, 274-275 
threshold of visibility, 274 
Target visibility 

detection of small targets, 271 
threshold size of a target, 278, 
281 

Targets for studying lens resolu¬ 
tion 

see Resolving-power targets for 
camera tests 
Tavistock theodolite, 457 
Technicolor Motion Picture Corpo¬ 
ration 

micrometer film holder, 164-165 
multiple slit focal plane camera 
shutter, 135-139 

rubber - shell antioscillation 
mounts, 513-515, 526-527 
shake table for testing periscope 
scanning device, 517 
Telephoto lenses, 44-50 
15-in., f/II; 537 

36-in., f/8, 9x18 wide-angle, 55- 
56, 70-76, 340 

36-in., f/11, 9x18 apochromatic, 
63-64, 69 

40-in., f/5, 9x9; 44-49, 155-156, 
167 

40-in., f/8; 67-69 
48-in., f/8, 3%x4%; 62 
60-in., f/5, 9x18; 48-50 
60-in., f/6, 9x9; 44-45, 48-49 



630 


INDEX 


disadvantages for color photog¬ 
raphy, 36-37 

disadvantages for phototheodo¬ 
lites, 536 

resolution, 69,153-155 
spherical achromatism, 340 
tests, 70 

wide-angle lenses, 55-56, 70-76, 
340 

Telescopes, 444-452 

antitank, 376-381, 451-452 
boresighting, 309 
coated optics, 257-258 
effect of vibrations, 511 
eyepieces, 435-437, 441-442, 448, 
450-451 
Galilean, 459 

KDC efficiency, 216-217, 258-260 
night requirements, 435 
objectives, 335-336, 444, 475-476 
precision theodolite telescopes, 
457-458 

recommendations, 470-471 
tank telescopes, 444-451, 471 
testing procedure, 238-239 
tracking telescopes for photo¬ 
theodolites, 528 
use of fluorite crystals, 312 
vibrations, 511 
wide-field systems, 435-442 
Telescopes, specific types, 437-442 
3x; 437-438, 441-442 
3x21; 440-441 
6x40; 365, 366 
6x42; 437, 440-441 
7x; 440 

7x50; 437, 441-442 
10x50; 437, 553 
M-17; 542-543 
M-70; 231-233 
M-71; 216-224,236 
M-72; 236 
M-76; 236 

T-108; 365-368, 376-381 
T-116; 376-381 
T-118; 365, 451-452 
Telestigmat lens, 65 
Telikon lens, 39-40, 62, 340 
10x50 binocular with 7-degree field, 
442 

10x50 telescope, 437 
p- tertiary butyl catechol, poly¬ 
merization inhibitor, 354 
Testing methods for optics 
see Optical testing methods 
Tetrazene for explosively-propelled 
binocular shutters, 563 
Theodolite telescopes 
C.T.S. level, 457 
design, 457-458 


eyepiece, 457 

optical characteristics, 458 
optical specifications, 457 
Tavistock T-65; 457 
Wild T-2 ; 457 
Zeiss II, 457 
Theodolites 

see Phototheodolites 
36-in. lenses 
f/8 apochromatic, 52-55 
f/8, 9x18 anastigmat, 56-57 
f/8, 9x18 wide-angle telephoto, 
55-56, 70-76, 340 

f/11, 9x18 apochromatic tele¬ 
photo, 63-64, 69 
3x tank telescope 

see Yerkes tank telescope 
3x telescopes, 437-438, 441-442 
3x21 monocular telescope, 440-441 
Threshold of visibility 

binoculars for night vision, 285 
definition, 267 
frequency of seeing, 267 
method of determining, 266-267 
target contrast, 274 
Tiffany Foundation project, naked 
eye data on target contrasts, 
281 

Titanium dioxide for high-reflec¬ 
tion films, 428 
Topogon lens, 52 
Topographic mapping 

see Mapping methods, aerial 
Transmission patterns 
flats, 383-387 
fluorite crystals, 326, 327 
natural fluorite, 327 
prisms, 387 

specifications for optical parts, 
243-244 

Triangulation, aerial, 178 
Trimetrogon photography, 177, 192 
Tri-X night film, 58 
12-in. lenses 

f/4.5, 9x18 anastigmat, 64 
f/5, 9x9 anastigmat, 57 
24-in. lenses 

Aero-Ektar, 155,167 
f/3.5, 9x18 (for night photog¬ 
raphy) , 64 

24-in. standard aerial camera 
A-8 mount, 112-114, 159-160 
K-17 camera shutter, 132-133 
resolution, 153, 155, 173 
II-c binocular mount, 519 
Two-mirror Schmidt camera 

see Schmidt aerial cameras, two- 
mirror 

Two-star navigating device 

see Navigating device, two-star 



Twyman - Green interferometer, 
383-387 

Twyman-Michelson interferometer 
see Michelson-Twyman interfer¬ 
ometer 

U. S. Management and Engineer¬ 
ing Company, stadiameters, 
506 

University of Chicago 
see Yerkes Observatory 

University of Michigan, exposure 
meter for aerial cameras, 
141 

University of Pennsylvania 
binocular tests, 276-282 
low brightness photometer, 272 

University of Rochester 

10x50 wide-field 9-degree tele¬ 
scope, 553 

antioscillation mounts, 510, 512- 
513, 516-522 
curved-field lens* 57-58 
f/1 aerial lens, 61 
figure-4 gunsights, 485, 490-491 
flightsight (reflex gunsight), 
437, 491-493 

low-power telescopes and binoc¬ 
ulars, 435-471 
night binoculars, 565-566 
polarizing beam splitters, 431 
prism method of harmonizing 
guns and sights, 290-295 
sky photometer, 268 
stadiameters, 505-509 
T-67 gunsight, 493-494 
T-94 gunsight, 488-489 

Vacuum film magazines, 172 

Vacuum spectrograph, 326 

Vane-driven camera shutter, 139 

Vibration of aircraft, effect on 
camera resolution, 156-164 
angular resolutions, 160 
exposure limitations, 156-157 
flight tests, 148 

frequency spectrum of vibration, 
172 

method of evaluating vibration 
magnitude, 156-158 
recommendations, 145, 171-172 
roll, pitch, and yaw motions, 158- 
159,162-163 
sources, 520 

Vibrations in optical systems, 510- 
512 

angular vibration, 511 
antioscillation mounts, 511-512 
camera shutters, 163 



INDEX 


631 


effectiveness of antivibration 
mounts, 511 

purpose of shock mountings, 510- 
511 

scanning devices, 551 
telescopes, 511 

Vignetting in aerial lenses, 34 
apochromatic lens, 52-54 
balancing of coma, 37 
dodging of prints, 38-39 
wide-angle telephoto lens, 55 
Vinyl benzoate, 349 
Viscous damping of optical sys¬ 
tems, 107-109, 523-526 
comparison with frictional damp¬ 
ing, 108-109, 522 
evaluation, 525, 526 
theory of vibration isolation, 
523-524 

Visibility of targets 

detection of small targets, 271 
threshold size of a target, 278, 
281 

Visibility threshold 

binoculars for night vision, 285 
definition, 267 
frequency of seeing, 267 
method of determining, 266-267 
target contrast, 274 
Visual aids for night use 

see Binoculars for night vision 
Visual physiology 
brightness levels, 276-277 
cone vision, 276 

fluctuations in pupil size, 276-277 
photopic vision, 265 
recommendations for future re¬ 
search, 282 
rod vision, 276 
scotopic vision, 265, 473 
Stiles-Crawford effect, 276 
tests with infrared photography, 
276-277 

Water depth determination, pho¬ 
tographic method 
aerial cameras, 200-202 
photogrammetric sounding, 178 
Wedge patterns for lenses 


Aero-Ektar lens, 70 
f/3.5 wide-angle, 78 
Harvard 36-in., f/8; 78 
Polaroid f/2.8; 70 
recommendations, 171 
Wedges, specifications, 213-215 
White glass lens, 7-in., f/2.5, 5x5; 
61 

Wide-angle binoculars 
10-mm exit pupil, 443 
dummy binocular, 443-444 
eyepieces, 442-444 
increased eye distance, 443 
reduced diameter at eye end, 442 
Wide-angle gunsights, 479-480 
Wide-angle photography, 40-44 
5.950-in., f/3.5 lens, 41, 78 
6-in., f/2.85 lens, 145 
concentric lenses, 64-65 
film, 40, 42 

fluorite lenses, 148, 444 
meniscus lenses, 339 
projection lens, 40 
recommendations, 41-42, 55, 145 
resolution, 41-42 
shutters, 40-41 
spherical crown lens, 40 
telephoto lens, 55-56, 70-76, 153- 
155, 340 

tests, 42-44, 65, 70 
Wide-angle telescopes, 435-442 
3x monocular, 441-442 
3x21 monocular, 440-441 
6x42 monocular, 440-441 
7x telescope, 440 
7x35 monocular, 441 
7x50 monocular, 441-442 
advantages, 435 
eyepieces, 435-437 

Wide-field photogrammetry, 178- 
179 

Windows, specifications, 214-215 
Wire method of harmonizing guns 
and sights, 290, 295-302 
evaluation, 307-308 
Mark II, 295-299 
Mark III wire method, 299-302 
recommendations for future re¬ 
search, 309-310 
suggested improvements, 309 


Wratten filter for periscope pho¬ 
tography, 575 
Wright Field, Ohio 

resolution patterns for aerial 
c'ameras, 149 

resolving-power targets, 148 

Yerkes Observatory 

aircraft periscopes, 464-469 
apo-periscope objective, 336-337 
high-resolution projection lens, 
473-474 

L9k gunsight, 488 
Lens-Mangin gunsight, 483-484, 
504 

M-16 reflex gunsight, 501 
panoramic scanner, 553-554 
spherical aberration modifier, 
249-251 

submarine periscopes, 458-464 
wide-field projector for dome 
trainer, 472-473 
Yerkes tank telescope 
aberrations, 445-449 
erecting system, 446-448 
Erfle eyepiece, 448 
lens, 446-448 
optical constants, 449 
Petzval sum, 447-448 
specifications, 445 
T-14.64; 449 
T-93;449 

Zeiss binoculars, 437, 442 
Zeiss theodolite, 457 
Zeiss-Telikon lens, 39-40, 62, 340 
Zinc sulfide 

dispersion power, 431 
high-reflection films, 427-428 
in polarizing beam splitters, 430 
refractive index, 430-431 
Zone errors (lens aberration), 33- 
34 

anastigmat lens, 51-52 
curved-field lens, 57-58 
varying as cube of aperture, 37- 
38 

varying as fifth power of aper¬ 
ture, 38 































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